Peter,
You really should:
1.) Raise and lower transmitter power by known dB amounts
and look at the signal level change in dB
2.) Add an external attenuator to the receiver (or analyzer)
and note the level change in dB.
3.) Observe the mixing signal level closely over time and
look for sudden drastic jumps or major changes in level.
Doing those simple tests, assuming you have calibrated the S
meter so you can see level change, will speak volumes about
where the problem is.
As in many Amps threads this has digressed to VHF repeaters,
which are very sensitive to noise from small connection
problems. For example climbing past a repeater antenna with
a pocket of keys or even loose coins can "noise up" a
repeater.
In HF systems you are much less likely to have connection
problems that appear remain stable over time because the
"antenna" or conductor area involved in the rectification
area is larger and current are often much higher. Also there
is no hard limiter at work in the receiver with FM creating
an illusion the level stays the same when it really varies a
lot. Almost all connection problems come and go or
drastically change level quite frequently, and it is very
obvious when they occur at HF when the system has an S meter
instead of a limiter.
In systems like you have, where you are duplexing with high
power HF transmitters and receivers, the single most common
problem is either receiver overload and mixing in the
receiving system or a bad connection in the receiving
system. A bad connection is almost always plainly
identifiable because the level will jump and the problem
will be intermittent.
You'll waste a lot of time and money if you don't go through
a logical series of tests that will only take an hour or
less.
73 Tom
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