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Re: [CQ-Contest] interference in so2r station

To: cq-contest@contesting.com
Subject: Re: [CQ-Contest] interference in so2r station
From: Jim Brown <k9yc@audiosystemsgroup.com>
Reply-to: k9yc@arrl.net
Date: Wed, 30 Mar 2022 01:43:21 -0700
List-post: <mailto:cq-contest@contesting.com>
On 3/29/2022 12:03 PM, Kostas SV1DPI wrote:
Any thought what to look for or what to test?
Yes. W3LPL identified the common problem of "passive intermod" that 
occurs outside the normal RF path by rectification in some non-linear 
device. Some common culprits are 1) the mechanical junction between the 
fixed and rotating part of antenna rotators; 2) switch-mode power 
supplies; 3) imperfect junctions between two conductors that could be 
ANYWHERE, and there's enough metal or wire connected to them that acts 
both as a receiving antenna for our transmitted RF and a transmitting 
antenna for the intermod products.
The intermod products are harmonics of the transmitted signal, and for 
obvious reasons, the harmonics of 80 to 40 CW, 40 to 20 CW are the most 
problematic, because of where they land, and because the most common 
SO2R operation is on adjacent bands.
The solution to the rotator is to bond around it; the solution to SMPS 
is to search and destroy (or replace). It generally takes two hams (at 
least) to find them. First, rotate the TX and RX antennas to find 
direction where it's worst. Then, one guy transmits, the other walks 
around with a battery operated radio looking for where the harmonic is 
strongest. If the antennas are on different towers with rotators, it can 
be possible to triangulate by also rotating the RX antenna.
Example of other IMD culprits -- I live in the mountains where wild 
fires are a major hazard, so I recently had a steel roof installed. LOTS 
imperfect junctions, massive passive intermod! The possibilities are 
endless.
73, Jim K9YC
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