I'm stuck on a very small lot with a pretty ugly RF environment. My
station is on the second floor of the house and the SteppIR is mounted
on a 10' roof tower directly over the station location. The feedlines
and control cables go down the roof and then enter under the eaves from
near ground level and are routed back up to the attic on the wall
immediately behind the radio station, where the antenna switching and
control cable interfaces are located.
I do have good quality chokes (Balun Designs) installed at the antenna
and also at each of the two outputs of my Double Ten switch that feed
the radios. For some weird reason I'm getting strong coupling only on 20
meters and only when the antenna is turned to around 120° to 150°
(exactly normal to the long axis of the house). None of the other wire
antennas or the SteppIR at other headings gives me any noticeable common
mode RF problems.
Unfortunately, in my case the shack IS part of the antenna field and
there's nothing I can do about it short of removing the antenna. When I
retire in a couple of years I fully intend to move to a location where I
can have the antennas properly separated from the living quarters and
radio shack. Until that happens I'll just have to deal with the RF as
best I can.
73...
Randy, W8FN
On 1/26/2014 6:38 PM, Don Wilhelm wrote:
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
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