[RFI] intermod/rectification

Stu Benner w3stu at myactv.net
Wed Oct 25 22:10:06 EDT 2006

I agree with much of what Earl says. There are usually lots of other poorly
bonded joints at places other than in your antenna system.

Consider using some sort of loop antenna with which you can DF the source
using a portable receiver. Use the nulls rather than the peaks to search. At
HF, you will likely have to be quite some distance from the source (whether
it be near you or near the transmitters) to accurately fix the direction.

You may want to investigate the broadcast stations that are causing your
problem. Determine frequencies and do the math to make sense of the spurious
signals that you're seeing. Determine if the stations are co-located and/or
are using the same antenna system.

Consider the possibility that other amateurs could be experiencing similar
problems due to other causes (e.g., receiver overload) before you believe
that data point. Are the other stations also seeing the identical pulsing as
are you?

Another very inefficient method of locating the problem if it's on or near
your property is to soak in water anything that you think might be the
non-linear junction.

I'll add one comment not relevant to locating or fixing the problem. In
order to mix two or more signals, a PN junction or rectification is not
required. All that is required (and what is usually going on in cases like
this) is the existence of some metallic couple with a non-linear transfer
function. (The knee of PN junction behaves approximately as a square-law

Keep us up to date, Mike. This one should be interesting once you find it.


-----Original Message-----
From: rfi-bounces at contesting.com [mailto:rfi-bounces at contesting.com] On
Behalf Of Morse, Earl (E.A.)
Sent: Tuesday, October 24, 2006 5:25 PM
To: rfi at contesting.com
Subject: Re: [RFI] intermod/rectification


The PN junction where the mixing is occuring may not be in your antennas but
rather close to you.  A metal fence or other object near you could have a
poor connection and be mixing signals strong enough to excite it.

The most likely places to look for external intermod is either close to the
transmitter or close to the receiver.  Since others are hearing the
interference it could be close to the transmitters.  Time to start mapping
it out, working your way away from your antennas.  Enlist the others in the
area to see if you can identify an area where it is strongest.  Go portable
and see if it disappears as you get away from your station.

A directional antenna would be nice so you can head in a logical direction
rather than wandering.  Do your beams, Beverages, and shorty yagi give you
any signal strength indications that indicate a particular direction.



Message: 1
Date: Mon, 23 Oct 2006 16:25:05 -0400
From: "W9RE" <w9re at comcast.net>
Subject: [RFI] Intermod/Rectification
To: <rfi at contesting.com>
Message-ID: <NCBBIPEKMKEIENBFHFHJKEAEJFAA.w9re at comcast.net>
Content-Type: text/plain;	charset="iso-8859-1"

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
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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