> If you have long runs of coax cable hooked to a monitor
you can
> actually visualize the common mode ingress watching the
hums bars roll
> across the screen. The 60 hz bar intensity may be directly
related to
> the amount of common mode ingress.
Unfortunately this doesn't apply at all to HF receiving. It
only means something at 60 Hz, where it affects video.
On 60 Hz the cable shield is not several depths thick. The
shield isn't thick enough to isolate the inside from the
outside. Naturally any increase in shield thickness helps
when the shield is too thin, mostly because it lowers shield
resistance. Those hum bars are for a large part caused by
potential differences in the equipment connected at both
ends. This potential difference is normally caused by
voltage drop in power line neutrals.
The same problem occurs with shielded audio cables. If the
shield grounds to two pieces of gear that have connections
to power line neutrals or different local grounds it
introduces hum in the audio. I even have that problem with
headphone leads in my shack.
When we wired BC studios we would only earth or ground
shields at one end of the run, generally the input end. We
would use the heaviest shield we could get on long runs.
On MF and higher the shield is electrically thick even when
it is a single layer of foil. What we see near dc doesn't
relate at all to the frequencies we use. A very clean shield
at 60Hz might have terrible common mode at 6MHz or it might
be the other way around.
73 Tom
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