[CQ-Contest] interference in so2r station

[Kostas SV1DPI]

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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