I recently had to add my computer to my single point grounding system.
Apparently the computer chassis potential was slightly different than the
chassis potential on my FT-7800 transceiver. I feed the audio output of the
FT-7800 into an audio stream for others to listen to on the internet. Over
the past year, a 60 cycle hum gradually increased in volume on this setup.
It was rather annoying. I theorized that there was a small AC current
traveling on the ground of the audio cable between FT-7800 and computer
sound card (ground loop). This presented itself as an AC hum to my
listeners. The FT-7800 was already attached to my single point ground panel
via the coax shield as the coax is attached to an ICE lightning arrestor
which bonds the coax shield to my single point ground panel. I guessed that
the AC voltage on computer chassis was was traveling on the audio
shield/ground to the FT-7800 transceiver where it then travelled on the coax
shield to the single point ground panel. I ran a braided wire from a screw
on computer chassis to the single point ground panel. Immediately there was
more hum. That was about a month ago and the hum is still gone.
My audio feed can be found on my web site which can be found here:
http://n2scj.net
Bob
-----Original Message-----
From: Pete Smith N4ZR
Sent: Sunday, July 01, 2012 10:28 AM
To: RFI List
Subject: [RFI] Single-point grounding
I'm not sure exactly where I should post this, but suspect that this
group probably has more relevant expertise than most, so please bear
with me.
In the last year, I have had two expensive episodes. In both cases, my
transceiver's RS-232 transceiver was fried during a lightning storm, and
several knowledgeable people suggested a difference in chassis potential
between computer and transceiver as the reason. Their prescription for
avoiding a repetition was simple - connect the chassis of all of the
units to a single-point ground.
My solution - which I'm asking people to critique - was to fabricate an
L-shaped, roughly 3x5 foot ground bus made from 3/4" copper tubing,
which I mounted on the back of my L-shaped operating desk. All joints
are silver-soldered. I then connected each of the affected units to the
bus with very short and heavy stranded wire, and connected the end of
the bus to my grounded shack entry panel (in a double-hung window).
The DC resistance of the ground bus is very low, but the length
approximates a quarter wave on 10 meters, and with the units connected
near the ends of the bus, I wonder if I'm feeling a false sense of
security about the likely behavior of the bus during a nearby lightning
event. Would I be better off (or no better) running heavy conductors
from each unit to the entry panel, even if they would have to be
similarly long? Or should I just give up and plan on disconnecting the
RS-232 connection whenever weather approaches?
73, Pete N4ZR
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