Really the best advice, as someone has already written in different
words, is to use a balanced antenna with a balanced feedline, so that
the net RF current flowing into the shack is zero. It's fine to run
an unbalanced transmission line such as coax into the shack if (1)
your antenna is balanced; (2) the antenna does not couple to the
feedline; (3) a good balun is used at the transition to the
unbalanced (usually coaxial) line; and (4) since the first three
conditions are never perfectly satisfied, the coax is loaded with
common-mode RF chokes.
I do all four, and although my second-floor shack has no RF grounding
(just safety grounding), I have no RF in the shack and my receiver
hears no AC-line or other artificial noise. Today at 2:30 AM while
working a weak QRP station on 80 meters, I observed that even with my
very sensitive (Icom IC-775DSP) transceiver's preamp switched in(!),
my S-meter rested at zero between the usual noise bursts from distant
lighting, and I could hear _no_ man-made noise. My computer was
running on the same table; it and three fluorescent lamps were
plugged into the same AC power branch circuit as my transceiver; and
in the room directly below me were three multi-hundred-watt
incandescent lamps, each lit but dimmed by an ordinary Triac/SCR-type
dimmer, a.k.a. QRO QRN generator.
Most of the AC-line buzz, digital-computer hash, etc., that most hams
hear is conducted to their antennas from their houses by common-mode
current on imperfectly balanced feedlines. I heard it, too, before I
learned to balance everything and eliminate the unbalanced, or
"common" mode.
-Chuck, W1HIS
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