Re: [Repeater-Builder] Re: Coax length, etc.
There is no way that impedance matching can be accomplished with changing the length of the transmission line in a 50 ohm system when the transmission line used is also of the same characteristic impedance without a compensating shunt XC or XL stub, at a location determined on a Smith Chart plot. See Electronic Applications of the Smith Chart in Waveguide, Circuit, and Component Analysis by Phillip H. Smith, Chapter 9, Waveguide and Impedance Matching, P. 97. Allan Crites wa9zzu --- On Wed, 8/18/10, n5qs ygr...@white-tiger.org wrote: From: n5qs ygr...@white-tiger.org Subject: [Repeater-Builder] Re: Coax length, etc. To: Repeater-Builder@yahoogroups.com Date: Wednesday, August 18, 2010, 9:04 PM This is taken from a Wacom tuning guide please excuse any typos Roger CABLE LENGTH BETWEEN TRANSMITTER AND DUPLEXER The length of the coaxial cable between the transmitter and the duplexer might be a critical length with some transmitters because of an impedance mismatch. A mismatch condition can exist between a transmitter, cavity filter/duplexer, and/or antenna since all of these components can vary from an absolute impedance of 50 Ohms. As an example, a transmitter, or duplexer, or antenna will probably be listed as having a nominal impedance of 50 Ohms and a VSWR (referenced to 50 Ohms) of 1.3 to 1 or 1.5 to 1. At a VSWR of 1.5 to 1, the antenna , or duplexer, or antenna could have an impedance of anything from 33 to 75 Ohms and still be within specifications. (At 1.3 to 1 VSWR, the impedance can be anything from 38 to 65 Ohms). In a worst-case-scenario, the transmitter might have an actual impedance of 33 Ohms, the duplexer 75 Ohms and the antenna 33 Ohms. As such, system performance will be degraded and yet, all components will individually meet manufactures specifications. This impedance mismatch problem can usually be resolved by optimizing the length of coax cable between transmitter and duplexer and/or by installing one of the impedance-matcher PI Networks available through Wacom and other sources. If a transmitter is overly sensitive to a mismatched load impedance, a variety of symptoms might appear, including one or more of the following: 1) The transmitter might generate numerous spurious radiations. 2) The transmitter output power might become erratic, either too high or too low as measured on a wattmeter. 3) The insertion loss of the duplexer might measure normal with sweep equipment but measure too high or too low on a wattmeter when connected to the transmitter. 4) The reflected power might change when the length of cable between transmitter and first cavity is changed. 5) Use of a ferrite isolator at output of transmitter solves the forward power and reflected power problem, and eliminates change in reflected power when length of cable between the isolator and the first cavity is changed. (In this instance, the ferrite isolator can be used to reveal the symptoms or as a solution to the problem.) The above problems might be reduced or minimized by optimizing the length of cable between the transmitter and first cavity of the duplexer. The optimum length of cable can be found by following procedure: 1) Tune the transmitter into a 50 Ohm dummy load according to the manufacturers instruction. 2) Connect the duplexer to the transmitter. The transmitter output signal should feed through the wattmeter, then through the duplexer then into a dummy load. If there is an impedance mismatch the duplexer will de-tune the transmitter and the cable length should be optimized. 3) Using short lengths of cable (no longer than 1 @ 900 MHz, 2 @ 460 MHz, 6 @ 160 MHz, 24 at 40 MHz) or right angle elbow connectors, gradually increase the length of the cable between the transmitter and duplexer until the optimum length (no de-tuning effect) is found. Up to four of these short cables should be tried. 4) When the proper cable length is determined, i.e., when the system is operating as it should, replace all the short and long lengths of cables, right angle elbow connectors and wattmeter with one continuous length of cable of equivalent electrical length. Do not overlook the fact that the wattmeter was part of the cable length between the transmitter and duplexer during this process. If the wattmeter is removed from the circuit, the cable length without the wattmeter must be lengthened by some amount in order to be the same equivalent electrical length as the cable with the wattmeter in the circuit.
RE: [Repeater-Builder] Re: Coax length, etc. GE Z-Matcher
HOLY CRAP I've been wondering that for some time. Guess the manual isn't always right. and maybe that's why the GE Z-match is there for the recruiting of true GE tuners.. Never-mind keeping your PA happy at 100% DC. I always wanted to ask but never did (for thought of flames from the GE manual) because I thought I heard somewhere that tuning procedure wasn't ideal for most cavity/duplexer applications (on this list in a unrelated post) so investigated. Now I always settled for the best low ratio of the two. PA current/TP1 from z-match. You'll see that the relationship of the two is not even close to linear. What I've found is you basically want to try to be on the edge of both curves knee's.. I think I have the right idea, for a 50mV gain from the TP1 z-match null I could drop PA current by 2500mA. And this was until now (a month ago) that I finally have a good HP8924 SM to help take the guess work out. Plus your PA and TX cavity temps go way down. Just glad to hear I was probably doing the right thing. Also I always did the final 1st 2nd pass cavity tuning this way. Retuning cavities for highest output power with least current vs. low TP1 on z-match. One tuning session on a friends machine with this method dropped TX cavity temps by more then 10deg to almost ambient temp while dropping PA temp, and current by 2 amps with no drop in output power or RX sense. This probably means the match in the cavity coupling or cabling was off but hey that PA will live longer. If I'm wrong or missing something here, let me know. 73's groupies Ross www.kc7rjk.net http://www.kc7rjk.net/ -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of larynl2 Sent: Saturday, August 14, 2010 8:17 AM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Re: Coax length, etc. GE Z-Matcher --- In Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com , Jeff DePolo j...@... wrote: c) Although not explictly described in GE's tuning procedures, significant improvement in efficiency can be obtained with proper tuning of the Z-matcher. Tuning for 50+j0 at the input to the Z-matcher is NOT necessarily the RIGHT match! Right, GE's instructions on tuning the Z-Matcher in their base stations are not correct. Instructions should instead describe a procedure that reduces current draw of the amplifier while simultaneously maintaining or increasing the output power. d) To charge more. I'm half-joking on this; I can't say I've statistically seen more or less failures on M2 PA's with or without the Z-matcher, so I'll give this answer half a smiley: .-, If the failures you've seen are in amps with Z-Matchers that were tuned following GE's Z-Matcher instructions, that *could* explain why you've given half a smiley... :-) Laryn K8TVZ
Re: [Repeater-Builder] Re: Coax length, etc.
Sid, I think I found your formula. Look on page 62 of: http://www.repeater-builder.com/antenna/pdf/ve2azx-duplexerinfo.pdf BTW, my guess was wrong. Length is expressed in inches. 73, Russ WB8ZCC On 8/13/2010 1:44 PM, Russ Hines wrote: Hmm, the formula is a bit off, but... 30 x 32.785 = 983.55. I'll also bet length is expressed in feet. Looks eerily like someone wants you to cut a one-wavelength piece of coax cut at the mean repeater frequency. Just a guess. 73, Russ WB8ZCC On 8/13/2010 11:38 AM, Sid wrote: I have a note in my file that I do not recall where it came from relative to cable length between the duplexer and the TX or between the duplexer and additional filter. Length = (30)(32.785)(vf/freq). 30 is for 30 degrees, vf is velocity factor, freq is the average of the pass and reject frequencies. If too short add 180 degrees. Don't know if this is good info or not. The article would be appreciated. Sid. --- In Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com, Nate Duehr n...@... wrote: On Aug 5, 2010, at 11:20 AM, Kevin Custer wrote: 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. There's already a great book on that topic, it's called the ARRL Antenna Handbook, and the chapter on transmission lines covers it in more detail than anyone will ever need to know in the real-world, who's not a practicing RF Engineer. That book if read cover-to-cover, is also damn good for insomnia. Or at least it'll keep you distracted while you can't sleep! :-) -- Nate Duehr n...@... facebook.com/denverpilot twitter.com/denverpilot
Re: [Repeater-Builder] Re: Coax length, etc.
Nate, I have both the 12th and 14th edition of the ARRL Antenna books, the 12th I acquired in 1974 and have read and re-read the section on transmission lines and impedance matching probabily more than anyone else has. I sometimes learn new things with each re-reading, as there is much to be learned. In my discussions with Kevin Custer about the length of the transmission line connecting the xmtr output and the input to the duplexer, he suggested and I accepted, to colaborate on an article explaining the problems associated with matching the output impedance of a solid state transmitter of somewhat different than the normal 50 Ohms, and the attempts made by a manufacturer of duplexers to adapt (read match) the xmtr output via certain lengths of transmission line and readjustment of the tuning of the cavity closest to the xmtr output to effect this matching, ignoring the possible degradation resulting to the pass and notch characteristics. The transmitter in our discussions was the HB GE Mastr 2 which, in the information available to me, appears to be having an output source impedance of 35+ or - (some unknown) reactance Ohms. Kevin commented that it appears that many hams are unaware of, or understand the methods needed, to do an appropriate job of impedance matching. Therefor we will be making this article for the benefit of those who don't understand the impedance matching necessary for optimum power transfer with a simple to understand way of impedance matching without the use of the infamous Smith Chart (which I have utilized for the past 50 yrs in all my impedance matching solutions and cannot be without). I agree that much information for impedance matching is contained in the ARRL Antenna Book but in my experience, real life adaptation of this information is and can be difficult to many hams. There is also another book I rely on and recommend, which is Electronic Applications of the Smith Chart by Philip Smith. Now, if you would like to contribute to our efforts I would gladly accept your contributions. Thanks for your input. Allan Crites wa9zzu From: Sid purvis...@yahoo.com To: Repeater-Builder@yahoogroups.com Sent: Fri, August 13, 2010 10:38:25 AM Subject: [Repeater-Builder] Re: Coax length, etc. I have a note in my file that I do not recall where it came from relative to cable length between the duplexer and the TX or between the duplexer and additional filter. Length = (30)(32.785)(vf/freq). 30 is for 30 degrees, vf is velocity factor, freq is the average of the pass and reject frequencies. If too short add 180 degrees. Don't know if this is good info or not. The article would be appreciated. Sid. --- In Repeater-Builder@yahoogroups.com, Nate Duehr n...@... wrote: On Aug 5, 2010, at 11:20 AM, Kevin Custer wrote: 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. There's already a great book on that topic, it's called the ARRL Antenna Handbook, and the chapter on transmission lines covers it in more detail than anyone will ever need to know in the real-world, who's not a practicing RF Engineer. That book if read cover-to-cover, is also damn good for insomnia. Or at least it'll keep you distracted while you can't sleep! :-) -- Nate Duehr n...@... facebook.com/denverpilot twitter.com/denverpilot
Re: [Repeater-Builder] Re: Coax length, etc.
Hmm, the formula is a bit off, but... 30 x 32.785 = 983.55. I'll also bet length is expressed in feet. Looks eerily like someone wants you to cut a one-wavelength piece of coax cut at the mean repeater frequency. Just a guess. 73, Russ WB8ZCC On 8/13/2010 11:38 AM, Sid wrote: I have a note in my file that I do not recall where it came from relative to cable length between the duplexer and the TX or between the duplexer and additional filter. Length = (30)(32.785)(vf/freq). 30 is for 30 degrees, vf is velocity factor, freq is the average of the pass and reject frequencies. If too short add 180 degrees. Don't know if this is good info or not. The article would be appreciated. Sid. --- In Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com, Nate Duehr n...@... wrote: On Aug 5, 2010, at 11:20 AM, Kevin Custer wrote: 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. There's already a great book on that topic, it's called the ARRL Antenna Handbook, and the chapter on transmission lines covers it in more detail than anyone will ever need to know in the real-world, who's not a practicing RF Engineer. That book if read cover-to-cover, is also damn good for insomnia. Or at least it'll keep you distracted while you can't sleep! :-) -- Nate Duehr n...@... facebook.com/denverpilot twitter.com/denverpilot
RE: [Repeater-Builder] Re: Coax length, etc.
Hi Allan, Do we really care what the output impedance of the transmitter is? Most transmitters do not present a pure 50 ohm output but are tuned to transfer maximum power into a 50 ohm load. This often comes out to something way different than a 50 ohm source impedance. As the source impedance does not affect SWR the system doesn't care what it is as long as the transmitter can transfer maximum power into 50 ohms. What the transmitter does sometimes care about is the reflected impedance from the first cavity (being hi Q) that is not on frequency. This presents a highly reactive load to the transmitter that can sometimes cause the transmitter to overheat or reduce output. Placing a cable of a different length between output and the first cavity can sometimes change the unwanted off frequency load seen by the transmitter. 73 Gary K4FMX _ From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of allan crites Sent: Friday, August 13, 2010 12:56 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Re: Coax length, etc. Nate, I have both the 12th and 14th edition of the ARRL Antenna books, the 12th I acquired in 1974 and have read and re-read the section on transmission lines and impedance matching probabily more than anyone else has. I sometimes learn new things with each re-reading, as there is much to be learned. In my discussions with Kevin Custer about the length of the transmission line connecting the xmtr output and the input to the duplexer, he suggested and I accepted, to colaborate on an article explaining the problems associated with matching the output impedance of a solid state transmitter of somewhat different than the normal 50 Ohms, and the attempts made by a manufacturer of duplexers to adapt (read match) the xmtr output via certain lengths of transmission line and readjustment of the tuning of the cavity closest to the xmtr output to effect this matching, ignoring the possible degradation resulting to the pass and notch characteristics. The transmitter in our discussions was the HB GE Mastr 2 which, in the information available to me, appears to be having an output source impedance of 35+ or - (some unknown) reactance Ohms. Kevin commented that it appears that many hams are unaware of, or understand the methods needed, to do an appropriate job of impedance matching. Therefor we will be making this article for the benefit of those who don't understand the impedance matching necessary for optimum power transfer with a simple to understand way of impedance matching without the use of the infamous Smith Chart (which I have utilized for the past 50 yrs in all my impedance matching solutions and cannot be without). I agree that much information for impedance matching is contained in the ARRL Antenna Book but in my experience, real life adaptation of this information is and can be difficult to many hams. There is also another book I rely on and recommend, which is Electronic Applications of the Smith Chart by Philip Smith. Now, if you would like to contribute to our efforts I would gladly accept your contributions. Thanks for your input. Allan Crites wa9zzu _ From: Sid purvis...@yahoo.com To: Repeater-Builder@yahoogroups.com Sent: Fri, August 13, 2010 10:38:25 AM Subject: [Repeater-Builder] Re: Coax length, etc. I have a note in my file that I do not recall where it came from relative to cable length between the duplexer and the TX or between the duplexer and additional filter. Length = (30)(32.785)(vf/freq). 30 is for 30 degrees, vf is velocity factor, freq is the average of the pass and reject frequencies. If too short add 180 degrees. Don't know if this is good info or not. The article would be appreciated. Sid. --- In Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com , Nate Duehr n...@... wrote: On Aug 5, 2010, at 11:20 AM, Kevin Custer wrote: 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. There's already a great book on that topic, it's called the ARRL Antenna Handbook, and the chapter on transmission lines covers it in more detail than anyone will ever need to know in the real-world, who's not a practicing RF Engineer. That book if read cover-to-cover, is also damn good for insomnia. Or at least it'll keep you distracted while you can't sleep! :-) -- Nate Duehr n...@... facebook.com/denverpilot twitter.com/denverpilot
Re: [Repeater-Builder] Re: Coax length, etc.
Gary, Perhaps you can give us some examples to illustrate your thoughts. Perhaps you can also explain why GE chose to include a pi network on the output of the HB M-2 base xmtr to match the xmtr output to 50 Ohms, the shunt capacitor values and the series inductor value used. I'm interested to hear your explaination on how you would determine the length of cable needed. AC From: Gary Schafer gascha...@comcast.net To: Repeater-Builder@yahoogroups.com Sent: Fri, August 13, 2010 2:36:23 PM Subject: RE: [Repeater-Builder] Re: Coax length, etc. Hi Allan, Do we really care what the output impedance of the transmitter is? Most transmitters do not present a pure 50 ohm output but are tuned to transfer maximum power into a 50 ohm load. This often comes out to something way different than a 50 ohm source impedance. As the source impedance does not affect SWR the system doesn’t care what it is as long as the transmitter can transfer maximum power into 50 ohms. What the transmitter does sometimes care about is the reflected impedance from the first cavity (being hi Q) that is not on frequency. This presents a highly reactive load to the transmitter that can sometimes cause the transmitter to overheat or reduce output. Placing a cable of a different length between output and the first cavity can sometimes change the unwanted off frequency load seen by the transmitter. 73 Gary K4FMX From:Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of allan crites Sent: Friday, August 13, 2010 12:56 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Re: Coax length, etc. Nate, I have both the 12th and 14th edition of the ARRL Antenna books, the 12th I acquired in 1974 and have read and re-read the section on transmission lines and impedance matching probabily more than anyone else has. I sometimes learn new things with each re-reading, as there is much to be learned. In my discussions with Kevin Custer about the length of the transmission line connecting the xmtr output and the input to the duplexer, he suggested and I accepted, to colaborate on an article explaining the problems associated with matching the output impedance of a solid state transmitter of somewhat different than the normal 50 Ohms, and the attempts made by a manufacturer of duplexers to adapt (read match) the xmtr output via certain lengths of transmission line and readjustment of the tuning of the cavity closest to the xmtr output to effect this matching, ignoring the possible degradation resulting to the pass and notch characteristics. The transmitter in our discussions was the HB GE Mastr 2 which, in the information available to me, appears to be having an output source impedance of 35+ or - (some unknown) reactance Ohms. Kevin commented that it appears that many hams are unaware of, or understand the methods needed, to do an appropriate job of impedance matching. Therefor we will be making this article for the benefit of those who don't understand the impedance matching necessary for optimum power transfer with a simple to understand way of impedance matching without the use of the infamous Smith Chart (which I have utilized for the past 50 yrs in all my impedance matching solutions and cannot be without). I agree that much information for impedance matching is contained in the ARRL Antenna Book but in my experience, real life adaptation of this information is and can be difficult to many hams. There is also another book I rely on and recommend, which is Electronic Applications of the Smith Chart by Philip Smith. Now, if you would like to contribute to our efforts I would gladly accept your contributions. Thanks for your input. Allan Crites wa9zzu From:Sid purvis...@yahoo.com To: Repeater-Builder@yahoogroups.com Sent: Fri, August 13, 2010 10:38:25 AM Subject: [Repeater-Builder] Re: Coax length, etc. I have a note in my file that I do not recall where it came from relative to cable length between the duplexer and the TX or between the duplexer and additional filter. Length = (30)(32.785)(vf/freq). 30 is for 30 degrees, vf is velocity factor, freq is the average of the pass and reject frequencies. If too short add 180 degrees. Don't know if this is good info or not. The article would be appreciated. Sid. --- In Repeater-Builder@yahoogroups.com, Nate Duehr n...@... wrote: On Aug 5, 2010, at 11:20 AM, Kevin Custer wrote: 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
RE: [Repeater-Builder] Re: Coax length, etc.
I'm going to take a stab at this, at the risk of possibly stepping on Gary's toes. 1. RF amplifiers in general (not only solid state) don't *have* a 50 ohm source impedance, they're (nominally) designed to work *into* a 50 ohm load. The difference is subtle, but significant. Transmitters aren't classic generators. 2. GE offered the matching network on station PA's for a number of reasons, among them: a) Amplifier circuit designs (solid-state or otherwise) have a finite bandwidth; a tuning network allows for some output matching adjustment b) Ideally the transmitter will be looking into a nice 50+j0 load (assuming that's what it was designed for), but the world isn't perfect, hence the adjustable output matching network to correct for *minor* load mis-match (strong emphasis on minor) c) Although not explictly described in GE's tuning procedures, significant improvement in efficiency can be obtained with proper tuning of the Z-matcher. Tuning for 50+j0 at the input to the Z-matcher is NOT necessarily the RIGHT match! d) To charge more. I'm half-joking on this; I can't say I've statistically seen more or less failures on M2 PA's with or without the Z-matcher, so I'll give this answer half a smiley: .-, 3. As far as Gary's comment about off-channel Z and its effect on transmitters, some sub-par (or damaged) PA's will go spurious when looking into a load that presents a bad match off-channel, even if it presents a nice flat load on-channel. Some manufacturers suggest playing with cable lengths to tame misbehaving PA's. Again, this is a shortcoming in the PA, and I, for one, am not into band-aid fixes for design flaws or defective equipment; I fix (or replace) the PA. When I walk off the site, I want to KNOW the PA is going to be stable in the future as the load changes, because it WILL change... As far as optimium power transfer, anyone that has passed their tech test probably already knows the textbook answer to that question (the maximum power theorem). But that's not really the issue here, is it OM? Again, we have to accept the fact that amplifiers aren't classic generators; we can't just look at the problem from the perspective of power transfer into a 50 ohm load. We have to look at the devices being used in the PA, the networks doing the impedance transformations, the behavior of the amplifier as a whole (including all cascaded gain stages), its behavior as voltages and temperature are varied, and, one of the most important parameters, efficiency. Just to back up a step, let's revisit the textbook answer of optimum power transfer, which again, is based on a classis generator. In such a case, the optimum power transfer is the *maximum* power that is received by the load. Well, in our little RF corner of the power transfer world, it's not that simple. We're not out eek the last watt out of our amplifier - that's not the goal (or at least it shouldn't be). We all know we can sometimes squeeze a fraction of a dB more out of an amplifier by purposefully mis-loading it, but is that a good thing? Does that make it an optimum match? Hell no. Among other things, we need to look at *efficiency*, and plotting that against power output if we want to find the sweet spot. Efficiency is a primary performance metric for RFPA matching, especially when it comes to continuous-duty solid-state RFPA's where heat is your worst enemy. As far as SS VHF/UHF amplifiers go, good RFPA design should dictate that you have adequate hardware headroom such that you're not stressing the devices or any support components to make rated output, so maximum power transfer should be the least of the worries for the tech tuning the equipment. Stability and spectral purity should be a given in a properly-designed RFPA. So the only parameter that should need to be monitored during fine-adjustment at the output is really efficiency/current draw if everything else was done right from the get-go. --- Jeff WN3A -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of allan crites Sent: Friday, August 13, 2010 5:41 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Re: Coax length, etc. Gary, Perhaps you can give us some examples to illustrate your thoughts. Perhaps you can also explain why GE chose to include a pi network on the output of the HB M-2 base xmtr to match the xmtr output to 50 Ohms, the shunt capacitor values and the series inductor value used. I'm interested to hear your explaination on how you would determine the length of cable needed. AC From: Gary Schafer gascha...@comcast.net To: Repeater-Builder@yahoogroups.com Sent: Fri, August 13, 2010 2:36:23 PM Subject: RE: [Repeater-Builder] Re: Coax length, etc. Hi Allan, Do we really care what the output impedance of the transmitter is? Most
Re: [Repeater-Builder] Re: Coax length, etc.
Allan, Wow... sounds like a great article you guys are working on! Now that I see the scope of what you're up to, it sounds like a great addition to the website. (Well, it would have been a great addition anyway... but wow!) You give me FAR too much credit, if you think I could add to it. My knowledge is barely able to keep up with most folks 'round here! I just mentioned the Antenna Book, 'cause I ended up with a copy one year (only thing I've ever won at a hamfest) and should have spent the $70 (at the time) to have bought a copy YEARS before I won that one. It really is an excellent book, well worth the $! -- Nate Duehr n...@natetech.com
Re: [Repeater-Builder] Re: Coax length, etc.
On Aug 13, 2010, at 7:22 PM, Jeff DePolo wrote: d) To charge more. I'm half-joking on this; I can't say I've statistically seen more or less failures on M2 PA's with or without the Z-matcher, so I'll give this answer half a smiley: .-, Lucky. I have. Learned that lesson... ;-) Got a pile of dead VHF MII PA's in the garage to prove it, too. :-) One particular site with a shared TX antenna system... hybrids even, so the darn thing has isolators sprouting from every orifice... Something about it just never sat well with the VHF MASTR II PAs. Z-matcher installed, running many years now... -- Nate Duehr n...@natetech.com
Re: [Repeater-Builder] Re: Coax length, etc.
Jeff, My internet service was interrupted again tonight for 2 hrs as well as last night and I just got off the phone with the tech in MX city for 35 min till he restored the service. He wants to get me a new modem to solve my problem but can't deliver it until Tuesday at which time I will be back in the US. So I respectfully decline to respond until then as there is no guarantee my service will continue uninterrupted. AC From: Jeff DePolo j...@broadsci.com To: Repeater-Builder@yahoogroups.com Sent: Fri, August 13, 2010 8:22:23 PM Subject: RE: [Repeater-Builder] Re: Coax length, etc. I'm going to take a stab at this, at the risk of possibly stepping on Gary's toes. 1. RF amplifiers in general (not only solid state) don't *have* a 50 ohm source impedance, they're (nominally) designed to work *into* a 50 ohm load. The difference is subtle, but significant. Transmitters aren't classic generators. 2. GE offered the matching network on station PA's for a number of reasons, among them: a) Amplifier circuit designs (solid-state or otherwise) have a finite bandwidth; a tuning network allows for some output matching adjustment b) Ideally the transmitter will be looking into a nice 50+j0 load (assuming that's what it was designed for), but the world isn't perfect, hence the adjustable output matching network to correct for *minor* load mis-match (strong emphasis on minor) c) Although not explictly described in GE's tuning procedures, significant improvement in efficiency can be obtained with proper tuning of the Z-matcher. Tuning for 50+j0 at the input to the Z-matcher is NOT necessarily the RIGHT match! d) To charge more. I'm half-joking on this; I can't say I've statistically seen more or less failures on M2 PA's with or without the Z-matcher, so I'll give this answer half a smiley: .-, 3. As far as Gary's comment about off-channel Z and its effect on transmitters, some sub-par (or damaged) PA's will go spurious when looking into a load that presents a bad match off-channel, even if it presents a nice flat load on-channel. Some manufacturers suggest playing with cable lengths to tame misbehaving PA's. Again, this is a shortcoming in the PA, and I, for one, am not into band-aid fixes for design flaws or defective equipment; I fix (or replace) the PA. When I walk off the site, I want to KNOW the PA is going to be stable in the future as the load changes, because it WILL change... As far as optimium power transfer, anyone that has passed their tech test probably already knows the textbook answer to that question (the maximum power theorem). But that's not really the issue here, is it OM? Again, we have to accept the fact that amplifiers aren't classic generators; we can't just look at the problem from the perspective of power transfer into a 50 ohm load. We have to look at the devices being used in the PA, the networks doing the impedance transformations, the behavior of the amplifier as a whole (including all cascaded gain stages), its behavior as voltages and temperature are varied, and, one of the most important parameters, efficiency. Just to back up a step, let's revisit the textbook answer of optimum power transfer, which again, is based on a classis generator. In such a case, the optimum power transfer is the *maximum* power that is received by the load. Well, in our little RF corner of the power transfer world, it's not that simple. We're not out eek the last watt out of our amplifier - that's not the goal (or at least it shouldn't be). We all know we can sometimes squeeze a fraction of a dB more out of an amplifier by purposefully mis-loading it, but is that a good thing? Does that make it an optimum match? Hell no. Among other things, we need to look at *efficiency*, and plotting that against power output if we want to find the sweet spot. Efficiency is a primary performance metric for RFPA matching, especially when it comes to continuous-duty solid-state RFPA's where heat is your worst enemy. As far as SS VHF/UHF amplifiers go, good RFPA design should dictate that you have adequate hardware headroom such that you're not stressing the devices or any support components to make rated output, so maximum power transfer should be the least of the worries for the tech tuning the equipment. Stability and spectral purity should be a given in a properly-designed RFPA. So the only parameter that should need to be monitored during fine-adjustment at the output is really efficiency/current draw if everything else was done right from the get-go. --- Jeff WN3A -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of allan crites Sent: Friday, August 13, 2010 5:41 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Re: Coax length, etc. Gary, Perhaps you can give us some examples to illustrate your thoughts. Perhaps you can also explain why GE chose to include a pi
Re: [Repeater-Builder] Re: Coax length, etc.
motarolla_doctor wrote: Kevin, I am using your coaxial matching section on a couple of antennas with good results. Great article on RB and not too hard to build MD, I wish I could take the credit, but the original concept was from a friend W8ZD - I just improved upon his concept. For those interested in what we are referring to, or, have availability of large (free) CATV hardline and want to use it in 50 Ohm service, go here: http://www.repeater-builder.com/projects/matchingstubs.html Thanks, Kevin
