[RFI] BCB Intermodulation and HV transmission lines

Tim Duffy K3LR k3lr at k3lr.com
Sat Jul 9 19:20:24 PDT 2011

Hello Ed:

I have seen this exact issue before here at K3LR. 2F1 + F2 from local AM BC
stations - the mix result was in the 80 meter band.

I was also able to create my own 2F1 (the AM station) + K3LR on 1810 mixing
product. That is when I knew I was on to something being wrong at the F1 AM

The source of the issue in my case was the F1 TX antenna system. A bad set
of relay contacts at the base of one of the 3 AM radiating towers had been
pitted due to a lightning hit. Once that relay was replaced - the mix went

I also noticed re-radiated of the mix coming from power lines - but that was
not the source of the mix.

Hope this helps.

Tim K3LR 

-----Original Message-----
From: rfi-bounces at contesting.com [mailto:rfi-bounces at contesting.com] On
Behalf Of Ed Richardson
Sent: Friday, July 08, 2011 1:03 PM
To: rfi at contesting.com
Subject: [RFI] BCB Intermodulation and HV transmission lines

I have been plagued by AM intermodulation products between 2 and 4 MHz. for
several years. I have verified they are not generated in my receivers.
Installed high pass and bandpass filters and this does not affect the level.
I have knocked the -10 dBm AM signals levels down to -90 dBm at the RX
antenna port.

With a handheld spectrum analyzer I have searched the neighborhood for the
source and the only potential source I have come up with is a High Voltage
transmission line that runs about 500m north of my tower. When directly
under one of the 3 parallel lines, I can detect the intermod products. If I
connect the telescopic antenna to transmission tower, the intermod product
rises to a -40 dBm. The lines are probably 30m overhead.

We are about 5-8 km from the AM transmission sites of at least 5

The intermod is primarily 3rd order, 2F1+/- F2.

Has anyone else seen this before and if so, were you able to get it


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