[Elecraft] KPA500 RFI Problem

[Randy Farmer]

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