[RFI] Tower Intermod - Control Lines

Jim Brown jim at audiosystemsgroup.com
Wed Jul 21 20:29:28 PDT 2010

On Wed, 21 Jul 2010 06:38:30 -0500, Ed Richardson wrote:

>With a portable SW receiver, I foud that the control lines used for the
>Rotor and remote antenna switch seem to carry this AM intermod and also
>a lot of noise in the lower HF band. Placed a couple chokes on the
>control line (5 turns through five 2.5" ferrites) at the base of the
>tower and it reduced the AM intermod detected around the base of the
>tower. The antennas on top still hear it.

Hi Ed,

First, when you say AM intermod, I assume you mean you're hearing 
multiple AM radio stations every 10 kHz up the band. 

I have a rather different take on this. The tower, and the control 
lines, are antennas, and as such, can RECEIVE that interference and re-
radiate it into the SW radio you are using as a probe. By putting a 
choke on those lines, you are reducing the antenna current, and thus 
their efficiency as antennas. 

SO -- if that's what's happening (and it OFTEN does), the intermod 
source may not be on the tower or the control lines, but somewhere else 
out in the environment. Indeed, it could even be in one of the 
transmitters that is involved in the intermod. 

A year or so ago, I had some noise show up on 160M, and I went looking 
for it with my 2M/440 talkie that has HF/MF receive capability. As I 
passed each power pole, I moved the talkie close to the ground downlead 
to look for noise. At one pole, I heard N7DD loud and clear calling CQ 
on 160. That ground lead wasn't radiating any noise, but it made a great 
RX antenna for my talkie!

And thanks for Q last week in NAQP RTTY! 

73, Jim Brown K9YC

More information about the RFI mailing list