> 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
