I've got a Wacom 6943-SP (yeah, I know) Broadband
Duplexer. It has four 4x4x12 inch bandpass sections on
each side. All interconnections are done internally
(i.e. no external coax jumpers between sections).
According to the label, it was originally tuned for
901-902MHz RX, 929-932MHz TX. When placed on a 900 MHz
repeater, I got excessive reflected power alarms and
the station cut back its transmit power. No real
surprise here. The TX side was operating on the lower
slope of the bandpass where there was over 3dB loss.
Using a spectrum analyzer with tracking generator, two
10dB pads, RG214 cables with crimped N-males, and a
good 50 ohm load on the third port (same setup I've
used for years to tune duplexers), I've tuned the RX
side to peak at 902.5 MHz with a passband loss of
about 1.1dB and a usable bandwidth of +/- 2 MHz. The
loss at 927.5 is 90dB. I tuned the TX side to peak at
927.5 MHz with a passband loss of about 1.0dB and a
usable bandwidth of +/- 3 MHz. The loss at 902.5 is
80dB. The bandpass ripple is just about non-existant.
On paper (and screen) this looks very good, although I
wish the TX side had about 10dB more rejection.
I then put my return loss bridge on the TX port and
found the best return loss up around 945 MHz. It took
a large amount of tweaking to get the best value down
to 927.5, and then of course the passband tuning was
all screwed up on the analyzer.
I haven't yet tried retuning it to the original center
freqs to see how the return loss is there. It may be
that it needs more than 25 MHz spread. I also haven't
tried it with a transmitter and wattmeter since
retuning it, but I suspect it won't present a good 50
ohm impedance as it sits now.
Am I doing something wrong? Anyone have suggestions
for success? Does anyone happen to have tuning
instructions or other info on this unit, or something
like it?
Bob M.
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