I think Mark said his problem is on all bands; not just 12 m. When I got
back on HF about 18 months ago after being off for many years, I ran my Omni
with a goldline mic right out the house er, tx site, to my vertical, a gap
titan. The omni was grounded to a cold water pipe with a thin run of tinned
braid, which I figured was good enough. It had worked in the old days with
my tube rigs. I'd soon learn that the new(er) solid state rigs are more
touchy about grounding. Working an old friend on 10 meters one day, he
remarked that my audio was fuzzy and told me I probably had rf feedback
which was a new thing to me. I blew it off since he was the only one to
tell me this (but a lot of hams will tell you you sound great when in fact
you sound awful--I attribute this to different standards) but when I put my
centurion on the air a few months later, I couldn't work anyone because the
rf had become too distorted for anyone to understand me with the higher
power level. After doing a lot of reading, phone calls to people at gap
antennas and radio works and experimenting, here's what I learned and what I
did at my place to fix my problem.
First, TT says in the Omni manual that a good ground is needed (or words to
that effect). They should put that in bold type. I've since talked to a
few other omni owners with the same problem and therefore think this is
going on in a number of QTHs and it all has to do with the installation.
The first thing to go was the cold water ground. I moved my shack to a
basement window, and ran 6.5 feet of copper cable outside to five 8' ground
rods, one in the center and 4 each connected directly to the center one (not
daisy chained). I had tried being lazy and running 22 feet of 2/0 cable
across the basement to the rods but quickly found the 22 ft. of ground run
was another antenna--no dice. Next I dumped the ground bus bar, a 6' copper
pipe (almost another antenna on 10 meters) and hooked a 1 foot copper pipe
to the ground cable coming in to the shack to use as a contact point. This
pipe sits right behind my transmatch and amp. they are connected to it with
6" runs of heavy gauge 1" wide braid. all other ground runs to the rigs,
p.s., are with this same type braid (available from cablexperts) coming way
from the pipe. Each piece of gear has its own separate run -- no daisy
chaining with one ground run. Next I wrapped the mic cable at the omni
input around 3 ferrite chokes and did the same with one or two chokes (which
are available from RF Parts, San Marcos Calif.; they have the best prices
and quality for split 1/2 inch i.d. chokes in plastic holders) on just about
every wire going in and out of the Omni. Lastly, I put Radio Works line
isolators in the feedline between the exciter and the amp, between the amp
and the transmatch, and at the feed point of the gap vertical. This was to
bust up ground loops circulating, between all the devices in the rf chain,
on the feed shields and ground braids. Now my rf is clean enough to eat off
of.
Overkill? Probably. The alternative (if you have the time) is to try each
supression technique one at a time while txing into a dummy load & listening
on a separate rx until you find the least number of measures that are
effective. Since I didn't have time (and patience) for that I went for
overkill, but I have to admit the other way would have been cheaper. Hope
this helps anyone with an Omni rf feedback problem.
73,
Rob Atkinson
k5uj@hotmail.com
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