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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