Tim,
I suspect that your amplifier failure came about as a result of the
duplexer being detuned. Even the best duplexers can gradually drift due
to a phenomenon called "rod pumping." This is far more pronounced at
sites where the duplexers are not in a temperature-controlled
environment. But, even in an air conditioned building, normal operation
of the duplexer causes significant heating. A typical 4-cavity duplexer
might have a 1.5 dB insertion loss, which means that a 100 watt
transmitter will lose around 31 watts in the duplexer. That amount of
heat in a high duty cycle repeater can make the cavities too hot to
touch. Despite the presence of Invar rods, the continuous cycling of
temperature will, over time, cause the duplexer to creep out of tune.
Let's not forget that the notch moves with the bandpass in a "BpBr"
duplexer, so the extremely important notch in the TX side that protects
the receiver from transmitter sideband noise is slowly becoming less
effective over time, causing desense to appear. Because the system
deterioration happens so slowly, it seldom rears its ugly head until
something breaks or the users complain about poor coverage.
The PA failure might have been coincidental, but I would give the
duplexer, isolator, feedline, and antenna a good going-over. I believe
that this inspection should happen on a regular basis, at least every
two years.
Tim Shephard wrote:
>
...Lately we have been having some trouble which has been nicely termed
the whale noise. ...Yesterday I checked everything and found I was
getting some desense, around 8 dB... I looked at the duplexer and it
looks like it has not been touched in around 25 years. The shafts are
rusty, cables look very fragile... What do you guys think?
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