Referring to W8JIT's suggestion to connect the coax shield to the power line
neutral: as a working journeyman electrician, I was alarmed to see this
suggestion. DO NOT DO THIS. IT IS UNSAFE. Here's why: If a load is on
the line downstream, and the neutral conductor fails (opens) between your
connection and the breaker panel, then your coax shield will be at 120
volts, minus the drop across the load. But, if the neutral has opened, there
will be no line current through the load and therefore its voltage drop will
be zero; and your coax shield will be at 120 volts to ground! And don't
count on the load resistance to drop the voltage to a safe level; it only
takes a few mils to kill someone.
Want more reasons? How about this script: someone goes to plug in the
modern appliance which is equipped with a polarized plug (one blade wider
than the other) but it won't fit the socket in this old house. No problem: a
couple of quick snips will remove the offending width of the neutral blade,
and now it plugs right in. And the antenna coax is now at 120 volts, this
time with NOTHING to limit the current!! Nuff said?
The National Electric Code, and any local code, clearly state that the
equipment grounding conductor (what we call the ground wire) and the
grounded conductor (what we call the neutral) are ONLY to be connected
together at the panel (fusebox or breaker panel). Anywhere else, the
neutral is a live conductor. Keep it insulated and don't connect anything
except load return wires to it.
You can follow W8JIT's suggestion safely by connecting the coax shield to
the neutral wire using a bypass capacitor (around 0.1 mf would be plenty big
enough) and thus have an RF connection without a 120 volt connection. Better
still, connect it to the ground wire and not the neutral. If you use too
large a capacitor, its capacitive reactance might be low enough to get a
shock from it. Those of us old enough to remember the days before equipment
grounding conductors became common in house wiring can also remember
touching our ham gear and getting a mild shock from it.
The other suggestions in Tom's posting to clear up the ITV (interference
from TV) are excellent. I'd like to add one more: the TV is on an outside
antenna. That antenna might be radiating the offending signals. Adding a
line amplifier (available from Radio Shack for about $10) would block
signals from going up the antenna wire.
If you have followed Tom's suggestions, and mine, and still haven't solved
the problem, you've sure got a stubborn case of RFI!
73
Larry, N9DX
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