RE: [Repeater-Builder] Cable formula
Sure, a UHF isolator will not protect the transmitter from VHF transmitter junk. But isn't the flip side that out of band VHF junk is less likely to produce UHF transmitter intermod than in band transmitter junk? Not necessarily. If it were the other way around (UHF coming back down the hose into a VHF transmitter), the harmonic filter built into the PA would prevent the VHF energy from getting to the devices. I had a UHF repeater (GE Mastr II 1/4 kW tube) with a VHF remote base (25 watt Micor mobile). The two antennas were about 20' apart from tip of the VHF to bottom of the UHF. I had mix problems in the tube PA that produces products at frequencies that intermod math would never predict to occur when the remote base Tx was keyed up. Adding a pass cavity to the repeater Tx cured it. And also, while a VHF band pass cavity might do its job resisting unwanted in band stuff, doesn't this cavity still easily pass undesired junk at frequency multiples? Sometimes yes. A quarter wave cavity will resonate just fine at odd multiples. The converse isn't true though; a UHF pass cavity will do a good job of keeping out VHF. --- Jeff
RE: [Repeater-Builder] Cable formula
The other ways to correct the problem, other than using a different transmitter that is not bothered by reactive loads as much, is to use a Z match or try different length cables that make the transmitter happier. But if the transmitter is bothered by the bad Z at frequencies outside of the pass, any matching device you put in line is only going to throw off the Z at the pass frequency. You can't have a duplexer that presents 50+j0 at the pass frequency, and then add a matching device between it and the PA and still have 50+j0 at the transmitter at the pass frequency. Sure, you can give the PA 50+j0 at some other off-channel frequency by doing this, but at the expense of messing up the pass performance, this seems like it's only creating new problems. Both of these transform the impedance/reactance presented by the duplexer to something more palatable to the transmitter and allow it to produce the power intended. If you have a PA that is happy with a good load at the carrier frequency, but isn't happy with the Z at other frequencies where it shouldn't be making power, I'd have to say you have a problem with the amplifier that needs to be fixed. The antenna system shouldn't have to be the cure for the amplifier's ills. I've never had a good PA (emphasis on good) that got unstable if it was looking into a matched load at the carrier frequency regardless of what was going on off-channel. There is no sin involved in using different length cables to make the system work properly. It is not a band-aid approach to mask other problems. Well, I guess we'll have to agree to disagree. The real problem is that some transmitters, because of the way they are designed, do not like reactive loads. They should like a good load at the carrier frequency, and be tolerant of strange load Z's off-channel. A PA that has a tendency to run away just because the load Z at frequencies well removed from carrier isn't perfect is an accident waiting to happen. Even if you mess with cable lengths and Z-matchers and walk away from the site with the PA running clean, there's a good chance the next time the antenna ices up or the HVAC fails that you'll be getting calls about spurs coming from your box. If I have a PA go spurious, for ANY reason, whether during commissioning or sometime during operation, it comes out of service until it can be fixed. That's just me. Close spaced duplexers will be the worst with off frequency reactance as the impedance has to change quickly as you move away from the wanted frequency in order for the duplexer to do its job. Mostly agree, except for notch-only duplexers with tend to have a fairly wide range of tolerable load Z outside the notch, and notch duplexers are fairly common in close-spaced situations. --- Jeff
Re: [Repeater-Builder] Cable formula
Doesn't the isolator typically installed at the transmitter output spin off any anything reflected from the duplexer (or the feedline) into it's load? In a message dated 7/1/2007 5:33:33 P.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: But at some off frequency that is not 50+j0 that impedance is going to get transformed into something yet again by the time the cable reaches the transmitter. ** See what's free at http://www.aol.com.
RE: [Repeater-Builder] Cable formula
Yes it does, if you have an isolator installed at the transmitter. With an isolator on the transmitter the transmitter will always see 50 ohms no matter what the load on the other end of the isolator is. There should be no problems with off frequency reactance when an isolator is used. But any reflected power into the isolators load (from on frequency signal) is lost in heat and never reaches the antenna. With an isolator, if the duplexer is not presenting a pure 50 ohms (at the wanted frequency) to the output of the isolator you could put a wattmeter between the isolator and the isolator load and change cable lengths between duplexer and isolator or tune a Z matcher if you have one, for minimum power into the isolators load. That will give you maximum power to the antenna and you will have a near perfect 50 ohm load on the transmitter always. 73 Gary K4FMX _ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] Sent: Sunday, July 01, 2007 8:30 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Cable formula Doesn't the isolator typically installed at the transmitter output spin off any anything reflected from the duplexer (or the feedline) into it's load? In a message dated 7/1/2007 5:33:33 P.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: But at some off frequency that is not 50+j0 that impedance is going to get transformed into something yet again by the time the cable reaches the transmitter. _ See what's free at AOL.com http://www.aol.com?ncid=AOLAOF0002000503 .
Re: [Repeater-Builder] Cable formula
That (isolator in place of tee) is how moto configured their Q package UHF Med Radio. The receive port gets the receive signal plus reflected energy that the preselector bounced back...never heard of that damaging an isolator, matter of fact, aside from burning up too small loads or lightning, I have never run into a damaged isolator, but I am sure there are instances - I have 14, 800 mhz smr boxes, and 80, 800 EDACS stations, plus 30 something UHF repeaters, I can only recall one piston capacitor failure on a telewave dual junction job. Steve NU5D. Gary Schafer wrote: Why would you ever want to do that? Unless you like destroying isolators. :) 73 Gary K4FMX -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:Repeater- [EMAIL PROTECTED] On Behalf Of Steve S. Bosshard (NU5D) Sent: Sunday, July 01, 2007 8:52 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Cable formula That would be the typical installation, unless you install the isolator at the output of the duplexer with the #1 (input) toward the TX cavities, #3 (load) toward the receive cavities, and #2 port (output) toward the antenna, used in place of the TEE fitting. Steve NU5D [EMAIL PROTECTED] wrote: Doesn't the isolator typically installed at the transmitter output spin off any anything reflected from the duplexer (or the feedline) into it's load? In a message dated 7/1/2007 5:33:33 P.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: But at some off frequency that is not 50+j0 that impedance is going to get transformed into something yet again by the time the cable reaches the transmitter.
RE: [Repeater-Builder] Cable formula
Hi Steve, I don't recall ever seeing that done. What was the purpose of using an isolator there? As to toasting the isolator, if you should loose the antenna etc. there would be a short at the tx frequency rather than a load (assuming there is some kind of cavity between isolator and receiver). 73 Gary K4FMX -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:Repeater- [EMAIL PROTECTED] On Behalf Of Steve S. Bosshard (NU5D) Sent: Sunday, July 01, 2007 9:35 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Cable formula That (isolator in place of tee) is how moto configured their Q package UHF Med Radio. The receive port gets the receive signal plus reflected energy that the preselector bounced back...never heard of that damaging an isolator, matter of fact, aside from burning up too small loads or lightning, I have never run into a damaged isolator, but I am sure there are instances - I have 14, 800 mhz smr boxes, and 80, 800 EDACS stations, plus 30 something UHF repeaters, I can only recall one piston capacitor failure on a telewave dual junction job. Steve NU5D. Gary Schafer wrote: Why would you ever want to do that? Unless you like destroying isolators. :) 73 Gary K4FMX -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:Repeater- [EMAIL PROTECTED] On Behalf Of Steve S. Bosshard (NU5D) Sent: Sunday, July 01, 2007 8:52 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Cable formula That would be the typical installation, unless you install the isolator at the output of the duplexer with the #1 (input) toward the TX cavities, #3 (load) toward the receive cavities, and #2 port (output) toward the antenna, used in place of the TEE fitting. Steve NU5D [EMAIL PROTECTED] wrote: Doesn't the isolator typically installed at the transmitter output spin off any anything reflected from the duplexer (or the feedline) into it's load? In a message dated 7/1/2007 5:33:33 P.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: But at some off frequency that is not 50+j0 that impedance is going to get transformed into something yet again by the time the cable reaches the transmitter. Yahoo! Groups Links
Re: [Repeater-Builder] Cable formula
I remember asking Lloyd at Wacom about using an isolator in the output - seems like he told me that it allowed more flexibility in cable lengths on the output - might make sense because the Q package had frequencies from 458 to 468 with the apcor carry in units. Ancient history - nightmares when they gave trouble.. If I get the chance I will call him on the phone - see how retirement is going, etc. Steve. Gary Schafer wrote: Hi Steve, I don't recall ever seeing that done. What was the purpose of using an isolator there? As to toasting the isolator, if you should loose the antenna etc. there would be a short at the tx frequency rather than a load (assuming there is some kind of cavity between isolator and receiver). 73 Gary K4FMX
RE: [Repeater-Builder] Cable formula
Doesn't the isolator typically installed at the transmitter output spin off any anything reflected from the duplexer (or the feedline) into it's load? The generic answer is yes, but the qualified answer is that isolators, like everything else, have a finite bandwidth, so if the energy is far removed from the design frequency of the isolator, it doesn't do its job as well. At far-removed frequencies, some of the power incident on the output power (i.e. what's coming back down the hose) gets reflected at the isolator due to a poor match, some will makes its way around to the reject load, and some will make it 240 degrees around to the transmitter. As an example of the limitations of an isolator, a UHF isolator on a UHF repeater isn't going to isolate your PA very well from highband signals coming down the line. A bandpass cavity between the isolator and the duplexer will. --- Jeff
RE: [Repeater-Builder] Cable formula
If your duplexer presents 50+j0 at its input at the operating frequency and you are using 50 ohm cable to connect it to the transmitter then the transmitter is always going to see 50+j0 at the operating frequency no matter what the cable length is. But at some off frequency that is not 50+j0 that impedance is going to get transformed into something yet again by the time the cable reaches the transmitter. And depending on how long or short you make that cable will depend on what transformation it will make to that off frequency impedance all the while still delivering 50+j0 at the operating frequency. So cable length can be used to change unwanted reactance of off frequency things. Of course. But if you stuck a Z-matcher between the duplexer and the PA, then what started out as 50+j0 on-channel is not going to be 50+j0 at the PA. That was my point - the Z-matcher - not cable lengths. Even if the duplexer is not a perfect 50 ohms, changing cable lengths can have enough of an effect on off frequency things to make the system work sometimes. The sometimes is what troubles me... Transistors are strange animals. The do not equate to tubes in there operation especially when broad band combining devices are used at the output of the transistor amplifiers as most do. The devices themselves do strange things in the presents of reactance. That's mostly why many amplifiers have swr shutdown circuits or power roll back. Yeah, but those foldback circuits usually rely on a directional coupler to monitor reflected power. If all of the energy (well, almost all of it anyway) is being produced on-channel, the directional coupler is going to sense nothing and the transmitter is going to run full bore and be happy. If the PA were to go spurious, and all of the off-channel garbage came back down the line, then it would fold back. But if the PA's clean, and it's looking into a well-matched load, there should be no reason to fold back. problems are generally not one of excess current because of reflected power but the excess current the devices draw is from the unwanted reactance that they see. You're talking about reactive currents flowing in the matching networks I presume. But those reactances are present *inside* the PA, before the point where it is connected to the outside 50+j0 world. Most bipolars will operate into an infinite VSWR without giving up the ghost. Excess reactive currents within the PA circuitry will manifest as higher current draw, which is yet one more reason why I always recommend watching current consumption when doing any kind of matching trickery. Sometimes a duplexer just doesn't present a perfect 50 ohms to the transmitter either. Some transmitters will start cutting back the power with little reflected power. Others may not be able to put out all the power it is supposed to if it does not see a perfect 50 ohms. These are some of the reasons most duplexer manufacturers recommend trying different cable lengths between the duplexer and transmitter. You can't convert to a perfect 50 ohms using cable lengths if the load isn't already 50 ohms. I've said before, and I'm saying it again. If your duplexer 50 ohms load, you can pull all of the 50 ohm cables you want out of your bag and you'll never get it back to 50 ohms at the PA. But if the duplexer is designed, built, and tuned right, it should be very close 50 ohms. I can't think of any duplexer I've ever tuned up on a VNA that didn't have at least 20 dB return loss at the pass frequency, with many often being 30 dB or more. Of course, that's only as good as your antenna load, but let's keep that out of the equation for now since we're talking about matching between the PA and duplexer only. All transmitters are not created equal. Amen to that brother. I do agree with you that gross problems that show up are signs of problems that need to be fixed other than changing cables. See, we do agree on something! --- Jeff
RE: [Repeater-Builder] Cable formula
Hi Steve, I don't recall ever seeing that done. What was the purpose of using an isolator there? Steve will probably reply too, but I'll give you the quick answer. UHF Micor mobiles all came stock with an isolator in the antenna network, just like their big brother base/repeater stations. When the radio was in Rx mode, a relay switch the reject port on the isolator to the receiver instead of the load. When it was in Tx, the relay switched the reject port back to the load. With this arrangement, you never had full PA power going through the T/R relay, only reject power, so presumably they did it that way as a means of prolonging the life of the relay contacts (just a guess). --- Jeff
RE: [Repeater-Builder] Cable formula
You can't convert to a perfect 50 ohms using cable lengths if the load isn't already 50 ohms. I've said before, and I'm saying it again. If your duplexer 50 ohms load, you can pull all of the 50 ohm cables you want out of your bag and you'll never get it back to 50 ohms at the PA. Boy, I really butchered the English language on that one. It's late. Rewritten: You can't convert to a perfect 50 ohms using cable lengths if the load isn't already 50 ohms. I've said it before, and I'm saying it again. If your duplexer isn't a 50 ohm load, you can pull all of the 50 ohm cables you want out of your bag and you'll never get it back to 50 ohms at the PA. --- Jeff
RE: [Repeater-Builder] Cable formula
I remember when the micro first came out. I thought I remembered that the isolator was in there to help with transmitter stability when a perfect load was not applied. It may have well been to preserve the relay as well. Seems that I remember something about relay problems in the early days of that radio?? 73 Gary K4FMX -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:Repeater- [EMAIL PROTECTED] On Behalf Of Jeff DePolo Sent: Sunday, July 01, 2007 10:47 PM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Cable formula Hi Steve, I don't recall ever seeing that done. What was the purpose of using an isolator there? Steve will probably reply too, but I'll give you the quick answer. UHF Micor mobiles all came stock with an isolator in the antenna network, just like their big brother base/repeater stations. When the radio was in Rx mode, a relay switch the reject port on the isolator to the receiver instead of the load. When it was in Tx, the relay switched the reject port back to the load. With this arrangement, you never had full PA power going through the T/R relay, only reject power, so presumably they did it that way as a means of prolonging the life of the relay contacts (just a guess). --- Jeff Yahoo! Groups Links
RE: [Repeater-Builder] Cable formula
-Original Message- From: Repeater-Builder@yahoogroups.com [mailto:Repeater- [EMAIL PROTECTED] On Behalf Of Jeff DePolo Sent: Sunday, July 01, 2007 10:51 PM To: 'Jeff DePolo'; Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Cable formula You can't convert to a perfect 50 ohms using cable lengths if the load isn't already 50 ohms. I've said before, and I'm saying it again. If your duplexer 50 ohms load, you can pull all of the 50 ohm cables you want out of your bag and you'll never get it back to 50 ohms at the PA. Boy, I really butchered the English language on that one. It's late. Rewritten: You can't convert to a perfect 50 ohms using cable lengths if the load isn't already 50 ohms. I've said it before, and I'm saying it again. If your duplexer isn't a 50 ohm load, you can pull all of the 50 ohm cables you want out of your bag and you'll never get it back to 50 ohms at the PA. --- Jeff Heh heh, I followed what you meant anyway Jeff. Yes I agree that you will never get it back to 50 ohms with a 50 ohm cable and I didn't mean to imply that you could although I guess that's what it sounded like as I wrote it. What I meant was that you can usually obtain some impedance that the transmitter likes better than what the duplexer might be supplying with a random length cable. Please note that most duplexer manufacturers recommend trying different cable lengths to cure the type of problems we have been discussing. 73 Gary K4FMX
Re: [Repeater-Builder] Cable formula
Sure, a UHF isolator will not protect the transmitter from VHF transmitter junk. But isn't the flip side that out of band VHF junk is less likely to produce UHF transmitter intermod than in band transmitter junk? And also, while a VHF band pass cavity might do its job resisting unwanted in band stuff, doesn't this cavity still easily pass undesired junk at frequency multiples? In a message dated 7/1/2007 8:49:50 P.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: As an example of the limitations of an isolator, a UHF isolator on a UHF repeater isn't going to isolate your PA very well from highband signals coming down the line. A bandpass cavity between the isolator and the duplexer will. ** See what's free at http://www.aol.com.
Re: [Repeater-Builder] Cable formula
OK, There was a long and detailed thread about z matchers on this group abou a year ago? Lots of interesting stuff about line matching emerged.. dont take my word for it have a look in the archives... I can only describe what I measure and that is cable leakage from a jumper between the transmitter and the duplexer when a DB products z matcher was used. (The z matcher was very nicley made with gold plated piston caps etc.) The cable leakage stopped when the z matcher was removed and the cable length was altered for optimum. Ive just dug out my line stretcher : 874-lk20L constant impedance adjustable line- general radio USA no striped paint here im afraid! Ian Ashford G8PWE - Original Message - From: [EMAIL PROTECTED] To: Repeater-Builder@yahoogroups.com Sent: Saturday, June 30, 2007 3:50 AM Subject: Re: [Repeater-Builder] Cable formula A good quality Z-Matcher has isolation caps on the trimmer ports so I don't think the matcher itself is producing any RF radiation. I don't understand your description of the z Matcher as introducing any mismatch. The mismatch is already there as a result of some disparity between the source, load and cable impedances. All the matcher does is permit you to match the source and cable impedances. In a message dated 6/29/2007 4:40:45 P.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: The z matcher is another option but in my practical experience it makes the jumper radiate RF you spend all that money on RG214/RG400 double silver plated shielding and then deliberatley mismatch it? Ian Ashford G8PWE -- See what's free at AOL.com.
RE: [Repeater-Builder] Cable formula
However, the load impedance of most PAs will vary significantly with the drive level, I think you meant source impedance. and the input impedance of a duplexer cavity is always reactive Not necessarily. You can tune a duplexer very close to 50+j0 at the pass frequency. It's at frequencies off the center frequency that the selective nature of the duplexer makes it appear as a Z other than 50+j0. However, a simpler approach is to install an impedance matching device, sometimes called a Z-Matcher, at the output of the PA and adjusting it for maximum forward power. I disagree with this and feel it is bad advice. The point where the transmitter makes maximum power may occur at some load Z other than what the transmitter was designed for. Just because you can eek out a few extra watts by futzing with the Z-matcher doesn't mean you've done anything to improve the stability of the amplifier, nor is it guaranteed to be operating at maximum efficiency, nor do you know if the increase in power you're seeing is due to new spurious/oscillation products being generated due to a bad match. If there is a means of adjusting the loading on the PA via a Z-matcher, be it internal or external to the PA, it should first be set flat at 50+j0 on a VNA or return loss bridge, then hooked up to the PA, and while monitoring current draw, power output, AND spectral purity, make minor adjustments if necessary to find the best balance. The Z-matcher itself performs as if it were a narrow-band device in the sense that it will only correctly match at a single frequency. The load Z of the duplexer, which varies as you get away from its tuned pass frequency, is going to present a varying load, which the Z-matcher is going to transform again, maybe for the better, maybe for the worse. So, at best, the Z-matcher is a band aid in my book. I would much rather see an isolator with a very good input return loss on the output of the PA if there are problems that can defintively be proven to be caused by the load Z of the duplexer and antenna system. I've always maintained that if you have amplifier stability problems or problems achieving rated specs for the PA (power out, current draw, spurious, etc.), then you've got fundamental problems that need to be fixed either in the antenna system or the amplifier itself. --- Jeff
RE: [Repeater-Builder] Cable formula
I don't think the cable cares whether the source and load impedances are 50 ohms resistive. I think the cable is indifferent to whether the load and source values are resistive or whether they present a complex impedance involving +/- J. as long as the composite value looks like 50 ohms. The cable only acts as a transformer if the *load* Z is not the same as the cable's characteristic Z. It doesn't care about the source Z; the mismatch that occurs at the source end only affects power transfer into the cable at that point. The conventional wisdom generally expressed is that as long as the cavities are properly tuned, that the interconnect length from the TX is immaterial. I question that: Properly tuned? When what's properly tuned and for what parameter? Is the pass section of the cavity(s) being tuned for maximum output or is it tuned for minimum reflection back to the TX source? Reflection (S11). Always always always tune pass or pass/reject cavity filters for best match (pounding on the desk as I type). Too many manufacturers' instructions say to tune for maximum power transfer or least insertion loss, probably because they assume the field techs don't have equipment for measuring return loss properly. That's just not good advice in my book. If the filter is designed and built right, the insertion loss minima and return loss maxima should conincide very closely. Even if they're off a tad and you can't get them to line up perfectly, you're always better off taking 10 dB more return loss in exchange for 0.1 dB of insertion loss degradation. That primarily applies to our little narrowband two-way radio world. In wideband applications, there are other things involved, such as group delay, that come into play, but for what we're talking about, return loss is the key. Only if the TX output impedance exactly matches the cavity impedance and the impedance of the interconnecting cable will the cavity tuning point be the same for either parameter. To assume that the TX output impedance is 50 ohms is optimistic and as you point out, altering the power level of the TX can affect TX output Z, the amount dependant on what TX stages are used to control TX output. Let's straighten something out here before we get off track. Most transmitters don't HAVE a 50 ohm source Z. They are designed to work INTO a 50 ohm Z. They have internal matching transformers (stripline or otherwise) to convert the very-low-Z output of the bipolar transistors to something approaching 50 ohms so that when it is connected to our external 50 ohm world that the devices are able to transfer power. Considering how nit-picky forum members are about designing and building their systems, (and I mean that in the best sense of the word), it seems inconsistent to be indifferent to how the duplexers might be affected by inserting what is potentially a radical impedance transformer between the TX and the cavities. In the absence of any way to measure any source and load mismatch, using a 1/2 wave (or half wave repeating) cable length will at least keep any existing mismatch status quo. It won't improve the match but at least it won't increase a mismatch because the 1/2 wave length simply repeats the TX output Z and does not act as a line transformer. But as the cable length departs from a 1/2 wave and approaches a 1/4 wave, the game changes and a 1/4 wave interconnect between a mismatched source and load can produce some eye opening shifts in the impedance reflected to the load and back to the source. Whether you have a half-wave or a quarter-wave cable terminated by a mismatched load, the VSWR remains the same. As as a simple example, assume the Z of the duplexer is 100 ohms. If you use a half-wave cable, the PA sees 100 ohms, a 2:1 VSWR. If the cable is a quarter wave, it transforms the 100 ohms to 25 ohms, again a 2:1 VSWR. Yes, the Z is not the same, but the VSWR is. You don't know whether the PA will be better off looking into the 25 ohm load versus the 100 ohm load, so why would you hold fast to the half-wave rule? For a given load Z, the VSWR remains constant no matter what cable length you use. A 50 ohm cable can't transform a non-50 ohm load to 50 ohms; it can only roll you around the Smith Chart at a constant VSWR, that being something other than 1:1. Round and round the Smith Chart we go, where she stops, nobody knows. Point being, if there is a mismatch, using a half wave cable does nothing to improve your chances of making your PA happy any more than would a quarter wave cable or any other random length. Without knowing the actual impedences involved, your odds of making an improvement using an X-length cable (pick your favorite value for X) are 50/50, nothing more, nothing less. Also keep in mind that the transformation the cable does in the case of a load mismatch is, for all practical purposes, random as you sweep across a range of
RE: [Repeater-Builder] Cable formula
OK so you have a reel of cable and two connectors to make up the jumper between transmitter and duplexer. The duplexer is tuned using 50 ohm test gear and the transmitter has been optimised into a 50 ohm load. Unfortunatley the output impedance of the transmitter is not 50 ohms and a length of cable to the duplexer will transform this impedance,detuning the first can in the transmit chain. No, it won't, you're putting the cart before the horse. The duplexer and antenna system is the load. You can't change the Z of the load by changing the Z of the source. Think about it this way. Get rid of your duplexer and antenna and replace them with a fixed resistor, pick a random value, say 1000 ohms, and use a perfect half-wave cable between the resistor and your PA. Now use three different amplifiers to transmit into this 1000 ohm load. Does each PA have the same source Z? No. Has your load Z changed?No. Has the Z at the input to the half-wave cable changed? No. No matter what you do at the source end, the load Z is 1000 ohms and will remain that way for ever and ever Amen. Let's look at it another way using a practical example. Say you have a single bandpass cavity filter that is properly tuned to present 50+j0 at its input port when its output port is terminated with a 50+j0 load. Now you hook it up to a PA that has a strange source impedence using a half-wave cable with a Bird wattmeter somewhere in the middle (its location in terms of distance from either end is immaterial). Will the Bird show any reflected power? Of course not; the Z on the transmission line is still 50 ohms, the E and I are still in-phase at a 50:1 amplitude ratio, the cavity hasn't been detuned at all. The Z on the line is 50+j0, the Z at the input to the cavity is 50+j0, the Z of the load Z is 50+j0. Now, if you look in the REVERSE direction, FROM the duplexer INTO the transmitter, as you change PA's the Z looking the other way is going to vary, but that's not what we're trying to match here. If we want to go down that road, I would argue just how BAD a Z-matcher could potentially be when used on the transmit side of a duplexer in terms of how it can throw off the termination Z and mess up the RECEIVE side performance. Ponder that for a bit (hint: what should be an open at the antenna tee no longer is). A Z-matcher is no substitute for an isolator. --- Jeff
Re: [Repeater-Builder] Cable formula
Do you recall if the leakage you observed was on channel or whether it was broadband noise? In a message dated 6/30/2007 3:53:42 A.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: The cable leakage stopped when the z matcher was removed and the cable length was altered for optimum. ** See what's free at http://www.aol.com.
Re: [Repeater-Builder] Cable formula
I don't think I ever suggested otherwise. I never said that using a half wave cable would improve anything. What I did say was that a half wave cable would repeat the prevailing condition neither making it better or worse and I further said that using any variation from a half wave cable could either mitigate the mismatch or aggravate it. Having said that, I still think that whatever measures you want to undertake to improve matching, utilizing a half-wave cable is the most coherent way to start. In a message dated 6/30/2007 8:54:45 A.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: Point being, if there is a mismatch, using a half wave cable does nothing to improve your chances of making your PA happy any more than would a quarter wave cable or any other random length. Without knowing the actual impedences involved, your odds of making an improvement using an X-length cable (pick your favorite value for X) are 50/50, nothing more, nothing less. ** See what's free at http://www.aol.com.
Re: [Repeater-Builder] Cable formula
Jeff You make some excellent points. Thanks! Bruce K7IJ ** See what's free at http://www.aol.com.
RE: [Repeater-Builder] Cable formula
I don't think I ever suggested otherwise. I never said that using a half wave cable would improve anything. What I did say was that a half wave cable would repeat the prevailing condition neither making it better or worse and I further said that using any variation from a half wave cable could either mitigate the mismatch or aggravate it. Having said that, I still think that whatever measures you want to undertake to improve matching, utilizing a half-wave cable is the most coherent way to start. What I was getting at was that the rule-of-thumb you recommended, i.e. sticking with a half-wave cable, doesn't give you any better or any worse of a chance in getting the right match. The rule could just as well be whatever cable is long enough to get from the transmitter to the duplexer and it would have just as good of a chance in making the PA happy. --- Jeff
Re: [Repeater-Builder] Cable formula
Can't argue with your analysis. My only point is that if you are intent on dealing with a TX to duplexer mismatch, a half wave cable replicates what ever mismatch exists. A random length cable can mask the real world condition by making the apparent mismatch better or worse than it really is. Do you have any thoughts on why or how a well designed Z match could produce cable radiation? In a message dated 6/30/2007 1:03:51 P.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: What I was getting at was that the rule-of-thumb you recommended, i.e. sticking with a half-wave cable, doesn't give you any better or any worse of a chance in getting the right match. The rule could just as well be whatever cable is long enough to get from the transmitter to the duplexer and it would have just as good of a chance in making the PA happy. ** See what's free at http://www.aol.com.
Re: [Repeater-Builder] Cable formula
I was thinking the same thing. I was wondering how the connection at the transmitter looked. My bet would be that this is where the real problem is. Chuck WB2EDV - Original Message - From: Jeff DePolo [EMAIL PROTECTED] To: Repeater-Builder@yahoogroups.com Sent: Saturday, June 30, 2007 6:22 PM Subject: RE: [Repeater-Builder] Cable formula Do you have any thoughts on why or how a well designed Z match could produce cable radiation? For the feedline to radiate, there would have to be currents flowing on the shield. It would seem to me the only way to get that to happen would be if there was an imperfect shield connection at the mating connector at one end or the other. Most Z-matchers I've dissected use a length of wire as an L, with shunt trimmer caps. Assuming the enclosure itself is bonded well to the connectors at either end, I can't come up with a good reason why such a device inserted in a transmission line would cause shield currents to flow absent a connector issue. --- Jeff
[Repeater-Builder] Cable formula
It looks like I am able to post here again. Don't know what happened. This subject was examined a little about a month ago. You can search for my comments back then but I will state again similar to what I said then. Some transmitters do not like to see any reactive load on them. They will cut back the power out and/or generate some spurs in the presence of a reactive load. Hooking a transmitter to a dummy load or an antenna with a pretty flat swr can present a good load to a transmitter as the load is broad band and so are most antennas fairly broad band. But when you put a cavity/duplexer on to a transmitter it is anything but broad band. It can have a good match/return loss/swr at the wanted frequency but at the same time be very reactive off frequency and the particular transmitter may not like that reactance. It is not just a matter of tuning the duplexer to present a good return loss at the wanted frequency as is done with a network analyzer or other method of tuning. The transmitter may still not like it for the reasons stated above. As Jeff has stated, the transmitter is not a 50 ohm source but is set up to deliver power into a 50 ohm load. Its output impedance may be far from 50 ohms but it doesn't matter as that won't affect the tuning of the duplexer. As long as the transmitter can deliver power to a 50 ohm load we really don't care what its output impedance is. However what impedance is presented to the transmitter as a load is important and that depends on the cavity tuning and whatever the interconnect cables does to the impedance if the cavity is not presenting a pure 50 ohms. The cable will transform that impedance to something else. This can be on frequency reactance and/or off frequency reactance. There are several ways to correct the problem of a transmitter not liking the load that a duplexer presents. The best and most expensive is to use an isolator at the transmitter. This always presents a flat 50 ohms to the transmitter. It may not transfer the most power if there is some (on frequency) mismatch at the duplexer though. Some of the power as the result of a mismatch at the cavity will go to the load on the isolator and be lost. Then there is the small inherent loss of the isolator and low pass filter too. As a side note here; an isolator can rob you of power into the antenna if the antenna/and or duplexer have reflected power. All the reflected power presented to an isolator goes to the isolator load and is lost in heat. Without an isolator nearly all the reflected power that is present will make it back to the antenna and be radiated as it gets re-reflected when it reaches the transmitter. That is of course if the transmitter is happy working into a reactive load. The other ways to correct the problem, other than using a different transmitter that is not bothered by reactive loads as much, is to use a Z match or try different length cables that make the transmitter happier. Both of these transform the impedance/reactance presented by the duplexer to something more palatable to the transmitter and allow it to produce the power intended. There is no sin involved in using different length cables to make the system work properly. It is not a band-aid approach to mask other problems. The real problem is that some transmitters, because of the way they are designed, do not like reactive loads. Some of the duplexer manufacturers tell you about trying different cable lengths in their tech notes for these very reasons. Close spaced duplexers will be the worst with off frequency reactance as the impedance has to change quickly as you move away from the wanted frequency in order for the duplexer to do its job. 73 Gary K4FMX
[Repeater-Builder] Cable formula
Does anyone know the formula for the cable length between a repeater and the duplexer? Thanks Don VE7EDA
RE: [Repeater-Builder] Cable formula
Don, The length of that jumper cable must consider both the length of any cable between the TX output jack and the power amplifier itself, as well as the length of the coupling loop inside of the duplexer cavity. If the output of the PA and the input of the duplexer were purely resistive, the cable length would be irrelevant. However, the load impedance of most PAs will vary significantly with the drive level, and the input impedance of a duplexer cavity is always reactive. Therefore, there is no pat formula for determining the optimum length of the jumper cable. In most instances, the TX jumper cable acts as an impedance transformer of sorts, and the optimum length can be determined by a laborious cut-and-try method, or by experimenting with the addition of elbow adapters. However, a simpler approach is to install an impedance matching device, sometimes called a Z-Matcher, at the output of the PA and adjusting it for maximum forward power. If you have a vector network analyzer, you can then measure the transformation value of the jumper and Z-matcher combination, and fabricate a new jumper cable that is equivalent. This may not be cost-effective, since the round trip back to the shop probably will cost a lot more in time and fuel than a Z-matcher costs. A VNA is not something that most installers want to carry around in the service truck. If your repeater changes to a lower power level when on a backup battery, be aware that the cable length that is a good match at full power will no longer be a good match when running on the backup battery. That may not be a problem for you, but it is something to keep in mind. 73, Eric Lemmon WB6FLY -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Don Morehouse Sent: Friday, June 29, 2007 8:25 AM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Cable formula Does anyone know the formula for the cable length between a repeater and the duplexer? Thanks Don VE7EDA
Re: [Repeater-Builder] Cable formula
The length from the repeater to the duplexer isno critical (or SHOULD not be when properly tuned). Therefore, there is no 'formula' other than you want it as short as possible yet allowing enough flex/extra that it won't get stressed. Joe M. Don Morehouse wrote: Does anyone know the formula for the cable length between a repeater and the duplexer? Thanks Don VE7EDA
Re: [Repeater-Builder] Cable formula
Sheesh. isno should have been is not. Joe M. mch wrote: The length from the repeater to the duplexer isno critical (or SHOULD not be when properly tuned). Therefore, there is no 'formula' other than you want it as short as possible yet allowing enough flex/extra that it won't get stressed. Joe M. Don Morehouse wrote: Does anyone know the formula for the cable length between a repeater and the duplexer? Thanks Don VE7EDA Yahoo! Groups Links
[Repeater-Builder] Cable formula
Why would the presence or absence of +/- J affect the determination of whether or not the feedline is functioning as an impedance transformer? When the source and load impedances are different, even though purely resistive, won't the connecting cable still act as a line transformer? In a message dated 6/29/2007 11:39:44 A.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: If the output of the PA and the input of the duplexer were purely resistive, the cable length would be irrelevant. ** See what's free at http://www.aol.com. ---BeginMessage--- Why would the presence or absence of +/- J affect the determination of whether or not the feedline is functioning as an impedance transformer? When the source and load impedances are different, even though purely resistive, won't the connecting cable still act as a line transformer? In a message dated 6/29/2007 11:39:44 A.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: If the output of the PA and the input of the duplexer were purely resistive, the cable length would be irrelevant. ** See what's free at http://www.aol.com. ---End Message---
[Repeater-Builder] Cable formula not required.
Roger, Roger, Roger.Steve NU5D mch wrote: *The length from the repeater to the duplexer is not critical (or SHOULD not be when properly tuned). Therefore, there is no 'formula' other than you want it as short as possible yet allowing enough flex/extra that it won't get stressed.* Joe M. Don Morehouse wrote: Does anyone know the formula for the cable length between a repeater and the duplexer? Thanks Don VE7EDA
Re: [Repeater-Builder] Cable formula not required.
I'll third that...even though duplexers are typically reactive, if it's right, length of the cables should not be an issue. If it is, I would look at either the duplexer or the antenna/feedline. As always, put a dummy load after the duplexer and see if all is well. If it is, it's an antenna system problem. If not, it's a duplexer problem. (barring repeater internal problems...) -- Jim Barbour WD8CHL Steve S. Bosshard (NU5D) wrote: Roger, Roger, Roger.Steve NU5D mch wrote: *The length from the repeater to the duplexer is not critical (or SHOULD not be when properly tuned). Therefore, there is no 'formula' other than you want it as short as possible yet allowing enough flex/extra that it won't get stressed.* Joe M. Don Morehouse wrote: Does anyone know the formula for the cable length between a repeater and the duplexer? Thanks Don VE7EDA
Re: [Repeater-Builder] Cable formula
OK so you have a reel of cable and two connectors to make up the jumper between transmitter and duplexer. The duplexer is tuned using 50 ohm test gear and the transmitter has been optimised into a 50 ohm load. Unfortunatley the output impedance of the transmitter is not 50 ohms and a length of cable to the duplexer will transform this impedance,detuning the first can in the transmit chain. A cable length can be found that minimises this effect this length can be found more easily by measurement than by calculation. I recommend a General Radio line stretcher as your next purchase from flea market/ebay. Using the stretcher an optimum electrical length can be found and copied to jumper length using the sweeper substitution method. All of this work can be done on site using a cheap sweeper and a line stretcher. Alternativley, a ferrite isolator can be put at the can end of the run, however the original posting is not in this area and isolator losses and sag make it an unattractive option. The z matcher is another option but in my practical experience it makes the jumper radiate RF you spend all that money on RG214/RG400 double silver plated shielding and then deliberatley mismatch it? Ian Ashford G8PWE
Re: [Repeater-Builder] Cable formula
I saw that line stretcher guy, Ian. He was next to the striped tower paint salesman. 73, Steve NU5D PS - I have used an isolator in place of the TEE on the output of a duplexer. Not very practical there, though, and it sure knocks down the receive if you put it in series with the antenna, else the load gets really warm if you hook it up backwards.steve IM Ashford wrote: OK so you have a reel of cable and two connectors to make up the jumper between transmitter and duplexer. The duplexer is tuned using 50 ohm test gear and the transmitter has been optimised into a 50 ohm load. Unfortunatley the output impedance of the transmitter is not 50 ohms and a length of cable to the duplexer will transform this impedance,detuning the first can in the transmit chain. A cable length can be found that minimises this effect this length can be found more easily by measurement than by calculation. I recommend a General Radio line stretcher as your next purchase from flea market/ebay.
Re: [Repeater-Builder] Cable formula
A good quality Z-Matcher has isolation caps on the trimmer ports so I don't think the matcher itself is producing any RF radiation. I don't understand your description of the z Matcher as introducing any mismatch. The mismatch is already there as a result of some disparity between the source, load and cable impedances. All the matcher does is permit you to match the source and cable impedances. In a message dated 6/29/2007 4:40:45 P.M. Pacific Daylight Time, [EMAIL PROTECTED] writes: The z matcher is another option but in my practical experience it makes the jumper radiate RF you spend all that money on RG214/RG400 double silver plated shielding and then deliberatley mismatch it? Ian Ashford G8PWE ** See what's free at http://www.aol.com.
