Sylvan,
You should try disconnecting as many non-rf cables as possible from your
radios. Essentially, you want to isolate the radios from all potential
common-mode return paths. I recently started to experiment with this kind of
noise reduction. I had found that noise levels didn't go down with the
introduction of a variety of filters in the rf path. But, as I disconnected DC
connections like the keyer cables, noises are reduced. This means there will be
a need for lots of ferrites on those DC and control cables. Even the power line
should be a suspect path.
Ground implementation is also going to have an impact on this kind of noise.
Changing the way your equipment grounds are routed to the station single-point
ground may have a positive effect. I recently made changes in grounding that
resulted in a couple dB of improvement.
You will probably find that improvements are realized in small increments.
There probably isn't one major source of coupling. You just have to gradually
find a lot of paths and address them one at a time.
73,
Jim
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