At 4:09 PM -0500 12/31/02, Pete Smith wrote:
>...Most of the noise I hear is being received on my antennas, not on
>the coax. This is particularly true since I got rid of my CRT
>monitor....
Of course there's no way for me to be sure without coming to your
station and experimenting, but the fact that you heard your CRT
monitor suggests that noise from it is being conducted to your
antenna, via your feedline(s), from your AC power wiring.
I have a 17-inch CRT monitor about six feet from my transceiver and
separated from it by no more than twelve feet of AC-power wiring, but
I don't hear it at all. This monitor is an Apple-branded Sony
Trinitron, to which I have done nothing to suppress RFI, although the
entire computer system including ADSL modem, ISDN modem, HP LaserJet,
ethernet hub, etc., is plugged into an AC line filter. Cables run
from this computer system into my shack and all over the house,
including near my coaxial ham-antenna feedline.
I also don't hear either of the TV sets in my house, one of which is
in a room directly abutting my shack; and my coaxial ham-antenna
feedline passes directly under, less than three feet below, the attic
antenna of the other.
IMO, based on a goodly amount of experimentation including sniffing
with an electrostatically shielded, B-field-sensing loop, the
number-one way by which RFI from computer monitors etc. gets to ham
antennas is not by free-space radiation, but by transmission-line or
guided-wave modes, most often via an unintended, unbalanced,
transmission line formed by a conductor above ground, with ground
return.
-Chuck, W1HIS
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