Thank you for helping
Interference is the same from R1 to R2 as from R2 to R1.
I am afraid that Jim hit the nail... I have a metallic roof, I changed
it 2 years ago, and the problem seems to be bigger the last time. I have
not combined it with the roof but this is possible... It is true also
that the noise is bigger when antenna looks the building. I hope it will
be easier to solve it...
73 Kostas SV1DPI
Στις 30/3/2022 11:43, ο/η Jim Brown έγραψε:
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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