OK, I think, for the most part, we're on the same page. I'm cuttin' and
trimmin' a lot here...
> And this is where I believe the duplexer manufacturers are
> covering their butt. They don't want the problem with
> complex reactance presented by the duplexer to be their
> problem. Not that I don't agree, because it's usually the
> transmitter that is really at fault.
I think that last sentence speaks volumes on the matter.
> Joe Ham buys a new duplexer and hooks it up to his 110 Watt
> MASTR II repeater and gets 50 watts out the antenna port. He
> does his homework and realizes that he should only be loosing
> 29% with the 1.5 dB of insertion loss stated in the paperwork
> - but he's loosing over 50%.
Ah, but is he really *losing* 50 percent in the duplexer, or is transmitter
not making the full 110 watts output to start with? Maybe his transmitter
is really only delivering 70 watts to the duplexer. Is it an issue of the
duplexer's loss being high, or is the problem the transmitter's not making
power? Seems to me it's really the latter.
> The duplexer manufacturer
> supposedly engineered and tuned it for a 50 Ohm system.
Well, kinda. Many duplexers are spec'ed for 1.5:1 (14 dB RL) input VSWR
max. Fortunately, I rarely see any that are that bad. I'll gladly trade
off a tenth of a dB of insertion loss for several (if not 10 or more) dB of
return loss improvement when I'm tuning on the VNA, but some hams are greedy
and don't think along those lines when they're tuning...
> He
> knows that the cable he connected to the transmitter is good,
> because when he disconnects the end going to the transmitter
> port of the duplexer and connects it to his Bird 43
> terminated with a good load - it reads 110 watts.
Yes, but did he have a second Bird between the Tx and the duplexer when he
was measuring power output? That would have told the real story.
> Now, is the transmitter becoming spurious
Now all bets are off.
> and the cable
> length being changed in length satisfies the match between
> the duplexer and transmitter - I don't know... All I can
> tell you is I have followed the suggestions written in the
> WACOM manual and it has worked. I had one instance of a ham
> radio club loosing PA's left and right on their 2M machine.
> They told me of the situation and I offered to do a little
> testing. The 110 watt PA would put out 110 watts into a Bird
> and dummy, but only 45 watts was coming out the antenna port
> of the duplexer. At the time I didn't own a spectrum
> analyzer. The repeater wouldn't duplex without desense. I
> changed the length of the line between the PA and duplexer
> until I got the power to read about 75 Watts as I remember.
> That was 13 years and they still have the same PA - no desense either.
Out of morbid curiosity, what kind of PA was it?
> You are changing the VSWR when tuning the cavity closest to
> the transmitter.
Yes, but once you've adjusted that cavity, from that point on, changing the
cable length doesn't vary the VSWR. That was my point - changing the cable
length doesn't change VSWR.
> I realize that impedance transformation
> cannot occur when you have a 50 Ohm cable (of any length) and
> a perfect 50 Ohm load - but I think you will agree that a
> duplexer doesn't, in any way shape or form, present a nice 50
> Ohm load.
Well, it can get pretty damn close. I can send you some VNA plots of
duplexers with input Z's well in excess of 30 dB return loss, some
approaching the limits of my test equipment. Of course, when hooked up to
an antenna instead of being terminated in a precision load, all bets are
off, but hey, that's not the fault of the duplexer...
> Some transmitters just cannot deal with it without
> some form of matching after the fact - like a Z-Matcher,
> Isolator, Circulator, or even a critical cable length.
I don't like those transmitters :-)
> GE MASTR II 110 watt 150.8 to 174 MHz PA and WACOM WP-641.
Thinking...thinking...no, haven't done that one.
> Motorola MICOR 150.8 to 162 MHz PA and WACOM WP-641.
Yes, have done that combo, several times that I can think of. Actually, one
of the repeaters was low-split from the factory (out of Canada) now that I
think about it, so that doesn't count, the others were all H split with no
PA mods. Didn't do anything special with cable lengths.
> Hamtronics 45 Watt 2M PA and Sinclair Q-202.
Haven't done any Hamtronics.
> Well, I cannot believe that I'm the only person on this list
> that has had success with optimizing the length of cable
> between the duplexer and transmitter/PA.
I don't doubt that others have seen positive (or negative) effects from
varying cable lengths - I just said I've never had to resort to doing it,
using the equipment that I've used, with the equipment tuned the way I've
tuned it.
> I'll get us some tickets for Vegas - Jeff.
I think ZZU has the right idea. He's down in MX-land right now, probably
sitting on a beach laughing at us working stiffs. The only things are on
his mind are hot senoritas and cold margaritas.
--- Jeff WN3A
