[RFI] Intermod/Rectification

Stu Benner w3stu at myactv.net
Mon Oct 23 18:10:17 EDT 2006


Although you stated that there seems to be some dependency on weather
conditions, I would recommend that you first verify that the problem is not
a function of your receiver. Have you observed the same signals on another
receiver? Consider adding an attenuator, say 10 dB, at the input the
receiver. If the undesired signal drops by more than 10 dB (say on the order
of 30 dB),  you're looking at a receiver overload problem. Another way to
evaluate this is by determining from license data the difference in day and
night field strength of the problem AM station and use that as your
calibrated signal difference. (The only caveat here is that there may be
other variables between the day and night signals that could have an effect
on your problem such as different transmitters, different audio processing,
how well the station complies with its licensed power and pattern, etc.)

You describe the signal as "pulsing." Is the observed variation coincident
with the AM station's modulation, is it seemingly random, or does it have
some other pattern?

That's enough to start with.

Stu Benner

-----Original Message-----
From: rfi-bounces at contesting.com [mailto:rfi-bounces at contesting.com] On
Behalf Of W9RE
Sent: Monday, October 23, 2006 4:25 PM
To: rfi at contesting.com
Subject: [RFI] Intermod/Rectification

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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