Hi Bob,
General tech info for contestors, I just hope it doesn't destroy the remote
station and potential cheating complaining that is so interesting and
exciting. :-)
> I redid the entire station grounding scheme after taking all equipment off
> of the bench, got behind the table and installed a grounding buss made
> from
> an unused shelf support bracket. I installed a ground from each piece of
> gear to this home brew grounding bar and took the bar to ground about 8
> feet
> from the shack to a grounding rod.
>
> The hum still persisted. I then realized I had not included the computer
> in
> the grounding scheme. After connecting the computer to the grounding bar
> the
> hum immediately went away.
Years ago, when we did broadcast studios, we all knew better than to have an
UNbalanced audio line that was grounded to chassis at both ends, in separate
pieces of gear. If you look at radios, even radio manufacturers figured that
out. They almost always float the low level audio line from chassis except
at one point along the path.
The problem is the common point to chassis should be at the cabinet entrance
in RF environments, or at least be bypassed heavily to chassis for RF at the
entrance. Otherwise, RF can get in the case on the mic lines.
Even if you look at microphones, the shield generally floats from any
potential chassis path to avoid ground loops.
The proper way to share unbalanced audio lines, if the shield cannot be
floated from ground safely at one end, is to use isolation transformers at
one end of the path, or somewhere along the path. This breaks the shield
path for low frequency currents.
The hum can be fixed sometimes by low impedance bonding, forcing the chassis
to chassis path through the binding cable to be many times less resistance
than the shield path, but that does not address the real problem. The real
problem is the chassis path on each end of the shield, which will
superimpose low frequency noise and hum laterally along the cable.
Consider if you have a rig with an external supply. You might have 20-30
peak amperes with modulation superimposed on the ground lead between the PS
and the rig. If the PS has a ground path back to the other gear, your
undecoded voice can modulate the audio line, making it sound like RF in the
audio, even though it is really a low frequency ground loop and not RF at
all.
It sounds to me like someone building equipment, or perhaps an error in the
wiring, has created some ground loops through a neglectful design. I'd
isolate grounds on cables, rather than depending on a low resistance bonding
path.
73 Tom
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