Randy,
You might have better success with applying the 2.4 inch mix 31 toroids
to your feedline rather than trying to eliminate RF intrusion on every
cable in the shack.
In other words, kill the common mode current at its source and you may
not have to add additional ferrites on all your shack cables. 3 to 5
turns through a stack of 5 2.4 inch mix 31 cores at the point where the
feedlines enter the dwelling may do the job nicely. Another similar
choke at the antenna feedpoint(s) may be helpful as well.
The place to deal with RF-in-the-Shack is at the antenna field, not in
the shack.
Just for the record, OCF antennas are infamous for these problems, and
with any antenna where the feedline are run under the radiator rather
than dropping at 90 degrees from the feedpoint for at least 1/4
wavelength can be expected to pick up radiation from the antenna and
feed it into the shack as common mode currents.
73,
Don W3FPR
On 1/26/2014 6:42 PM, Randy Farmer wrote:
Thanks to all the folks who replied with advice, both on and off the
list. I spent the better part of the weekend working the problem and
thought I'd update y'all on what I found.
It turns out there were two major problem areas. The first was RF
getting into the K3 via the ACC jack from the connections between the
radio and the KPA500, the XV144 and the microHAM MK2R+ box. The KPA500
is NOT affected by the common mode RF on the ACC cable. The power
gyrations were clearly due to RFI effects within the K3. I *mostly*
fixed this by building a short extension cable for the ACC jack and
wrapping six turns of it around a 2.4" mix 31 toroid. This lets me use
up to 500W output at the problem antenna heading without making the
radio go nuts. If I increase power slightly it still begins to act up,
so I haven't completely cured the problem. It's easier to just back
the power down a few watts than it is to figure out how to get another
few dB suppression on this path. I plan to live with it.I still have a
little RF getting into the receive audio line at the worst case
antenna heading and causing a bit of hum on the audio, but this is not
really a big deal.
The second problem turned out to be the 13.8V power supply I use for
the antenna switching relays and powering the Station Masters. This
thing is a cheap knockoff of an Astron supply that uses a 723 voltage
regulator. RF was apparently getting into the supply and modulating
the DC output. This caused the antenna relays in the Station Master to
buzz. This was undoubtedly the source of the "buzz" on my signal
reported by listeners. I took several turns of the output DC cable
around another mix 31 toroid near the supply output terminals and this
seems to have cured the problem.
Thanks again to those who weighed in with suggestions. You gave me a
lot of troubleshooting approaches that let me home in on the problem
pretty quickly once I got started.
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