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" <[email protected]> To: <[email protected]> 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 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 making a living playing > blackjack... > > --- Jeff WN3A
