> I never tried performing any 'intermodulation' tests to
measure
> the 'dynamic range' of that old coax; your post brought
this
> little observation back to mind and just what problems
like
> ohmic, rusted, oxidized copper conductors can create.
Repeaters are an entirely different case. In the case of a
repeater duplexing from one antenna, you have all of the
transmitter power conveyed on the system that is also
receiving at noise floor, and noise floor at VHF is normally
very low compared to HF when the bands are open.
When my business maintained multiple repeaters in large
communications systems, we would even have problems from
loose hardware on or near towers causing noise like you
describe. Any less than perfectly solid connection would
"noise up" a repeater. Virtually all of the problems were
connectors or other pressure contact points such as guy line
to tower contact areas. We always used standard braided
cables like RG-8 in various parts of systems, but never had
a problem like that that I'm aware of. While I've never
seen the effect you describe, I'm certain it can happen if
the shield is tarnished or if the lays are not compacted to
make good connections. I know that climbing past a repeater
antenna with metal tools or even car keys rubbing will noise
up a repeater while it is transmitting.
This has little to nothing to do with Ray's problem, which
apparently is that someplace in his system he has severe
common mode ingress into the cables.
I have a fairly complex system in a very quiet rural
location and have had some fairly noisy switching supplies
and lights in my shack without undue problems. Any RF
ingress is virtually never "ground loops". It is some other
flaw such as poor antenna feedline or feedpoint construction
or engineering or bad connections at RF connectors that do
the deed. I don't use any special cabling. I do make good
shield connections.
73 Tom
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