[RFI] Intermod/Rectification

W9RE w9re at comcast.net
Mon Oct 23 16:25:05 EDT 2006

I have a few loud (s9 to 20db over) intermod AM broadcast signals on 3640
and 3690 and  these signals are heard on the Southside of Indianapolis by
several other hams (at a lower level).  My problem is that these signals
pulse to 20db over 9 and sound like I have a loose connection someplace that
is causing rectification (which also causes other images across the band).
I cannot track down the loose  connection.   I’ve tried monitoring SWR while
transmitting, monitoring with a MFJ-259B, listening to the antennas with a
pair of headphones (like a crystal radio with/without a diode), monitoring
with a scope, monitoring with an analog AC voltmeter,  transmitting into
antennas and listening on others, etc. with no luck.  I have three 80 meter
antennas- a 40 meter double extended zepp, a sloping V Beam (both these fed
with open wire line) and a shortened 2 element yagi, the noise is loudest on
the zepp and the V Beam.  I also notice the noise on Beverages and short
vertical receiving systems.

At night the signal levels are not as strong (AM stations changing
power/pattern) and when it rains the problem is not observed.  Transmitting
on 160 with a KW stops the pulsing from S9 to 20 over but it returns when I
stop transmitting.

Last week I thought I had isolated and found it when I was listening on 3690
to a pair of 10 meter yagis (that are not used very often) I found the
intermod to be S9 and pulsing but after disconnecting the antennas up on the
tower the problem still remains on the other antennas.

Any suggestions as far as isolating the problem?


Mike W9RE

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