Re: Properly designed PAs (was: Re: [Repeater-Builder] Coax length, etc.)
Nate, My sincerest thoughts are with you in this time. Don, KD9PT - Original Message - From: Nate Duehr To: Repeater-Builder@yahoogroups.com Sent: Tuesday, August 17, 2010 12:46 AM Subject: Re: Properly designed PAs (was: Re: [Repeater-Builder] Coax length, etc.) On Aug 15, 2010, at 7:06 AM, Jeff DePolo wrote: Whoever said time is money was an idiot. Time is worth inifinitely times more than money. You can make more money. You can even borrow money. Hell, if you were desparate you could even steal money. You can't do any of those things with time. Time is the one resource you can't make more of. With the passing of a loved-one in my family today, truer words could not be spoken. Money won't even buy an additional 5 minutes of idle chit-chat with someone you care about. I'll jump back into the conversation later... family's already on the way here and it's going to be a busy rest of the week. My wife and I are supposed to sing at the funeral, and I'm a pall-bearer. Didn't want you to think I'd disappeared on you Jeff. Appreciate the TIME you took to share your experiences with PAs. Won't have any time to respond with my thoughts for a little while, though. Best Regards, -- Nate Duehr, WY0X n...@natetech.com
Re: Properly designed PAs (was: Re: [Repeater-Builder] Coax length, etc.)
Nate, Please accept my sympathies and condolences to you and your family. I agree with you on the time aspect entirely. With Prayers - John Hymes La Rue Communications 10 S. Aurora Street Stockton, CA 95202 http://tinyurl.com/2dtngmn - Original Message - From: Nate Duehr To: Repeater-Builder@yahoogroups.com Sent: Monday, August 16, 2010 10:46 PM Subject: Re: Properly designed PAs (was: Re: [Repeater-Builder] Coax length, etc.) On Aug 15, 2010, at 7:06 AM, Jeff DePolo wrote: Whoever said time is money was an idiot. Time is worth inifinitely times more than money. You can make more money. You can even borrow money. Hell, if you were desparate you could even steal money. You can't do any of those things with time. Time is the one resource you can't make more of. With the passing of a loved-one in my family today, truer words could not be spoken. Money won't even buy an additional 5 minutes of idle chit-chat with someone you care about. I'll jump back into the conversation later... family's already on the way here and it's going to be a busy rest of the week. My wife and I are supposed to sing at the funeral, and I'm a pall-bearer. Didn't want you to think I'd disappeared on you Jeff. Appreciate the TIME you took to share your experiences with PAs. Won't have any time to respond with my thoughts for a little while, though. Best Regards, -- Nate Duehr, WY0X n...@natetech.com
Re: Properly designed PAs (was: Re: [Repeater-Builder] Coax length, etc.)
On Aug 15, 2010, at 7:06 AM, Jeff DePolo wrote: Whoever said time is money was an idiot. Time is worth inifinitely times more than money. You can make more money. You can even borrow money. Hell, if you were desparate you could even steal money. You can't do any of those things with time. Time is the one resource you can't make more of. With the passing of a loved-one in my family today, truer words could not be spoken. Money won't even buy an additional 5 minutes of idle chit-chat with someone you care about. I'll jump back into the conversation later... family's already on the way here and it's going to be a busy rest of the week. My wife and I are supposed to sing at the funeral, and I'm a pall-bearer. Didn't want you to think I'd disappeared on you Jeff. Appreciate the TIME you took to share your experiences with PAs. Won't have any time to respond with my thoughts for a little while, though. Best Regards, -- Nate Duehr, WY0X n...@natetech.com
Properly designed PAs (was: Re: [Repeater-Builder] Coax length, etc.)
On Aug 14, 2010, at 9:45 PM, Jeff DePolo wrote: I disagree. I would accept the notion that the transmitter may not be happy (and I put that in quotes not to mock you, but becuase I can't come up with a better word either) because it is not *properly matched* when looking into a 50+j0 load. This indicates a deficiency in the amplifier; if it were designed and working right, it *should* make rated power when terminated in a 50 ohm load on-channel. Okay, I'd love to go off on a side tangent here for a moment. (By the way, loving the discussion. Learning from it.) Jeff, out of all the PAs you've seen out there, both commonly used and not-so-common... which ones (in your opinion) are properly designed (when working right)? I have this feeling that most, if not all, have various problems... but you've seen a heck of a lot more of them in-service than I have. Which ones behave the best, as regards to this other discussion that's going on about making them happy. In other words, which ones have you bought/used that you hooked up, and forgot about them completely because you knew they'd just work. I ask, because this is always the kind of mature, well-developed tech I'm looking for. Price is still a factor, but when you find something that just works... it's truly grand in the tech world, for all sorts of reasons that tend to degrade what something was intended to be, vs. what it really ended up being. I'm also curious to see if your recommendations are new gear, or 20+ year old gear. I really like MASTR II Stations, but I will admit to some consternation over how the PAs *sometimes* act. We've had 'em run for a decade, and we've had 'em pop like light bulbs every few months. Yes, the problem is often in the duplexer/feedline/antenna system when this happens, but it's also often subtle and not exactly easy to find. I'm wondering to myself, (and now out loud)... Is there a PA out there that wouldn't have cared or been un-happy? Bonus points for it monitoring its own happiness and turning on an alarm light, closing a contact, etc. Is the answer to this question the Crescend amps perhaps? How did their acquisition of Vocom affect their quality? I haven't looked lately, did they mix up the model line and keep the Vocom stuff? Just some questioning thoughts, not very well thought out, at almost 1AM... -- Nate Duehr, WY0X n...@natetech.com
Re: [Repeater-Builder] Coax length, etc.
On Aug 14, 2010, at 9:45 PM, Jeff DePolo wrote: Well, kinda. Many duplexers are spec'ed for 1.5:1 (14 dB RL) input VSWR max. Fortunately, I rarely see any that are that bad. I'll gladly trade off a tenth of a dB of insertion loss for several (if not 10 or more) dB of return loss improvement when I'm tuning on the VNA, but some hams are greedy and don't think along those lines when they're tuning... Actually I think that even though Service Monitors have finally become *relatively* commonplace in the Ham Shack, the VNA is not something most hams have seen or know how to use. (I would count myself in that group.) Like Service Monitors used to be before the flood of HPs on eBay in the last few years, I hear rumors of great deals on VNAs, and yet never see them in any way plentiful, easy to acquire, or affordable, but then again I'm also not exactly looking that hard, and perhaps I'm missing one of those everyone knows about Bob's VNA Warehouse! kinds of sources for such things. I do find it interesting (sorry another side-thought) that a great many *professionals* don't seem to have access to them, nor can convince their employers to purchase them. I have heard the excitement in professional RF Engineer's voices when their company finally acquires one... and seen 'em load up the car with stuff they've always wanted to put on the VNA. A few years ago, anyway. Are they more common than I think? From Jeff's comments and other's off-list, they're obviously the right tool for the job, and tuning duplexers without them seems similar to messing around with taking the engine out of a car without an engine hoist, but unlike engine hoists -- professional RF shops don't even seem to regularly own a VNA or have anyone on staff qualified/trained on how to use one. Can't count the number of cell site and other RF techs I've talked to over the years who were just happy as clams when they finally got TDR equipment to check cables too. What's up with the RF industry not buying these things by the truckload? Too spendy? -- Nate Duehr, WY0X n...@natetech.com
Re: [Repeater-Builder] Coax length, etc.
Hi Kevin: Regarding temperature, our club has a site, no A/C or heat, where temperatures inside the shelter can get below +20 deg F in winter, and well over 130 deg F in the summer heat. I can't imagine filter tuning not changing under such conditions, Invar or not. I can see over time where tuning might walk off the reservation. But I bow to your greater experience with cavity duplexers. Another chance? Which part, erroneous readings, don't directly measure power, or the voltmeter part? Sure, what the heck. ;-) I've had Bird 43's, and calibrated line sections with matched elements for that matter, give erroneous reflected power readings depending upon what was going on with the transmission line. By erroneous, I mean it was usually a reading that was, for example, excessively high versus what we knew was going on, such as a straight piece of rigid line or coax terminated into a known good load. On rare occasion, we found we slipped a bullet or had a bad connector. More often, relocating the instrument somewhere else along the line resolved those bad readings. RF calorimeters can measure power directly. But unless they've one hidden in them somewhere, ThruLine meters can not. Just because the Commission might accept wattmeter readings, or Bird says so, doesn't make it so. As for the voltmeter part, check out page 6 of the Bird 43 manual (page 18 of the PDF), a copy of which you'll recall is here: http://www.repeater-builder.com/bird/pdf/bird-43-wattmeter-2004.pdf I respectfully submit what is shown is a schematic/diagram of a directional coupler attached to a voltmeter as an indicator. An induced RF voltage sample is rectified, filtered and applied through a dropping resistor to a shunt-connected ammeter. By definition, a voltmeter is the shunt-connected ammeter with series resistor part. But don't take my word for it. Take a peek at Chapter 25 in any recent ARRL Handbook (this works for my 2007 copy anyway). Is it less a voltmeter because the induced voltage tracks current on the line? Want to call it an ammeter or current meter then, after all that's what the actual meter movement is? I submit this particular voltmeter happens to be calibrated to read average power at 50 ohms impedance, and it does this quite well within its limitations. I now await your thrashing. Please be gentle. ;-) Like the manual says, the Bird 43 is fast, convenient and accurate. I agree it's fast and convenient. I'll agree it's accurate with the caveats expressed. It beats lugging a slotted line around, and it beats every other meter like it, IMHO, including my old Daiwa dual-metered POS wattmeter. ;-) Oh, BTW, the emperor has no clothes either. :-P 73, Russ WB8ZCC On 8/14/2010 10:11 PM, Kevin Custer wrote: Russ Hines wrote: Some related comments, if you don't mind. Temperature changes seem to be the biggest detuner of largely mechanical devices like cavity duplexers. We often send our repeaters off to live in less-than-ideal environments, then expect cavity input/output impedances to remain as we measured them in the shop? Don't think so. I largely disagree. Most modern duplexer designs (within the last 25 years or so) use compensating elements to make the duplexer or cavity temperature stable. Invar is a nickel-steel alloy that exhibits about 1/10 the thermal expansion as a common carbon steel counterpart. Invar is used to make the tuning rod - many times it's threaded. The rest of the duplexer or cavity is usually made of similar metals and generally thermal expansion occurs across these components equally, resulting in extremely low frequency drift over its rated operating temperature. Our in-line power meters, like our trusted Bird 43, do not directly measure power. They're really voltage meters calibrated in watts at a specific impedance. That's why they can be fooled into displaying an erroneous reflected power reading, perhaps lulling us into a sense of security that the VSWR on the line is acceptable when it may not be. What? Maybe you would like to have another chance at that one Kevin Custer
Re: [Repeater-Builder] Coax length, etc.
I don't know about that. Anritsu SiteMaster and CellMaster test sets are fairly common test equipment available to cell techs here in Connecticut. Whether they use them (or know how) is another thing. Joe On 8/15/2010 2:59 AM, Nate Duehr wrote: What's up with the RF industry not buying these things by the truckload? Too spendy?
RE: [Repeater-Builder] Coax length, etc.
But why? If all of the power (or, let's hope, at least 99.99% of it) is on-channel, *should* a properly-designed and properly-functioning transmitter misbehave due to the poor match a duplexer presents at frequencies far removed from the channel center? Well yes, properly designed transmitter. But how much do you want to pay for it? Me personally? I'll pay for a transmitter that works, and works right. The way I see it, repeaters are like cars. You have to get your car inspected for safety. Your car doesn't pass safety inspection? You can't drive it on the public roads, lest you'd be putting other people at risk. Same with a repeater transmitter. If it's unstable and has the potential for causing interference other systems (ham repeaters, public safety, aviation, etc.), it shouldn't be on the air. Either fix it, or if you can't afford to fix it, take it down. I don't want some scmuck driving a beat-up 1972 pickup down the interstate in front of me and having his rear bumper fall off any more than I want somebody putting some clunker up on a mountantop and having it go spurious and interfering with EMS or ATC. That's just the way I see it, sorry if that rubs some people the wrong way. A built in isolator will solve all of those problems as an example. Maybe. An isolator will help flatten the load on, and around, the carrier frequency, but isolators, too, have a finite VSWR bandwidth, they won't provide a perfect load across the entire spectrum. And if you can afford an isolator, you can probably afford a better PA. It is almost impossible for a high Q cavity to not present some reactance away from the tuned frequency. It's not almost impossible, it's definately impossible. If it didn't then it would not have any selectivity. Right. The random length cable of course transforms that reactance to something that the transmitter may or may not be comfortable with as discussed above. Just to clarify, the complex Z is being transformed (both R and jX), not just the reactive component. The thing with random-length cables is just that - they're random. How do we know what cable length is going to make the transmitter happy? Does the transmitter like more XL or more XC, or bigger R's or smaller R's, and at what frequency, because as I'm sure you know, the complex Z is going vary wildly at different frequencies, due to the duplexer's Z, its behavior as a transformer with respect to the load Z at the antenna port, the antenna feedline acting as a transformer with respect to the antenna feedpoint Z, and the cable between the PA and the duplexer also acting as a transformer, so you end up with this complex system of cascaded transformers. Chances are if the PA is that picky, its behavior may also change with temperature, voltage, who knows what else. Antenna feedpoint Z's change with environmental conditions (precipitation, icing, etc.). Feedline electrical lengths (phase) change with temperature, so the resulting Z at the duplexer antenna port is also going to change. There are *so many variables* that will constantly be changing over time that what may seem to work when you walk off the site may fail miserably months, days, maybe even hours later after you think you've found that magic cable length. At least with an isolator we've taken the bulk of those external variables out of the equation - I can agree with that. But, call me a fundamentalist, I still believe that a PA should work, and work right, when it sees 50 ohms on-channel no matter what's happening off-channel. --- Jeff WN3A
RE: [Repeater-Builder] Coax length, etc.
Actually I think that even though Service Monitors have finally become *relatively* commonplace in the Ham Shack, the VNA is not something most hams have seen or know how to use. For $100, Rick's (Amtronix) return loss bridge is a must-have for anyone that has a SM with a SA/TG. With it, there's no longer any excuse for not being able to tune cavities properly for maximum return loss. Like Service Monitors used to be before the flood of HPs on eBay in the last few years, I hear rumors of great deals on VNAs, and yet never see them in any way plentiful, easy to acquire, or affordable, but then again I'm also not exactly looking that hard, and perhaps I'm missing one of those everyone knows about Bob's VNA Warehouse! kinds of sources for such things. Hey, I didn't say they were cheap, nor that everybody can or should own one. There's nothing more enjoyable than tuning up a $100 duplexer from Dayton on a $50,000 network analyzer, especially when it's a 3-porter and you don't even have to swap cables around :-) --- Jeff WN3A
RE: Properly designed PAs (was: Re: [Repeater-Builder] Coax length, etc.)
Jeff, out of all the PAs you've seen out there, both commonly used and not-so-common... which ones (in your opinion) are properly designed (when working right)? I think a lot of them, generally speaking, are properly designed. That's not to say that some of them don't have some downsides or specific, recurring points of failure (to wit: the beloved Mastr II output strap connection failure). I'd name manufacturers that are on my $^!+ list, but I'd rather not do that here, but I will say that most of them are the made-for-amateur brands. I've had great luck with just about anything Micor (and, I have to say, significantly better long-term results with Micor over M2, sorry GE fans). Crescend and *newer* TPL amps have been good to me. EFJ CR1010 PA's have also been workhorses. I have this feeling that most, if not all, have various problems... but you've seen a heck of a lot more of them in-service than I have. Well, I dunno, there are probably others on this list in the two-way business that have seen more than me. I do broadcast for a living; I'm generally an RF guy, my interest in repeaters is just a subset of that. I have a bunch of ham repeaters (20-some I think), and maintain a bunch for other individuals/clubs, and have built or maintained many for others over the years, but I'm sure there are others that do two-way on a daily basis that can give more points of reference as far as recurring problems with other brands/models that I'm not as familiar with. I ask, because this is always the kind of mature, well-developed tech I'm looking for. Price is still a factor, but when you find something that just works... it's truly grand in the tech world, for all sorts of reasons that tend to degrade what something was intended to be, vs. what it really ended up being. To me, the cost of the radio hardware is the least of my worries. I'm not saying that to sound like an alpha-hotel. I look at it this way. I've got all of these repeaters to deal with. I have no free time the way it is. When one breaks, that means I have to take a day off work (or away from family, or away from something else) to go deal with it. It probably means a few hundred miles of driving. And, more than likely, if it's a major failure, I'm probably going to have to make a return trip, doubling the time/cost. So do I really want to take a chance on low-grade hardware up front? No way. Whoever said time is money was an idiot. Time is worth inifinitely times more than money. You can make more money. You can even borrow money. Hell, if you were desparate you could even steal money. You can't do any of those things with time. Time is the one resource you can't make more of. And, for me, I've never had enough time to get everything done that I want to get done. Life's too short to waste time on high-maintenance equipment. I'm also curious to see if your recommendations are new gear, or 20+ year old gear. Both. While I still believe the glory days of two-way turned out the best damned equipment ever made, there is still some decent stuff being made today. I really like MASTR II Stations, but I will admit to some consternation over how the PAs *sometimes* act. We've had 'em run for a decade, and we've had 'em pop like light bulbs every few months. With the exception of the PA's, they generally just run. 100 watt UHF M2 PA's have been rather disappointing for me, both with and without matching networks, with or without isolators. 75 watters seem to run forever. Highband and lowband, much fewer problems. I have a bunch of the 200 watt solid state M2 stations, and have pulled them all out (except for one, which is coming out in a week or two), they're just a nightmare to keep all three PA's working all the time. Is the answer to this question the Crescend amps perhaps? I've been happy with them. I have 7 or 8 of the previous-vintage UHF Crescend/Milcoms (the gold-alodined ones that you're probably familiar with) on the air, and they've been fine, running in the 150-175 watt range. I ordered a couple 100 watt highband amps for a local club about a year ago, they seem OK. I have a bunch of their 900 MHz linear amps in use on STL's and they've been solid. I wouldn't hesitate to buy them. How did their acquisition of Vocom affect their quality? They did change their design, and talking to their engineers a few months ago, they're doing some re-designs due to some of the devices they had been using going on EOL, so more changes will be forthcoming. Some of the older pre-Crescend Vocom amps weren't very good. I haven't looked lately, did they mix up the model line and keep the Vocom stuff? They still have the Vocom line which they market as a lower-cost alternative. I like the TPL RXR series because they are extremely simple. They also have one device per board, so in the event that you have a device fail or burn up a collector trace or something, you only have
RE: [Repeater-Builder] Coax length, etc.
I've brought that issue up a few times, and usually get the blank radio shack salesman type of stare. -- Original Message -- Received: Sat, 14 Aug 2010 08:45:47 PM PDT From: Jeff DePolo j...@broadsci.com There is no simple rule of thumb, and if anybody tells you that there is, ask them how do you account for the unknown-length of coax that's *inside* your transmitter/amplifier before it gets to the antenna jack. --- Jeff WN3A
RE: [Repeater-Builder] Coax length, etc.
Russ, Of course the Bird 43 does not measure power directly. But it does sample voltage AND current on the line in amounts that are combined to indicate power. It is a directional coupler. The only time you will have a problem with it deviating from its accuracy is when the directivity becomes too low such as when the line impedance is way off from its design 50 ohms. As I said before it will read power accurately even if the transmission line is no a 50 ohm line. The manual even tells you that you can use it to measure line loss with an open at the far end of the line. Please read chapter 2 theory of operation of the Bird manual that you show the reference to. Then read it again! 73 Gary K4FMX Another chance? Which part, erroneous readings, don't directly measure power, or the voltmeter part? Sure, what the heck. ;-) I've had Bird 43's, and calibrated line sections with matched elements for that matter, give erroneous reflected power readings depending upon what was going on with the transmission line. By erroneous, I mean it was usually a reading that was, for example, excessively high versus what we knew was going on, such as a straight piece of rigid line or coax terminated into a known good load. On rare occasion, we found we slipped a bullet or had a bad connector. More often, relocating the instrument somewhere else along the line resolved those bad readings. RF calorimeters can measure power directly. But unless they've one hidden in them somewhere, ThruLine meters can not. Just because the Commission might accept wattmeter readings, or Bird says so, doesn't make it so. As for the voltmeter part, check out page 6 of the Bird 43 manual (page 18 of the PDF), a copy of which you'll recall is here: http://www.repeater-builder.com/bird/pdf/bird-43-wattmeter-2004.pdf I respectfully submit what is shown is a schematic/diagram of a directional coupler attached to a voltmeter as an indicator. An induced RF voltage sample is rectified, filtered and applied through a dropping resistor to a shunt-connected ammeter. By definition, a voltmeter is the shunt-connected ammeter with series resistor part. But don't take my word for it. Take a peek at Chapter 25 in any recent ARRL Handbook (this works for my 2007 copy anyway). Is it less a voltmeter because the induced voltage tracks current on the line? Want to call it an ammeter or current meter then, after all that's what the actual meter movement is? I submit this particular voltmeter happens to be calibrated to read average power at 50 ohms impedance, and it does this quite well within its limitations. I now await your thrashing. Please be gentle. ;-) Like the manual says, the Bird 43 is fast, convenient and accurate. I agree it's fast and convenient. I'll agree it's accurate with the caveats expressed. It beats lugging a slotted line around, and it beats every other meter like it, IMHO, including my old Daiwa dual-metered POS wattmeter. ;-) Oh, BTW, the emperor has no clothes either. :-P 73, Russ WB8ZCC
RE: [Repeater-Builder] Coax length, etc.
Agreed!! Mark - N9WYS -Original Message- From: Repeater-Builder@yahoogroups.com On Behalf Of Eric Lemmon (major snippage) This discussion is both informative and quite entertaining! 73, Eric Lemmon WB6FLY
Re: [Repeater-Builder] Coax length, etc.
I see some folks are heading for the Advil. My apologies. Thanks, Gary, for admitting the 43 doesn't measure power directly. One myth down. Of course, it is a directional coupler, no argument. That makes it a reflectometer, it enables the instrument to isolate forward/reflected samples to some degree of reliability. What's the rest of the circuit? ;-) IMHO, what makes the 43 better than most (if not all) meters of its type, is the directional coupler is a true transmission line coupler, not a ferrite transformer, directly connected capacitor, etc. As far as rereading the manual, I have been. Bird's explanation requires the reader to suspend a standing wave viewpoint of transmission line theory, and buy into their traveling wave viewpoint. Uh, okay. But that kind of thing sends up red flags for me. I shouldn't have to suspend accepted transmission line theory to understand how their meter works. As it turns out, I don't. When line impedances get away from 50 ohms, accuracy falls and the meter behaves like you'd expect. It tracks whatever current is on the line at that (the meter's) point in the line without regard for impedance. Since it's just not calibrated for whatever that impedance might be, how can it be accurate? If the meter did as you suggest, then it would show what the voltage and current are at any point in the line, and therefore be able to tell you what the impedance is at that point, all with some level of accuracy. It simply can't do all that. Yes, Bird describes what happens when using 70 ohm lines with the meter under less-than-perfect conditions. IMHO, it's really messy. It can't tell the difference between a 1:1 VSWR and a 2:1 VSWR (both will calculate out to 1.4:1) on a 70 ohm line. That's not accuracy, that's nearly useless. BTW, my POS Daiwa can show me a 100% reflected condition, just like the Bird. And just like the Bird, it doesn't indicate if that's an open or a short. I believe Bird wants us to believe that their meter is faster and more convenient (it is) yet as accurate as a slotted line and calorimeter (sorry, nope). It's a calibrated voltmeter, not a network analyzer. For most everyday, mundane RF chores, it's just dandy as we don't really need high accuracy. And as long as line impedances stay reasonably close to 50 ohms, it turns out accuracy is pretty good, too. Certainly not bad for a portable instrument, and that's the point. If we remember what its limitations are, we should be good to go. That's why I own one and want more. Okay, I'm done picking nits. It's the next yahoo's turn. ;-) 73, Russ WB8ZCC On 8/15/2010 2:08 PM, Gary Schafer wrote: Russ, Of course the Bird 43 does not measure power directly. But it does sample voltage AND current on the line in amounts that are combined to indicate power. It is a directional coupler. The only time you will have a problem with it deviating from its accuracy is when the directivity becomes too low such as when the line impedance is way off from its design 50 ohms. As I said before it will read power accurately even if the transmission line is no a 50 ohm line. The manual even tells you that you can use it to measure line loss with an open at the far end of the line. Please read chapter 2 theory of operation of the Bird manual that you show the reference to. Then read it again! 73 Gary K4FMX Another chance? Which part, erroneous readings, don't directly measure power, or the voltmeter part? Sure, what the heck. ;-) I've had Bird 43's, and calibrated line sections with matched elements for that matter, give erroneous reflected power readings depending upon what was going on with the transmission line. By erroneous, I mean it was usually a reading that was, for example, excessively high versus what we knew was going on, such as a straight piece of rigid line or coax terminated into a known good load. On rare occasion, we found we slipped a bullet or had a bad connector. More often, relocating the instrument somewhere else along the line resolved those bad readings. RF calorimeters can measure power directly. But unless they've one hidden in them somewhere, ThruLine meters can not. Just because the Commission might accept wattmeter readings, or Bird says so, doesn't make it so. As for the voltmeter part, check out page 6 of the Bird 43 manual (page 18 of the PDF), a copy of which you'll recall is here: http://www.repeater-builder.com/bird/pdf/bird-43-wattmeter-2004.pdf I respectfully submit what is shown is a schematic/diagram of a directional coupler attached to a voltmeter as an indicator. An induced RF voltage sample is rectified, filtered and applied through a dropping resistor to a shunt-connected ammeter. By definition, a voltmeter is the shunt-connected ammeter with series resistor part. But don't take my word for it. Take a peek at Chapter 25 in any recent ARRL Handbook (this works
RE: [Repeater-Builder] Coax length, etc.
Hi again Russ, _ From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Russ Hines Sent: Sunday, August 15, 2010 4:54 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Coax length, etc. I see some folks are heading for the Advil. My apologies. Thanks, Gary, for admitting the 43 doesn't measure power directly. One myth down. Of course, it is a directional coupler, no argument. That makes it a reflectometer, it enables the instrument to isolate forward/reflected samples to some degree of reliability. What's the rest of the circuit? ;-) IMHO, what makes the 43 better than most (if not all) meters of its type, is the directional coupler is a true transmission line coupler, not a ferrite transformer, directly connected capacitor, etc. But it works the same way. As far as rereading the manual, I have been. Bird's explanation requires the reader to suspend a standing wave viewpoint of transmission line theory, and buy into their traveling wave viewpoint. Uh, okay. But that kind of thing sends up red flags for me. I shouldn't have to suspend accepted transmission line theory to understand how their meter works. There are no standing waves that you can measure directly with the Bird meter. In order to truly measure standing waves you need to have a line length greater than a half wave length and measure where the nulls are along the line. Swr is calculated from forward and reflected power at one point on the line with a Bird type of meter. As it turns out, I don't. When line impedances get away from 50 ohms, accuracy falls and the meter behaves like you'd expect. It tracks whatever current is on the line at that (the meter's) point in the line without regard for impedance. Since it's just not calibrated for whatever that impedance might be, how can it be accurate? The Bird is set up so that the ratio of voltage and current that are detected work out to the power at 50 ohms. When the line is not 50 ohms that voltage/current ratio change that the meter detects. So you can no longer simply look at the scale on the meter and directly read power. For ANY reflected power reading you must subtract the reflected power shown from the forward power shown to find the true power delivered to the load. This holds true no matter what the impedance of the line is. If the meter did as you suggest, then it would show what the voltage and current are at any point in the line, and therefore be able to tell you what the impedance is at that point, all with some level of accuracy. It simply can't do all that. With the Bird meter you don't care what the impedance is because it measures voltage (by way of capacitive coupling) and current (by way of inductive coupling). Both create voltages that add together in the proper ratio to give the meter reading that represents power level for that combination of voltage and current. Yes, Bird describes what happens when using 70 ohm lines with the meter under less-than-perfect conditions. IMHO, it's really messy. It can't tell the difference between a 1:1 VSWR and a 2:1 VSWR (both will calculate out to 1.4:1) on a 70 ohm line. That's not accuracy, that's nearly useless. Yes it gets a little tricky to find VSWR with a non 50 ohm line. But most of the time we really don't care what it is. I say we don't care because it is rare that the 50 ohm Bird meter gets used in a non 50 ohm transmission line. With a 50 ohm line things work out nicely to find power and VSWR no matter what kind of reflection the load presents. BTW, my POS Daiwa can show me a 100% reflected condition, just like the Bird. And just like the Bird, it doesn't indicate if that's an open or a short. So what? If you need to know that then you are using the wrong instrument. I believe Bird wants us to believe that their meter is faster and more convenient (it is) yet as accurate as a slotted line and calorimeter (sorry, nope). It's a calibrated voltmeter, not a network analyzer. Try doing the same thing with a voltmeter. :) No one claims it to be anything other than a simple wattmeter. It is not a super accurate at measuring power either. It is claimed to be 5% of full scale reading. That means with a 100 watt slug the best accuracy that you can depend on is +- 5 watts anywhere on the scale. So at 25 watts on the meter scale it could be as low as 20 watts or as much as 30 watts. But for what it is it works very well. For most everyday, mundane RF chores, it's just dandy as we don't really need high accuracy. And as long as line impedances stay reasonably close to 50 ohms, it turns out accuracy is pretty good, too. Again, line impedance doesn't matter for power measurement. 73 Gary K4FMX Certainly not bad for a portable instrument, and that's the point. If we remember what its limitations are, we should be good to go. That's why I own one and want more. Okay, I'm done picking
Re: [Repeater-Builder] Coax length, etc.
Last round. Hi again, Gary. ;-) On 8/15/2010 7:09 PM, Gary Schafer wrote: Hi again Russ, *From:* Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] *On Behalf Of *Russ Hines *Sent:* Sunday, August 15, 2010 4:54 PM *To:* Repeater-Builder@yahoogroups.com *Subject:* Re: [Repeater-Builder] Coax length, etc. I see some folks are heading for the Advil. My apologies. Thanks, Gary, for admitting the 43 doesn't measure power directly. One myth down. Of course, it is a directional coupler, no argument. That makes it a reflectometer, it enables the instrument to isolate forward/reflected samples to some degree of reliability. What's the rest of the circuit? ;-) IMHO, what makes the 43 better than most (if not all) meters of its type, is the directional coupler is a true transmission line coupler, not a ferrite transformer, directly connected capacitor, etc. But it works the same way. Yeah, and? The Bird does it better. As far as rereading the manual, I have been. Bird's explanation requires the reader to suspend a standing wave viewpoint of transmission line theory, and buy into their traveling wave viewpoint. Uh, okay. But that kind of thing sends up red flags for me. I shouldn't have to suspend accepted transmission line theory to understand how their meter works. There are no standing waves that you can measure directly with the Bird meter. In order to truly measure standing waves you need to have a line length greater than a half wave length and measure where the nulls are along the line. Swr is calculated from forward and reflected power at one point on the line with a Bird type of meter. That's correct. As I said, the 43 isn't a slotted line. Regarding VSWR, all in-line meters make an attempt at this, some have fancy cross-needle indicators where VSWR is represented at the intersection of the needles. How else would you do determine VSWR with such a device? That was a rhetorical question. ;-) As it turns out, I don't. When line impedances get away from 50 ohms, accuracy falls and the meter behaves like you'd expect. It tracks whatever current is on the line at that (the meter's) point in the line without regard for impedance. Since it's just not calibrated for whatever that impedance might be, how can it be accurate? The Bird is set up so that the ratio of voltage and current that are detected work out to the power at 50 ohms. When the line is not 50 ohms that voltage/current ratio change that the meter detects. So you can no longer simply look at the scale on the meter and directly read power. For ANY reflected power reading you must subtract the reflected power shown from the forward power shown to find the true power delivered to the load. This holds true no matter what the impedance of the line is. Thanks, Gary, that's right. The meter is calibrated at 50 ohms impedance. When the line impedance isn't 50 ohms, you can't just look at the meter, the meter scale is no longer accurate, is it? Subtracting reflected from forward is a given, and never at issue here. Well, impedance does matter. At the characteristic impedance of the meter, line, load, etc., seems a waste of time to subtract nothing, you'll see right away there's no reflected power. ;-) If the meter did as you suggest, then it would show what the voltage and current are at any point in the line, and therefore be able to tell you what the impedance is at that point, all with some level of accuracy. It simply can't do all that. With the Bird meter you don't care what the impedance is because it measures voltage (by way of capacitive coupling) and current (by way of inductive coupling). Both create voltages that add together in the proper ratio to give the meter reading that represents power level for that combination of voltage and current. Gary, you seem to be contradicting yourself. A paragraph ago you said the ratio of voltage and current work out to the power at 50 ohms. Now we don't care what the impedance is? We either do or don't. As for me, I choose to care 'cuz that's the kind of guy I am. ;-) I understand the coupling, both are present, agreed. But if impedance didn't matter, then the meter would indicate power accurately regardless of line impedance. That's simply not so. The Bird manual even says it's not so. It's limited by its own line section. Yes, Bird describes what happens when using 70 ohm lines with the meter under less-than-perfect conditions. IMHO, it's really messy. It can't tell the difference between a 1:1 VSWR and a 2:1 VSWR (both will calculate out to 1.4:1) on a 70 ohm line. That's not accuracy, that's nearly useless. Yes it gets a little tricky to find VSWR with a non 50 ohm line. But most of the time we really don't care what it is. I say we don't care because it is rare that the 50
Re: [Repeater-Builder] Coax length, etc.
Russ Hines wrote: Hi Kevin: Regarding temperature, our club has a site, no A/C or heat, where temperatures inside the shelter can get below +20 deg F in winter, and well over 130 deg F in the summer heat. I can't imagine filter tuning not changing under such conditions, Invar or not. I can see over time where tuning might walk off the reservation. But I bow to your greater experience with cavity duplexers. Duplexer tuning is not the only thing you'd need to worry about with temperature extremes you reference. The radio set might have a hard time with +130 shelter temp. That could easily relate to radio temperatures exceeding the ability of some equipment manufacturers to remain stable. Filters will change with temperature - it's a fact. The amount they change is, I suppose, what's under question. Most commercially made units have a temperature rating of -30 to +60 degrees C. This means the duplexer will remain within its ratings between those extremes. Most of us engineer our repeater systems to have some amount of isolation headroom. Headroom is necessary for several reasons, temperature variations are likely the biggest reason, along with icing of the antenna. I have a site using a Wacom WP-641 and 250 watt transmitter. This site sees -25 degree F temperatures in the winter and +85 degree F temperatures in the summer - outside shelter. The shelter is not climate controlled. The duplexer loss is 1.5 dB or 29%. This relates to 72.5 watts going up as heat. If the repeater is in transmit for a long time, the temperature in the building can approach +100 degrees ambient. Now, add the heat generated by the loss of the duplexer, and the duplexer becomes very hot to the touch. Even at these temperature extremes, the repeater is completely happy with the isolation provided by the duplexer. You state that over time the tuning might walk off the reservation. This seems to allude to the duplexer changing tuning and not coming back to its settings after it has returned to the temperature it was tuned at. If this is the case, the duplexer design is faulty - period. Another chance? Which part, erroneous readings, don't directly measure power, or the voltmeter part? Sure, what the heck. ;-) I've had Bird 43's, and calibrated line sections with matched elements for that matter, give erroneous reflected power readings depending upon what was going on with the transmission line. By erroneous, I mean it was usually a reading that was, for example, excessively high versus what we knew was going on, such as a straight piece of rigid line or coax terminated into a known good load. On rare occasion, we found we slipped a bullet or had a bad connector. More often, relocating the instrument somewhere else along the line resolved those bad readings. If you were able to move the meter and have differing determined power readings, something IS/WAS very wrong. But, maybe you aren't using the instrument correctly? To paraphrase the manual, the reflected power must be subtracted from the forward power to determine the actual power delivered to the load. If you move the meter about the line, it is possible that the power shown on the meter will change, but, if you read the reflected at the same spot, and determine the power - it should always subtract to the same determined power reading. RF calorimeters can measure power directly. But unless they've one hidden in them somewhere, ThruLine meters can not. Just because the Commission might accept wattmeter readings, or Bird says so, doesn't make it so. While I'll agree that the Bird doesn't measure power directly, it still measures power, quite accurately, no matter the impedance of the connected line. Bird claims the meter to be accurate within 5% of full scale reading. While it might not be as accurate as a VNA, it doesn't cost as much either. Let's revisit what you originally wrote: /Our in-line power meters, like our trusted Bird 43, do not directly measure power./ No argument here... / They're really voltage meters calibrated in watts at a specific impedance. / When you were reading the manual, you missed something - read on... As for the voltmeter part, check out page 6 of the Bird 43 manual (page 18 of the PDF), a copy of which you'll recall is here: http://www.repeater-builder.com/bird/pdf/bird-43-wattmeter-2004.pdf I respectfully submit what is shown is a schematic/diagram of a directional coupler attached to a voltmeter as an indicator. An induced RF voltage sample is rectified, filtered and applied through a dropping resistor to a shunt-connected ammeter. By definition, a voltmeter is the shunt-connected ammeter with series resistor part. But don't take my word for it. Take a peek at Chapter 25 in any recent ARRL Handbook (this works for my 2007 copy anyway). Is it less a voltmeter because the induced voltage tracks current on the line? Want
RE: [Repeater-Builder] Coax length, etc.
I don't know if you really don't get it or you are just trying to be controversial. I tend to think a little of both. Either way, I give up. 73 Gary K4FMX _ From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Russ Hines Sent: Sunday, August 15, 2010 7:37 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Coax length, etc. Last round. Hi again, Gary. ;-) On 8/15/2010 7:09 PM, Gary Schafer wrote: Hi again Russ, _ From: mailto:Repeater-Builder@yahoogroups.com Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Russ Hines Sent: Sunday, August 15, 2010 4:54 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Coax length, etc. I see some folks are heading for the Advil. My apologies. Thanks, Gary, for admitting the 43 doesn't measure power directly. One myth down. Of course, it is a directional coupler, no argument. That makes it a reflectometer, it enables the instrument to isolate forward/reflected samples to some degree of reliability. What's the rest of the circuit? ;-) IMHO, what makes the 43 better than most (if not all) meters of its type, is the directional coupler is a true transmission line coupler, not a ferrite transformer, directly connected capacitor, etc. But it works the same way. Yeah, and? The Bird does it better. As far as rereading the manual, I have been. Bird's explanation requires the reader to suspend a standing wave viewpoint of transmission line theory, and buy into their traveling wave viewpoint. Uh, okay. But that kind of thing sends up red flags for me. I shouldn't have to suspend accepted transmission line theory to understand how their meter works. There are no standing waves that you can measure directly with the Bird meter. In order to truly measure standing waves you need to have a line length greater than a half wave length and measure where the nulls are along the line. Swr is calculated from forward and reflected power at one point on the line with a Bird type of meter. That's correct. As I said, the 43 isn't a slotted line. Regarding VSWR, all in-line meters make an attempt at this, some have fancy cross-needle indicators where VSWR is represented at the intersection of the needles. How else would you do determine VSWR with such a device? That was a rhetorical question. ;-) As it turns out, I don't. When line impedances get away from 50 ohms, accuracy falls and the meter behaves like you'd expect. It tracks whatever current is on the line at that (the meter's) point in the line without regard for impedance. Since it's just not calibrated for whatever that impedance might be, how can it be accurate? The Bird is set up so that the ratio of voltage and current that are detected work out to the power at 50 ohms. When the line is not 50 ohms that voltage/current ratio change that the meter detects. So you can no longer simply look at the scale on the meter and directly read power. For ANY reflected power reading you must subtract the reflected power shown from the forward power shown to find the true power delivered to the load. This holds true no matter what the impedance of the line is. Thanks, Gary, that's right. The meter is calibrated at 50 ohms impedance. When the line impedance isn't 50 ohms, you can't just look at the meter, the meter scale is no longer accurate, is it? Subtracting reflected from forward is a given, and never at issue here. Well, impedance does matter. At the characteristic impedance of the meter, line, load, etc., seems a waste of time to subtract nothing, you'll see right away there's no reflected power. ;-) If the meter did as you suggest, then it would show what the voltage and current are at any point in the line, and therefore be able to tell you what the impedance is at that point, all with some level of accuracy. It simply can't do all that. With the Bird meter you don't care what the impedance is because it measures voltage (by way of capacitive coupling) and current (by way of inductive coupling). Both create voltages that add together in the proper ratio to give the meter reading that represents power level for that combination of voltage and current. Gary, you seem to be contradicting yourself. A paragraph ago you said the ratio of voltage and current work out to the power at 50 ohms. Now we don't care what the impedance is? We either do or don't. As for me, I choose to care 'cuz that's the kind of guy I am. ;-) I understand the coupling, both are present, agreed. But if impedance didn't matter, then the meter would indicate power accurately regardless of line impedance. That's simply not so. The Bird manual even says it's not so. It's limited by its own line section. Yes, Bird describes what happens when using 70 ohm lines with the meter under less-than-perfect conditions. IMHO
RE: [Repeater-Builder] Coax length, etc.
I know I'm going to regret stepping into this one, but since when has that stopped me before... Thanks, Gary, for admitting the 43 doesn't measure power directly. What do you mean by measure power directly? If you're talking about comparing a thruline measurement against absorptive/calorimetric techniques, then that's apples and oranges, one is measuring power in a transmission line (either with or without reflections present), the other is measuring power absorbed into a load, big difference. Please clarify what you mean by measuring power directly so at least we're all on the same page. Of course, it is a directional coupler, no argument. That makes it a reflectometer No, it's not a reflectometer, it can't do forward and reverse measurements concurrently. If the meter did as you suggest, then it would show what the voltage and current are at any point in the line, and therefore be able to tell you what the impedance is at that point Not without knowing the phase between the two it couldn't. BTW, my POS Daiwa can show me a 100% reflected condition, just like the Bird. And just like the Bird, it doesn't indicate if that's an open or a short. A Bird isn't a VSWR bridge, it's a directional wattmeter. Yes, it can be used in a roundabout way to measure/calculate VSWR, but it's not a VSWR meter. Sidebar. I grit my teeth when I hear someone on the radio say my SWR meter shows I'm putting out 100 watts. Since when does a SWR meter measure power!???! Do you use your bathroom scale to check your blood pressure? Egads. I'm not taking a stance here (at least not yet) on the relative merits of the Bird 43 or other thruline-type wattmeter line sections or elements, I'm just trying to get a handle on the matter that is the subject of debate... --- Jeff WN3A
Re: [Repeater-Builder] Coax length, etc.
Sorry, Gary. I have a bad tendency to question what's put in front of me. That includes what I call the girl copy I read in product manuals and brochures. FWIW, girl copy refers to the rarely-true supposed personal information about the particular lady-of-the-month in certain men's magazines. ;-) I appreciated the banter, take care, Gary. 73, Russ WB8ZCC On 8/15/2010 9:20 PM, Gary Schafer wrote: I don't know if you really don't get it or you are just trying to be controversial. I tend to think a little of both. Either way, I give up. 73 Gary K4FMX *From:* Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] *On Behalf Of *Russ Hines *Sent:* Sunday, August 15, 2010 7:37 PM *To:* Repeater-Builder@yahoogroups.com *Subject:* Re: [Repeater-Builder] Coax length, etc. Last round. Hi again, Gary. ;-) On 8/15/2010 7:09 PM, Gary Schafer wrote: Hi again Russ, *From:* Repeater-Builder@yahoogroups.com mailto:Repeater-Builder@yahoogroups.com [mailto:Repeater-Builder@yahoogroups.com mailto:Repeater-Builder@yahoogroups.com] *On Behalf Of *Russ Hines *Sent:* Sunday, August 15, 2010 4:54 PM *To:* Repeater-Builder@yahoogroups.com mailto:Repeater-Builder@yahoogroups.com *Subject:* Re: [Repeater-Builder] Coax length, etc. I see some folks are heading for the Advil. My apologies. Thanks, Gary, for admitting the 43 doesn't measure power directly. One myth down. Of course, it is a directional coupler, no argument. That makes it a reflectometer, it enables the instrument to isolate forward/reflected samples to some degree of reliability. What's the rest of the circuit? ;-) IMHO, what makes the 43 better than most (if not all) meters of its type, is the directional coupler is a true transmission line coupler, not a ferrite transformer, directly connected capacitor, etc. But it works the same way. Yeah, and? The Bird does it better. As far as rereading the manual, I have been. Bird's explanation requires the reader to suspend a standing wave viewpoint of transmission line theory, and buy into their traveling wave viewpoint. Uh, okay. But that kind of thing sends up red flags for me. I shouldn't have to suspend accepted transmission line theory to understand how their meter works. There are no standing waves that you can measure directly with the Bird meter. In order to truly measure standing waves you need to have a line length greater than a half wave length and measure where the nulls are along the line. Swr is calculated from forward and reflected power at one point on the line with a Bird type of meter. That's correct. As I said, the 43 isn't a slotted line. Regarding VSWR, all in-line meters make an attempt at this, some have fancy cross-needle indicators where VSWR is represented at the intersection of the needles. How else would you do determine VSWR with such a device? That was a rhetorical question. ;-) As it turns out, I don't. When line impedances get away from 50 ohms, accuracy falls and the meter behaves like you'd expect. It tracks whatever current is on the line at that (the meter's) point in the line without regard for impedance. Since it's just not calibrated for whatever that impedance might be, how can it be accurate? The Bird is set up so that the ratio of voltage and current that are detected work out to the power at 50 ohms. When the line is not 50 ohms that voltage/current ratio change that the meter detects. So you can no longer simply look at the scale on the meter and directly read power. For ANY reflected power reading you must subtract the reflected power shown from the forward power shown to find the true power delivered to the load. This holds true no matter what the impedance of the line is. Thanks, Gary, that's right. The meter is calibrated at 50 ohms impedance. When the line impedance isn't 50 ohms, you can't just look at the meter, the meter scale is no longer accurate, is it? Subtracting reflected from forward is a given, and never at issue here. Well, impedance does matter. At the characteristic impedance of the meter, line, load, etc., seems a waste of time to subtract nothing, you'll see right away there's no reflected power. ;-) If the meter did as you suggest, then it would show what the voltage and current are at any point in the line, and therefore be able to tell you what the impedance is at that point, all with some level of accuracy. It simply can't do all that. With the Bird meter you don't care what the impedance is because it measures voltage (by way of capacitive coupling) and current (by way of inductive coupling). Both create voltages that add together in the proper ratio to give the meter reading that represents power level for that combination of voltage and current. Gary, you
RE: [Repeater-Builder] Coax length, etc.
Because the impedance is not matched between the transmitter and duplexer, the 'apparent' loss of the duplexer is greater than the manufacturers stated loss of the duplexer. Changing the cable length is not changing the loss of the duplexer, it's changing the power that is accepted at the transmitter port of the duplexer by matching the output impedance of the transmitter to the input impedance of the transmitter port of the duplexer. But if the duplexer is tuned to 50 ohms, and the cable is 50 ohms, varying the cable length isn't going to change the Z seen by the transmitter. Or are you suggesting the duplexer is purposely de-tuned from 50 ohms? And also that by varying the cable length between the transmitter and the duplexer that you can vary the reflected power on that same line? Yes. With all due respect, that's not possible, regardless of what the Z is of the duplexer. The only time it could have an effect on the reflected power would be if the transmitter/PA were spurious, and the amplitude/frequency of the spurs changed with varying load Z, and I think we can both agree that if that's the case, we have bigger fish to fry. Not to belabor the point, but whatever the VSWR is on a length of transmission line, that's the VSWR that's on the line *regardless of length*. You can't change the VSWR by changing the length of the line. As you vary the length, you go round n' round the Smith Chart in a constant VSWR circle, with the Z repeating cyclicly every half-wavelength, but you've still got a complex Z that nets a 1:5:1 VSWR relative to 50 ohms at the end of whatever length of line you choose (cable loss effects notwithstanding). There are an infinite number of complex Z's that yield a 1.5:1 VSWR - cut the line to any random length and you'll hit one of them. In a situation where the duplexer and transmitter have differing impedances, and a cable optimized in length matches these impedances, the mismatch at the duplexer is minimized, therefore the power reflected by the duplexer is minimized. I think what you're really saying is that the mismatch at the *input to the matching section* (i.e. the cable between the PA and the duplexer), NOT the mismatch at the duplexer, is minimized. The duplexer's input Z isn't changing; you can't change that unless you re-tune the cavities or change the load at the antenna port. Whether or that the transmitter likes/dislikes the different Z it sees as you change cable lengths is, I guess, what's up for debate... I have found that when you get a transmitter that is 'picky' about the length of interconnecting cable, power being read at the output port of the duplexer is low and you cannot alter the tuning of the cavity closest to the transmitter to make things right. In other words, the place where lowest VSWR and maximum power transfer occurs is at two completely different places, and power transfer is not up where it should be (transmitter makes 100 watts into a dummy load but only shows 50 watts on the output port of the duplexer that has a stated 1.5 dB loss (29 %)). That would imply that either duplexer is presenting a load Z substantially far removed from 50+j0, OR the transmitter doesn't like a 50 ohm load, or something inbetween, would it not? As you get close to the 'optimum' cable length, the lowest VSWR and maximum power transfer occur near the same place when tuning the cavity closest to the transmitter. But again, *you're NOT changing the VSWR*! You can't change the VSWR by varying the length of the line! I just want to make sure we're on the same page - the VSWR on a transmission line doesn't vary with length (loss notwithstanding). I usually pay more attention to what is coming out the antenna port of the duplexer - first. Then, when things are right, comparing forward power going to the duplexer and power going to a good dummy load will be very close the same, since matching the impedance of the transmitter to the impedance of the duplexer was accomplished by some means. Can you give me some real-world examples of what combinations of duplexers and transmitters you've run across that just didn't want to play nice without having to resort to changing cable lengths? Like a highband Micor 110 watt H split paired with a Q2220E or whatever. I'm just curious if I've done any of the same combinations. I think you know me well enough by now Kevin that I'm not looking to pick a fight, I'm just a hard-ass when it comes to basing technique on solid engineering foundation. I can't say I've ever had to play with cable lengths to either get a transmitter/PA to make rated power, or to get the apparent loss of a duplexer to meet spec. Have I just been lucky? Maybe. But if I'm *that* lucky, I'm in the wrong business, I shouldn't be sitting a hotel room in Harrisburg on a Saturday waiting for a tower crew to show up, I should be living the good life in Vegas
Re: [Repeater-Builder] Coax length, etc.
FWIW, TX/RX Systems talks about adverse length cable between the transmitter and the duplexer in their technical papers. Chuck WB2EDV - Original Message - From: Jeff DePolo j...@broadsci.com To: Repeater-Builder@yahoogroups.com Sent: Saturday, August 14, 2010 8:44 AM Subject: RE: [Repeater-Builder] Coax length, etc. Because the impedance is not matched between the transmitter and duplexer, the 'apparent' loss of the duplexer is greater than the manufacturers stated loss of the duplexer. Changing the cable length is not changing the loss of the duplexer, it's changing the power that is accepted at the transmitter port of the duplexer by matching the output impedance of the transmitter to the input impedance of the transmitter port of the duplexer. But if the duplexer is tuned to 50 ohms, and the cable is 50 ohms, varying the cable length isn't going to change the Z seen by the transmitter. Or are you suggesting the duplexer is purposely de-tuned from 50 ohms? And also that by varying the cable length between the transmitter and the duplexer that you can vary the reflected power on that same line? Yes. With all due respect, that's not possible, regardless of what the Z is of the duplexer. The only time it could have an effect on the reflected power would be if the transmitter/PA were spurious, and the amplitude/frequency of the spurs changed with varying load Z, and I think we can both agree that if that's the case, we have bigger fish to fry. Not to belabor the point, but whatever the VSWR is on a length of transmission line, that's the VSWR that's on the line *regardless of length*. You can't change the VSWR by changing the length of the line. As you vary the length, you go round n' round the Smith Chart in a constant VSWR circle, with the Z repeating cyclicly every half-wavelength, but you've still got a complex Z that nets a 1:5:1 VSWR relative to 50 ohms at the end of whatever length of line you choose (cable loss effects notwithstanding). There are an infinite number of complex Z's that yield a 1.5:1 VSWR - cut the line to any random length and you'll hit one of them. In a situation where the duplexer and transmitter have differing impedances, and a cable optimized in length matches these impedances, the mismatch at the duplexer is minimized, therefore the power reflected by the duplexer is minimized. I think what you're really saying is that the mismatch at the *input to the matching section* (i.e. the cable between the PA and the duplexer), NOT the mismatch at the duplexer, is minimized. The duplexer's input Z isn't changing; you can't change that unless you re-tune the cavities or change the load at the antenna port. Whether or that the transmitter likes/dislikes the different Z it sees as you change cable lengths is, I guess, what's up for debate... I have found that when you get a transmitter that is 'picky' about the length of interconnecting cable, power being read at the output port of the duplexer is low and you cannot alter the tuning of the cavity closest to the transmitter to make things right. In other words, the place where lowest VSWR and maximum power transfer occurs is at two completely different places, and power transfer is not up where it should be (transmitter makes 100 watts into a dummy load but only shows 50 watts on the output port of the duplexer that has a stated 1.5 dB loss (29 %)). That would imply that either duplexer is presenting a load Z substantially far removed from 50+j0, OR the transmitter doesn't like a 50 ohm load, or something inbetween, would it not? As you get close to the 'optimum' cable length, the lowest VSWR and maximum power transfer occur near the same place when tuning the cavity closest to the transmitter. But again, *you're NOT changing the VSWR*! You can't change the VSWR by varying the length of the line! I just want to make sure we're on the same page - the VSWR on a transmission line doesn't vary with length (loss notwithstanding). I usually pay more attention to what is coming out the antenna port of the duplexer - first. Then, when things are right, comparing forward power going to the duplexer and power going to a good dummy load will be very close the same, since matching the impedance of the transmitter to the impedance of the duplexer was accomplished by some means. Can you give me some real-world examples of what combinations of duplexers and transmitters you've run across that just didn't want to play nice without having to resort to changing cable lengths? Like a highband Micor 110 watt H split paired with a Q2220E or whatever. I'm just curious if I've done any of the same combinations. I think you know me well enough by now Kevin that I'm not looking to pick a fight, I'm just a hard-ass when it comes to basing technique on solid engineering foundation. I can't say I've ever had to play
Re: [Repeater-Builder] Coax length, etc.
On 8/14/2010 8:44 AM, Jeff DePolo wrote: But if the duplexer is tuned to 50 ohms, and the cable is 50 ohms, varying the cable length isn't going to change the Z seen by the transmitter. Or are you suggesting the duplexer is purposely de-tuned from 50 ohms? I use a Network Analyzer to tune duplexers. Although I can usually get an impedance of 50 ohms, many times the L or C reactance is not perfect. Maybe the transmitter is responding more to the reactance mismatch rather than the impedance mismatch. This area of RF black magic very quickly gets me lost in the ether. 73, Joe, k1ike
Re: [Repeater-Builder] Coax length, etc.
Jeff DePolo wrote: Because the impedance is not matched between the transmitter and duplexer, the 'apparent' loss of the duplexer is greater than the manufacturers stated loss of the duplexer. Changing the cable length is not changing the loss of the duplexer, it's changing the power that is accepted at the transmitter port of the duplexer by matching the output impedance of the transmitter to the input impedance of the transmitter port of the duplexer. But if the duplexer is tuned to 50 ohms, and the cable is 50 ohms, varying the cable length isn't going to change the Z seen by the transmitter. Or are you suggesting the duplexer is purposely de-tuned from 50 ohms? Purposely, accidentally, by lack of good design - people not having the right equipment to tune it correctly - whatever. And also that by varying the cable length between the transmitter and the duplexer that you can vary the reflected power on that same line? Yes. With all due respect, that's not possible, regardless of what the Z is of the duplexer. The only time it could have an effect on the reflected power would be if the transmitter/PA were spurious, and the amplitude/frequency of the spurs changed with varying load Z, and I think we can both agree that if that's the case, we have bigger fish to fry. And this is where I believe the duplexer manufacturers are covering their butt. They don't want the problem with complex reactance presented by the duplexer to be their problem. Not that I don't agree, because it's usually the transmitter that is really at fault. Joe Ham buys a new duplexer and hooks it up to his 110 Watt MASTR II repeater and gets 50 watts out the antenna port. He does his homework and realizes that he should only be loosing 29% with the 1.5 dB of insertion loss stated in the paperwork - but he's loosing over 50%. The duplexer manufacturer supposedly engineered and tuned it for a 50 Ohm system. He knows that the cable he connected to the transmitter is good, because when he disconnects the end going to the transmitter port of the duplexer and connects it to his Bird 43 terminated with a good load - it reads 110 watts. Now, is the transmitter becoming spurious and the cable length being changed in length satisfies the match between the duplexer and transmitter - I don't know... All I can tell you is I have followed the suggestions written in the WACOM manual and it has worked. I had one instance of a ham radio club loosing PA's left and right on their 2M machine. They told me of the situation and I offered to do a little testing. The 110 watt PA would put out 110 watts into a Bird and dummy, but only 45 watts was coming out the antenna port of the duplexer. At the time I didn't own a spectrum analyzer. The repeater wouldn't duplex without desense. I changed the length of the line between the PA and duplexer until I got the power to read about 75 Watts as I remember. That was 13 years and they still have the same PA - no desense either. Not to belabor the point, but whatever the VSWR is on a length of transmission line, that's the VSWR that's on the line *regardless of length*. You can't change the VSWR by changing the length of the line. As you vary the length, you go round n' round the Smith Chart in a constant VSWR circle, with the Z repeating cyclicly every half-wavelength, but you've still got a complex Z that nets a 1:5:1 VSWR relative to 50 ohms at the end of whatever length of line you choose (cable loss effects notwithstanding). There are an infinite number of complex Z's that yield a 1.5:1 VSWR - cut the line to any random length and you'll hit one of them. In a situation where the duplexer and transmitter have differing impedances, and a cable optimized in length matches these impedances, the mismatch at the duplexer is minimized, therefore the power reflected by the duplexer is minimized. I think what you're really saying is that the mismatch at the *input to the matching section* (i.e. the cable between the PA and the duplexer), NOT the mismatch at the duplexer, is minimized. Sorry - that is what I meant to say. Many of us use converted commercial gear in the ham band. Many don't take the time to properly convert the receiver and especially the transmitter to properly operate in the adjacent ham band. So, when you run a 150.8 to 174 MHz amplifier in the 2M ham band or a 450 to 470 MHz amplifier in the UHF ham band is it going to represent a good 50 Ohm impedance? Likely not... We need to realize that most duplexer manufacturers know what they are doing and their products are presenting a 50 ohm match on its intended frequencies - unless somebody has adjusted on it. But, because the duplexer is not a perfect load, it creates reactance and the transmitter/PA may not like it. If it doesn't like it, it may become spurious. If it becomes spurious, it isn't putting out all of its power
Re: [Repeater-Builder] Coax length, etc.
Kevin Custer wrote: Joe Ham buys a new duplexer and hooks it up to his 110 Watt MASTR II repeater and gets 50 watts out the antenna port. He does his homework and realizes that he should only be loosing 29% Wow -* loosing -* that should have been losing - that's what I get for being in a hurry
Re: [Repeater-Builder] Coax length, etc.
Kevin Custer wrote: I had one instance of a ham radio club loosing PA's left and right on their 2M machine. Indeed - I am loosing my mind - grin K
RE: [Repeater-Builder] Coax length, etc.
Kevin, Don't feel bad- look at the number of postings that use you instead of your, your instead of you're, mhz instead of MHz, Khz instead of kHz, it's instead of its... the list is endless! But, back to the thread... it's refreshing to see that more than a few list members know that a dummy load is purely resistive, while a duplexer cavity is reactive- explaining why a transmitter that works perfectly when feeding a dummy load can be unstable when connected to a duplexer. This discussion is both informative and quite entertaining! 73, Eric Lemmon WB6FLY -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Kevin Custer Sent: Saturday, August 14, 2010 10:15 AM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Coax length, etc. Kevin Custer wrote: Joe Ham buys a new duplexer and hooks it up to his 110 Watt MASTR II repeater and gets 50 watts out the antenna port. He does his homework and realizes that he should only be loosing 29% Wow - loosing - that should have been losing - that's what I get for being in a hurry
RE: [Repeater-Builder] Coax length, etc.
So will someone post a simple rule of thumb. If you have the option of optimizing cable length from PA to first cavity, IE you haven't made them yet what's the best simple rule of thumb to follow to build them to avoid reactance. 1/2wl if allowed minus coupling loop depth? Or is that past a simple thumb. Also, This will obviously not work well for 220 or 440 or a most vhf repeater setups. So what would the next ideal cable wl be? And so forth. The reason I ask, if your building new cables why not? Answers on here seem to range a lot. Ross kc7rjk
RE: [Repeater-Builder] Coax length, etc.
FORGOT to multiply cable VF then subtract coupling loop depth!!! Forgive me :-) -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Ross Johnson Sent: Saturday, August 14, 2010 11:59 AM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Coax length, etc. So will someone post a simple rule of thumb. If you have the option of optimizing cable length from PA to first cavity, IE you haven't made them yet what's the best simple rule of thumb to follow to build them to avoid reactance. 1/2wl if allowed minus coupling loop depth? Or is that past a simple thumb. Also, This will obviously not work well for 220 or 440 or a most vhf repeater setups. So what would the next ideal cable wl be? And so forth. The reason I ask, if your building new cables why not? Answers on here seem to range a lot. Ross kc7rjk
Re: [Repeater-Builder] Coax length, etc.
That's because there are as many rules as there are thumbs. ;-) I don't know about anyone else, but I can tell you about the highly scientific method I use. I start with a multiple of 1/2 electrical wavelength and trim as necessary. I'd stay away from an odd-multiple of 1/4 wavelength in this application... no good reason, just because (black magic and all that). Try cutting the transmitter-to-duplexer line using the receive frequency length, and vice versa. If that doesn't work out, you can swap them. I know, on a 2m amateur system, the length difference is about 1/4. In that case, make a cable 1-2 shorter and see what happens. An alternative is to use multiple short lengths of coax connected together to find a happy length, then replace with a single coax cut to that length. As I said, highly scientific. :-P 73, Russ WB8ZCC On 8/14/2010 2:59 PM, Ross Johnson wrote: So will someone post a simple rule of thumb... If you have the option of optimizing cable length from PA to first cavity, IE you haven't made them yet what's the best simple rule of thumb to follow to build them to avoid reactance. 1/2wl if allowed minus coupling loop depth? Or is that past a simple thumb. Also, This will obviously not work well for 220 or 440 or a most vhf repeater setups. So what would the next ideal cable wl be? And so forth... The reason I ask, if your building new cables why not? Answers on here seem to range a lot... Ross kc7rjk
Re: [Repeater-Builder] Coax length, etc.
Some related comments, if you don't mind. Temperature changes seem to be the biggest detuner of largely mechanical devices like cavity duplexers. We often send our repeaters off to live in less-than-ideal environments, then expect cavity input/output impedances to remain as we measured them in the shop? Don't think so. IMHO, we're making the same mistake I made in a post the other day, saying VSWR when what we really mean is reflected power as indicated on a meter. Jeff is correct, VSWR along a transmission line doesn't change if source, load and line impedances are stable, the ratio remains the same. What does change, and what is affected by line length, are actual impedances along the line under not-so-perfect-or-stable conditions; the actual impedances along the line change but the ratio does not. For example, 100+j0, 25+j0, 40+j30, and 40-j30, are different impedances yet all exhibit a VSWR of 2:1 in a 50-ohm impedance system. Voltage is proportional to impedance. We can't really have a voltage standing wave ratio greater than 1:1 without a voltage differential, and that really can't happen if impedances along the line remain the same. Our friends at Agilent have put together a Java applet demonstrating what happens along a transmission line. Maybe you're aware of it, it's really kind of cool. The applet allows you to change the load impedance of the model and see the changes, so have fun with it. http://education.tm.agilent.com/index.cgi?CONTENT_ID=6 Our in-line power meters, like our trusted Bird 43, do not directly measure power. They're really voltage meters calibrated in watts at a specific impedance. That's why they can be fooled into displaying an erroneous reflected power reading, perhaps lulling us into a sense of security that the VSWR on the line is acceptable when it may not be. With most transmitters I'm familiar with, a high VSWR condition is detected from a reflected RF sample from a directional coupler at the transmitter's output, so it's not a real VSWR measurement per se, it's a voltage measurement. Worse, these couplers tend not to be very selective, so out-of-channel and even out-of-band energy can cause high VSWR trips even when our measurements indicate all is well on our frequency of interest. Great discussion, keep it going. If I repeated what was already mentioned, my apologies. 73, Russ WB8ZCC On 8/14/2010 12:53 PM, Kevin Custer wrote: Jeff DePolo wrote: Because the impedance is not matched between the transmitter and duplexer, the 'apparent' loss of the duplexer is greater than the manufacturers stated loss of the duplexer. Changing the cable length is not changing the loss of the duplexer, it's changing the power that is accepted at the transmitter port of the duplexer by matching the output impedance of the transmitter to the input impedance of the transmitter port of the duplexer. But if the duplexer is tuned to 50 ohms, and the cable is 50 ohms, varying the cable length isn't going to change the Z seen by the transmitter. Or are you suggesting the duplexer is purposely de-tuned from 50 ohms? Purposely, accidentally, by lack of good design - people not having the right equipment to tune it correctly - whatever. And also that by varying the cable length between the transmitter and the duplexer that you can vary the reflected power on that same line? Yes. With all due respect, that's not possible, regardless of what the Z is of the duplexer. The only time it could have an effect on the reflected power would be if the transmitter/PA were spurious, and the amplitude/frequency of the spurs changed with varying load Z, and I think we can both agree that if that's the case, we have bigger fish to fry. And this is where I believe the duplexer manufacturers are covering their butt. They don't want the problem with complex reactance presented by the duplexer to be their problem. Not that I don't agree, because it's usually the transmitter that is really at fault. Joe Ham buys a new duplexer and hooks it up to his 110 Watt MASTR II repeater and gets 50 watts out the antenna port. He does his homework and realizes that he should only be loosing 29% with the 1.5 dB of insertion loss stated in the paperwork - but he's loosing over 50%. The duplexer manufacturer supposedly engineered and tuned it for a 50 Ohm system. He knows that the cable he connected to the transmitter is good, because when he disconnects the end going to the transmitter port of the duplexer and connects it to his Bird 43 terminated with a good load - it reads 110 watts. Now, is the transmitter becoming spurious and the cable length being changed in length satisfies the match between the duplexer and transmitter - I don't know... All I can tell you is I have followed the suggestions written in the WACOM manual and it has worked. I had one instance of a ham radio club
RE: [Repeater-Builder] Coax length, etc.
One correction here; the Bird power meter is not just a voltage measuring meter. It does in fact measure voltage and current to calculate power. It will give true power even if used in a non 50 ohm circuit. But you must always subtract reflected power from indicated forward power to find true power delivered to the load. When measuring SWR you must always calculate it (or use the chart) and compare reflected to forward indicated on the meter. It is easy to be fooled as indicated forward power also drops as reflected power drops. 73 Gary K4FMX _ From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Russ Hines Sent: Saturday, August 14, 2010 4:30 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Coax length, etc.73 Some related comments, if you don't mind. Temperature changes seem to be the biggest detuner of largely mechanical devices like cavity duplexers. We often send our repeaters off to live in less-than-ideal environments, then expect cavity input/output impedances to remain as we measured them in the shop? Don't think so. IMHO, we're making the same mistake I made in a post the other day, saying VSWR when what we really mean is reflected power as indicated on a meter. Jeff is correct, VSWR along a transmission line doesn't change if source, load and line impedances are stable, the ratio remains the same. What does change, and what is affected by line length, are actual impedances along the line under not-so-perfect-or-stable conditions; the actual impedances along the line change but the ratio does not. For example, 100+j0, 25+j0, 40+j30, and 40-j30, are different impedances yet all exhibit a VSWR of 2:1 in a 50-ohm impedance system. Voltage is proportional to impedance. We can't really have a voltage standing wave ratio greater than 1:1 without a voltage differential, and that really can't happen if impedances along the line remain the same. Our friends at Agilent have put together a Java applet demonstrating what happens along a transmission line. Maybe you're aware of it, it's really kind of cool. The applet allows you to change the load impedance of the model and see the changes, so have fun with it. http://education.tm.agilent.com/index.cgi?CONTENT_ID=6 Our in-line power meters, like our trusted Bird 43, do not directly measure power. They're really voltage meters calibrated in watts at a specific impedance. That's why they can be fooled into displaying an erroneous reflected power reading, perhaps lulling us into a sense of security that the VSWR on the line is acceptable when it may not be. With most transmitters I'm familiar with, a high VSWR condition is detected from a reflected RF sample from a directional coupler at the transmitter's output, so it's not a real VSWR measurement per se, it's a voltage measurement. Worse, these couplers tend not to be very selective, so out-of-channel and even out-of-band energy can cause high VSWR trips even when our measurements indicate all is well on our frequency of interest. Great discussion, keep it going. If I repeated what was already mentioned, my apologies. 73, Russ WB8ZCC _._,___
Re: [Repeater-Builder] Coax length, etc.
Russ Hines wrote: Some related comments, if you don't mind. Temperature changes seem to be the biggest detuner of largely mechanical devices like cavity duplexers. We often send our repeaters off to live in less-than-ideal environments, then expect cavity input/output impedances to remain as we measured them in the shop? Don't think so. I largely disagree. Most modern duplexer designs (within the last 25 years or so) use compensating elements to make the duplexer or cavity temperature stable. Invar is a nickel-steel alloy that exhibits about 1/10 the thermal expansion as a common carbon steel counterpart. Invar is used to make the tuning rod - many times it's threaded. The rest of the duplexer or cavity is usually made of similar metals and generally thermal expansion occurs across these components equally, resulting in extremely low frequency drift over its rated operating temperature. Our in-line power meters, like our trusted Bird 43, do not directly measure power. They're really voltage meters calibrated in watts at a specific impedance. That's why they can be fooled into displaying an erroneous reflected power reading, perhaps lulling us into a sense of security that the VSWR on the line is acceptable when it may not be. What? Maybe you would like to have another chance at that one Kevin Custer
Re: [Repeater-Builder] Coax length, etc.
Ross Johnson wrote: So will someone post a simple rule of thumb... If you have the option of optimizing cable length from PA to first cavity, IE you haven't made them yet what's the best simple rule of thumb to follow to build them to avoid reactance. 1/2wl if allowed minus coupling loop depth? Or is that past a simple thumb. Simple rule - there is none. The length can be determined experimentally as outlined in several publications of various duplexer manufacturers. I use the length I need to do a good job - then, if the transmitter is unhappy, I build a cable long enough to do the job and satisfy the transmitter. Kevin
RE: [Repeater-Builder] Coax length, etc.
Jeff, you aren't stepping on my toes at all. Glad to see your comments. OK, good. Since you've never met me, I can assure you, you definately DO NOT want me stepping on your toes, it would be painful. I do have to agree with Kevin that most duplexer manufacturers recommend different cable length trials between the transmitter and the duplexer when full power can not be reached into the duplexer. Ah, but the crux of the matter is that we're not changing the performance of the duplexer, we're just getting the transmitter to transfer more power into the line. Over the years I have been a manufacturers rep for TX-RX, Sinclair and Telewave. All of them recommend the same thing. Again, it's a CYA measure as Kevin pointed out. PA won't make power? Don't blame us, try mucking with the cable length, see if that helps. I am not a transmitter expert but it is my understanding that the problem is not one of the duplexer not presenting 50 ohms at the wanted frequency but the impedance that it presents off frequency to the transmitter finals. Some solid state devices do not like to see high reactance, even off frequency. But why? If all of the power (or, let's hope, at least 99.99% of it) is on-channel, *should* a properly-designed and properly-functioning transmitter misbehave due to the poor match a duplexer presents at frequencies far removed from the channel center? For one thing the reactance causes them to draw more current than normal. Again, why? This may be why you find that tuning for minimum pa current and maximum power out don't exactly agree with one another. I can promise you they almost never do, but that's not any great mystery. You are probably finding a balance between the off frequency reactance and the on frequency wanted load that the finals see. No, that's not it. The off-frequency Z issue is a totally separate topic from the efficiency vs maximum output subject. Let's keep those two topics separate for the sake of this discussion. If you have the duplexer properly tuned to provide 50 ohms at its input port, the transmitter may still not be happy because of the off frequency reactance presented by the duplexer. I disagree. I would accept the notion that the transmitter may not be happy (and I put that in quotes not to mock you, but becuase I can't come up with a better word either) because it is not *properly matched* when looking into a 50+j0 load. This indicates a deficiency in the amplifier; if it were designed and working right, it *should* make rated power when terminated in a 50 ohm load on-channel. Changing the cable length in this case really does nothing for the on frequency load between the duplexer and transmitter, when the duplexer is presenting 50 ohms, but it can change the off frequency impedance transformation that the transmitter sees. Yes, but again, I argue that this all points back to a PA problem. Or the input Z of the duplexer really isn't 50 ohms and the line is acting as a transformer. Detuning the duplexer and or changing cable length to get the transmitter power up is the wrong way to go here. First the transmitter should be optimized into a 50 ohm load. Then optimize the duplexer input for 50 ohms input. Yes, yes, yes, amen! Someone asked about a rule of thumb for transmitter to duplexer cable length. There is none! Yes there is. You take out a tape measure and the distance from the transmitter to the duplexer. You make the cable at least that length. The cable length between multiple cavities is predictable. As an example between two notch cavities; the first notch presents a very low impedance. With a quarter wave line to the next cavity that low impedance is transformed to a high impedance at the input to the next cavity. That high impedance is then presented with a very low impedance of the second cavity. This critical length cable increases the ultimate notch depth because the high impedance that the cable presents and the low impedance of the cavity form a voltage divider. The greater the ratio the better the rejection. 'zactly. When done right, you can pick up close to 6 dB additional net notch depth when cascading notch (or pass/notch) cavities when the intra-cavity cables are cut this way. Jeff WN3A
RE: [Repeater-Builder] Coax length, etc.
So will someone post a simple rule of thumb. If you have the option of optimizing cable length from PA to first cavity, IE you haven't made them yet what's the best simple rule of thumb to follow to build them to avoid reactance. 1/2wl if allowed minus coupling loop depth? Or is that past a simple thumb. Also, This will obviously not work well for 220 or 440 or a most vhf repeater setups. So what would the next ideal cable wl be? And so forth. The reason I ask, if your building new cables why not? Answers on here seem to range a lot. There is no simple rule of thumb, and if anybody tells you that there is, ask them how do you account for the unknown-length of coax that's *inside* your transmitter/amplifier before it gets to the antenna jack. --- Jeff WN3A
RE: [Repeater-Builder] Coax length, etc.
OK, I think, for the most part, we're on the same page. I'm cuttin' and trimmin' a lot here... And this is where I believe the duplexer manufacturers are covering their butt. They don't want the problem with complex reactance presented by the duplexer to be their problem. Not that I don't agree, because it's usually the transmitter that is really at fault. I think that last sentence speaks volumes on the matter. Joe Ham buys a new duplexer and hooks it up to his 110 Watt MASTR II repeater and gets 50 watts out the antenna port. He does his homework and realizes that he should only be loosing 29% with the 1.5 dB of insertion loss stated in the paperwork - but he's loosing over 50%. Ah, but is he really *losing* 50 percent in the duplexer, or is transmitter not making the full 110 watts output to start with? Maybe his transmitter is really only delivering 70 watts to the duplexer. Is it an issue of the duplexer's loss being high, or is the problem the transmitter's not making power? Seems to me it's really the latter. The duplexer manufacturer supposedly engineered and tuned it for a 50 Ohm system. Well, kinda. Many duplexers are spec'ed for 1.5:1 (14 dB RL) input VSWR max. Fortunately, I rarely see any that are that bad. I'll gladly trade off a tenth of a dB of insertion loss for several (if not 10 or more) dB of return loss improvement when I'm tuning on the VNA, but some hams are greedy and don't think along those lines when they're tuning... He knows that the cable he connected to the transmitter is good, because when he disconnects the end going to the transmitter port of the duplexer and connects it to his Bird 43 terminated with a good load - it reads 110 watts. Yes, but did he have a second Bird between the Tx and the duplexer when he was measuring power output? That would have told the real story. Now, is the transmitter becoming spurious Now all bets are off. and the cable length being changed in length satisfies the match between the duplexer and transmitter - I don't know... All I can tell you is I have followed the suggestions written in the WACOM manual and it has worked. I had one instance of a ham radio club loosing PA's left and right on their 2M machine. They told me of the situation and I offered to do a little testing. The 110 watt PA would put out 110 watts into a Bird and dummy, but only 45 watts was coming out the antenna port of the duplexer. At the time I didn't own a spectrum analyzer. The repeater wouldn't duplex without desense. I changed the length of the line between the PA and duplexer until I got the power to read about 75 Watts as I remember. That was 13 years and they still have the same PA - no desense either. Out of morbid curiosity, what kind of PA was it? You are changing the VSWR when tuning the cavity closest to the transmitter. Yes, but once you've adjusted that cavity, from that point on, changing the cable length doesn't vary the VSWR. That was my point - changing the cable length doesn't change VSWR. I realize that impedance transformation cannot occur when you have a 50 Ohm cable (of any length) and a perfect 50 Ohm load - but I think you will agree that a duplexer doesn't, in any way shape or form, present a nice 50 Ohm load. Well, it can get pretty damn close. I can send you some VNA plots of duplexers with input Z's well in excess of 30 dB return loss, some approaching the limits of my test equipment. Of course, when hooked up to an antenna instead of being terminated in a precision load, all bets are off, but hey, that's not the fault of the duplexer... Some transmitters just cannot deal with it without some form of matching after the fact - like a Z-Matcher, Isolator, Circulator, or even a critical cable length. I don't like those transmitters :-) GE MASTR II 110 watt 150.8 to 174 MHz PA and WACOM WP-641. Thinking...thinking...no, haven't done that one. Motorola MICOR 150.8 to 162 MHz PA and WACOM WP-641. Yes, have done that combo, several times that I can think of. Actually, one of the repeaters was low-split from the factory (out of Canada) now that I think about it, so that doesn't count, the others were all H split with no PA mods. Didn't do anything special with cable lengths. Hamtronics 45 Watt 2M PA and Sinclair Q-202. Haven't done any Hamtronics. Well, I cannot believe that I'm the only person on this list that has had success with optimizing the length of cable between the duplexer and transmitter/PA. I don't doubt that others have seen positive (or negative) effects from varying cable lengths - I just said I've never had to resort to doing it, using the equipment that I've used, with the equipment tuned the way I've tuned it. I'll get us some tickets for Vegas - Jeff. I think ZZU has the right idea. He's down in MX-land right now, probably sitting on a beach laughing at us working
RE: [Repeater-Builder] Coax length, etc.
-Original Message- From: Repeater-Builder@yahoogroups.com [mailto:Repeater- buil...@yahoogroups.com] On Behalf Of Jeff DePolo Sent: Saturday, August 14, 2010 10:45 PM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Coax length, etc. Jeff, you aren't stepping on my toes at all. Glad to see your comments. OK, good. Since you've never met me, I can assure you, you definately DO NOT want me stepping on your toes, it would be painful. I do have to agree with Kevin that most duplexer manufacturers recommend different cable length trials between the transmitter and the duplexer when full power can not be reached into the duplexer. Ah, but the crux of the matter is that we're not changing the performance of the duplexer, we're just getting the transmitter to transfer more power into the line. Yes! I fully agree. Over the years I have been a manufacturers rep for TX-RX, Sinclair and Telewave. All of them recommend the same thing. Again, it's a CYA measure as Kevin pointed out. PA won't make power? Don't blame us, try mucking with the cable length, see if that helps. But it is not necessarily the duplexer's problem. I am not a transmitter expert but it is my understanding that the problem is not one of the duplexer not presenting 50 ohms at the wanted frequency but the impedance that it presents off frequency to the transmitter finals. Some solid state devices do not like to see high reactance, even off frequency. But why? If all of the power (or, let's hope, at least 99.99% of it) is on-channel, *should* a properly-designed and properly-functioning transmitter misbehave due to the poor match a duplexer presents at frequencies far removed from the channel center? Well yes, properly designed transmitter. But how much do you want to pay for it? A built in isolator will solve all of those problems as an example. For one thing the reactance causes them to draw more current than normal. Again, why? Not sure why. I have been told by device engineers that is a characteristics of some devices. This may be why you find that tuning for minimum pa current and maximum power out don't exactly agree with one another. I can promise you they almost never do, but that's not any great mystery. You are probably finding a balance between the off frequency reactance and the on frequency wanted load that the finals see. No, that's not it. The off-frequency Z issue is a totally separate topic from the efficiency vs maximum output subject. Let's keep those two topics separate for the sake of this discussion. If what you find in tuning happens directly into a 50 ohm load I agree. If you have the duplexer properly tuned to provide 50 ohms at its input port, the transmitter may still not be happy because of the off frequency reactance presented by the duplexer. I disagree. I would accept the notion that the transmitter may not be happy (and I put that in quotes not to mock you, but becuase I can't come up with a better word either) because it is not *properly matched* when looking into a 50+j0 load. This indicates a deficiency in the amplifier; if it were designed and working right, it *should* make rated power when terminated in a 50 ohm load on-channel. Yes it would be a transmitter problem. Maybe as designed. Changing the cable length in this case really does nothing for the on frequency load between the duplexer and transmitter, when the duplexer is presenting 50 ohms, but it can change the off frequency impedance transformation that the transmitter sees. Yes, but again, I argue that this all points back to a PA problem. Or the input Z of the duplexer really isn't 50 ohms and the line is acting as a transformer. Again I agree. In this instance I was describing a duplexer that did present 50 ohms at the operating frequency and still the transmitter was not happy. Because of the off frequency impedance being transformed to something that the transmitter does not like. It is almost impossible for a high Q cavity to not present some reactance away from the tuned frequency. If it didn't then it would not have any selectivity. The random length cable of course transforms that reactance to something that the transmitter may or may not be comfortable with as discussed above. Detuning the duplexer and or changing cable length to get the transmitter power up is the wrong way to go here. First the transmitter should be optimized into a 50 ohm load. Then optimize the duplexer input for 50 ohms input. Of course I am talking about when the duplexer is presenting a good 50 ohm input impedance at the operating frequency. Yes, yes, yes, amen! Someone asked about a rule of thumb for transmitter to duplexer cable length. There is none! Yes there is. You take out a tape measure and the distance from the transmitter to the duplexer. You make
RE: [Repeater-Builder] Coax length, etc.
I must have missed some posts - my inbox ran out of space (I'm on the road and not checking email as often as I usually do), so my apologies if I'm asking questions that have already been answered... Allan Crites and I are currently in discussion which will be used as the basis of a RB web article that will explain exactly what is happening, why it happens, and why an 'optimized' cable length can be used to transfer power ending up with the stated loss of the duplexer and have little reflected power toward the transmitter - so long as the duplexer is tuned properly and exhibits good return loss on the frequency it's designed to pass. Maybe I'm not understanding right. Are you saying that by varying the cable length between the transmitter and the duplexer that you can affect the insertion loss of the duplexer? And also that by varying the cable length between the transmitter and the duplexer that you can vary the reflected power on that same line? Please tell me I'm reading this wrong...I've been on the road a long time and working a lot of long hours, so it's quite possible... --- Jeff WN3A
Re: [Repeater-Builder] Coax length, etc.
Jeff DePolo wrote: Maybe I'm not understanding right. Are you saying that by varying the cable length between the transmitter and the duplexer that you can affect the insertion loss of the duplexer? No. Because the impedance is not matched between the transmitter and duplexer, the 'apparent' loss of the duplexer is greater than the manufacturers stated loss of the duplexer. Changing the cable length is not changing the loss of the duplexer, it's changing the power that is accepted at the transmitter port of the duplexer by matching the output impedance of the transmitter to the input impedance of the transmitter port of the duplexer. And also that by varying the cable length between the transmitter and the duplexer that you can vary the reflected power on that same line? Yes. In a situation where the duplexer and transmitter have differing impedances, and a cable optimized in length matches these impedances, the mismatch at the duplexer is minimized, therefore the power reflected by the duplexer is minimized. Please tell me I'm reading this wrong...I've been on the road a long time and working a lot of long hours, so it's quite possible... --- Jeff WN3A I have found that when you get a transmitter that is 'picky' about the length of interconnecting cable, power being read at the output port of the duplexer is low and you cannot alter the tuning of the cavity closest to the transmitter to make things right. In other words, the place where lowest VSWR and maximum power transfer occurs is at two completely different places, and power transfer is not up where it should be (transmitter makes 100 watts into a dummy load but only shows 50 watts on the output port of the duplexer that has a stated 1.5 dB loss (29 %)). As you get close to the 'optimum' cable length, the lowest VSWR and maximum power transfer occur near the same place when tuning the cavity closest to the transmitter. I usually pay more attention to what is coming out the antenna port of the duplexer - first. Then, when things are right, comparing forward power going to the duplexer and power going to a good dummy load will be very close the same, since matching the impedance of the transmitter to the impedance of the duplexer was accomplished by some means. I use a compensation cable along with my Bird so that the samplers transmission line length is nullified in the line under test. This compensation cable results in exactly 1/2 WL including the samplers transmission line. Compensation cable lengths are outlined in the manual for the Bird 43. Kevin
RE: [Repeater-Builder] Coax length, etc.
The cable length issue is a brother to if you don't like your VSWR, change the point along the transmission line where you're measuring it. I don't know what that's supposed to mean. The VSWR on the line is the same no matter where along the line you measure it. If you're using a meter that reads a different VSWR depending where on the line you put it, you need a new meter... --- Jeff WN3A
Re: [Repeater-Builder] Coax length, etc.
And a new perspective on transmission lines. I didn't think it was worth responding to, Jeff. AC WA9ZZU. From: Jeff DePolo j...@broadsci.com To: Repeater-Builder@yahoogroups.com Sent: Fri, August 6, 2010 8:23:09 AM Subject: RE: [Repeater-Builder] Coax length, etc. The cable length issue is a brother to if you don't like your VSWR, change the point along the transmission line where you're measuring it. I don't know what that's supposed to mean. The VSWR on the line is the same no matter where along the line you measure it. If you're using a meter that reads a different VSWR depending where on the line you put it, you need a new meter... --- Jeff WN3A
Re: [Repeater-Builder] Coax length, etc.
Grab your Smith chart! LOL - Original Message - From: allan crites To: Repeater-Builder@yahoogroups.com Sent: Friday, August 06, 2010 12:46 PM Subject: Re: [Repeater-Builder] Coax length, etc. And a new perspective on transmission lines. I didn't think it was worth responding to, Jeff. AC WA9ZZU. -- From: Jeff DePolo j...@broadsci.com To: Repeater-Builder@yahoogroups.com Sent: Fri, August 6, 2010 8:23:09 AM Subject: RE: [Repeater-Builder] Coax length, etc. The cable length issue is a brother to if you don't like your VSWR, change the point along the transmission line where you're measuring it. I don't know what that's supposed to mean. The VSWR on the line is the same no matter where along the line you measure it. If you're using a meter that reads a different VSWR depending where on the line you put it, you need a new meter... --- Jeff WN3A
Re: [Repeater-Builder] Coax length, etc.
Jeff was correct to question. I was vague. What I should have said was indicated reflected power, not VSWR. But good luck trying to determine an accurate VSWR based on erroneous reflected readings. Let the boo birds squawk. Keep questioning, Jeff. 73, Russ WB8ZCC On 8/6/2010 1:07 PM, Steven M Hodell wrote: Grab your Smith chart! LOL - Original Message - *From:* allan crites mailto:wa9...@arrl.net *To:* Repeater-Builder@yahoogroups.com mailto:Repeater-Builder@yahoogroups.com *Sent:* Friday, August 06, 2010 12:46 PM *Subject:* Re: [Repeater-Builder] Coax length, etc. And a new perspective on transmission lines. I didn't think it was worth responding to, Jeff. AC WA9ZZU. *From:* Jeff DePolo j...@broadsci.com mailto:j...@broadsci.com *To:* Repeater-Builder@yahoogroups.com mailto:Repeater-Builder@yahoogroups.com *Sent:* Fri, August 6, 2010 8:23:09 AM *Subject:* RE: [Repeater-Builder] Coax length, etc. The cable length issue is a brother to if you don't like your VSWR, change the point along the transmission line where you're measuring it. I don't know what that's supposed to mean. The VSWR on the line is the same no matter where along the line you measure it. If you're using a meter that reads a different VSWR depending where on the line you put it, you need a new meter... --- Jeff WN3A
Re: [Repeater-Builder] Coax length, etc.
Thanks, guys, a good topic and one that always seems to come up. And it sparks more questions and comments, of course. The cable length issue is a brother to if you don't like your VSWR, change the point along the transmission line where you're measuring it. By changing the length of the line, we're creating a transmission line transformer (a good thing) but we're limited by its length (not so good). It seems to me the mentioned circulator/isolator at the output of the xmtr is a better fix, as reflections coming back from the duplexer is absorbed by the circulator's load, the xmtr is generally happy, and we're no longer limited where we can put things in a rack or elsewhere. For amateurs, coming up with usable VHF circulators seems to be difficult and usually expensive, and coax always seems to be cheaper. Has anyone had luck finding a source for reasonbly priced VHF circulators, or success in rolling their own? Also, I noted in the pamphlet Kevin referenced that the unused duplexer port was left open (Figs. 1 2). I guess if the isolation is already greater than the load's return loss, it doesn't matter, at least at the reject frequency. But it seems to me one could possibly create problems for oneself by not terminating the unused open port. Just a thought. Maybe I work better knowing there's a load there. ;-) Your comments, please. 73, Russ WB8ZCC On 8/5/2010 10:19 AM, Doug Hutchison wrote: Hi Kevin and all who responded to my question. Thank you, good info in the link provided by Kevin along with other interesting guidelines. More for the file. Regards, Doug - GM7SVK On 04/08/2010 11:04 PM, Kevin Custer wrote: Doug Hutchison wrote: Does the length of coax connecting cable between repeater and filters matter? Yes - and no. Please read the note about cabling lengths between the repeater and the duplexer in the section on page 4 of the following document: http://www.repeater-builder.com/wacom/wp6xx-vhf-tuning-instructions-remec.pdf Watch for word wrap... Kevin Custer Yahoo! Groups Links
Re: [Repeater-Builder] Coax length, etc.
Russ Hines wrote: Thanks, guys, a good topic and one that always seems to come up. And it sparks more questions and comments, of course. The cable length issue is a brother to if you don't like your VSWR, change the point along the transmission line where you're measuring it. By changing the length of the line, we're creating a transmission line transformer (a good thing) but we're limited by its length (not so good). It seems to me the mentioned circulator/isolator at the output of the xmtr is a better fix, as reflections coming back from the duplexer is absorbed by the circulator's load, the xmtr is generally happy, and we're no longer limited where we can put things in a rack or elsewhere. For amateurs, coming up with usable VHF circulators seems to be difficult and usually expensive, and coax always seems to be cheaper. Has anyone had luck finding a source for reasonbly priced VHF circulators, or success in rolling their own? Also, I noted in the pamphlet Kevin referenced that the unused duplexer port was left open (Figs. 1 2). I guess if the isolation is already greater than the load's return loss, it doesn't matter, at least at the reject frequency. But it seems to me one could possibly create problems for oneself by not terminating the unused open port. Just a thought. Maybe I work better knowing there's a load there. ;-) Your comments, please. 73, Russ WB8ZCC I think we all agree that a real impedance matching device is the best approach, but hams (generally speaking) are cheap. Many will spend two days hacking on a piece of RG-214 before spending fifty or a hundred bucks on a different (better?) solution. Allan Crites and I are currently in discussion which will be used as the basis of a RB web article that will explain exactly what is happening, why it happens, and why an 'optimized' cable length can be used to transfer power ending up with the stated loss of the duplexer and have little reflected power toward the transmitter - so long as the duplexer is tuned properly and exhibits good return loss on the frequency it's designed to pass. Kevin Custer
Re: [Repeater-Builder] Coax length, etc.
Thanks for the reply, Kevin. I'm looking forward to seeing the article. 73, Russ WB8ZCC On 8/5/2010 1:20 PM, Kevin Custer wrote: Russ Hines wrote: Thanks, guys, a good topic and one that always seems to come up. And it sparks more questions and comments, of course. The cable length issue is a brother to if you don't like your VSWR, change the point along the transmission line where you're measuring it. By changing the length of the line, we're creating a transmission line transformer (a good thing) but we're limited by its length (not so good). It seems to me the mentioned circulator/isolator at the output of the xmtr is a better fix, as reflections coming back from the duplexer is absorbed by the circulator's load, the xmtr is generally happy, and we're no longer limited where we can put things in a rack or elsewhere. For amateurs, coming up with usable VHF circulators seems to be difficult and usually expensive, and coax always seems to be cheaper. Has anyone had luck finding a source for reasonbly priced VHF circulators, or success in rolling their own? Also, I noted in the pamphlet Kevin referenced that the unused duplexer port was left open (Figs. 1 2). I guess if the isolation is already greater than the load's return loss, it doesn't matter, at least at the reject frequency. But it seems to me one could possibly create problems for oneself by not terminating the unused open port. Just a thought. Maybe I work better knowing there's a load there. ;-) Your comments, please. 73, Russ WB8ZCC I think we all agree that a real impedance matching device is the best approach, but hams (generally speaking) are cheap. Many will spend two days hacking on a piece of RG-214 before spending fifty or a hundred bucks on a different (better?) solution. Allan Crites and I are currently in discussion which will be used as the basis of a RB web article that will explain exactly what is happening, why it happens, and why an 'optimized' cable length can be used to transfer power ending up with the stated loss of the duplexer and have little reflected power toward the transmitter - so long as the duplexer is tuned properly and exhibits good return loss on the frequency it's designed to pass. Kevin Custer
