[TowerTalk] Fwd: BCB Intermodulation

[hanslg at aol.com]

It appears as you have a case of intermodulation?generated in?a semiconducting junction somewhere. You obviously have a couple of strong AM stations close by. Did?this started recently or have you had this for a long (or all)?time?

This phenomena can happened in many different ways, bad connection or corrosion in a connector, corrosion in a junction between two metal objects such as the sheet metal in a metal roof, connection between two pipes in an antenna mast etc. The possibilities are endless.

The offending junction is probably relatively close by indicated by the strength of the interference. You should look for what I tried to describe within a few hundred feet from your antennas. Hopefully you will be able to find something that can be easily fixed.

Good Luck and 73 de N2JFS, Hans


-----Original Message-----
From: Ed Richardson <ed_richardson at shaw.ca>
To: 'Tower' <towertalk at contesting.com>
Sent: Thu, 8 Jan 2009 10:02 pm
Subject: [TowerTalk] BCB Intermodulation



Just wondering if anyone has experienced intermodulation caused by some
component in an antenna system? I am getting severe broadcast band
intermodulation across the 80m band and have ruled out the receiver.

The intermodulation takes the form of (2x Frequency 1) + frequency 2. I
have no less than 6 strong AM stations that all interact with each other
to basically wipe out most of 80m. The intermod products vary from S9 to
S9+20dB.

I have installed a high pass filter in line that reduced the
fundamentals at least 40 dB but the intermod remained unchanged. Added a
30 dB pad on the input, no change in intermod except being reduced by 30
dB.

I have a single feedline (7/8" Heliax) with a jumper to the radio
connected to a RATPAK tower mounted switch. Switching between multiple
antennas yields intermod with slightly varying levels. When selecting an
open connector port, Intermod drops to almost nil but is just slightly
detectable.

Any suggestions where to look next?

Ed
VE4EAR  

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