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: [email protected] [mailto:[email protected]] On Behalf Of allan crites Sent: Friday, August 13, 2010 12:56 PM To: [email protected] 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 <[email protected]> To: [email protected] 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 [email protected] <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 >
