The receiver is already in a (steel) waterproof box. Tomorrow I will try
moving the receiver as far away as I can. If that doesn't fix the
problem, I'll try the stub antenna notch filter solution. I have a BNC
Tee. Can anyone point me to the 1/4 wave length formula for 120.6 MHz?
Does 0.591 meters (23 5/16") sound right?
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For the stub to work properly at the intended (notched) frequency , you must
take into account the velocity factor of the coax. For a number of common
coax types the velocity factor is about 0.67

For a freq of 120.6 MHz a quarter wavelength is 0.62 metres, so with coax
having the above velocity factor the physical length is 0.62 * 0.67 equals
0.42 metres equals 16.4 inches.
However ,as mentioned before in this thread you could use odd multiples of
this length

When you really want the stub 'as meant for the job' try to find someone
with a spectrum analyser (with tracking generator) , cut the coax a bit
longer than its calculated length and snip tiny bits off until the spectrum
analyser shows the notch at 120.6 MHz.

On a different but related topic ; interference to a UHF TV sig by a nearby
144 MHz tx sig , I made a high pass filter and a downstream quarter wave
stub which was subsequently optimised with the aid of a spectrum analyser.
As expected there was also a clear notch at 432 MHz ,since the stub is a 3
quarter (odd)wavelength for that frequency. BTW the actual notch was at
144.4xx MHz being the part of the 2m band used for SSB operation in the UK
, with SSB (varying RF amplitude) being a 'high risk' mode for interference.
This HP filter/stub combination now permits the relevant radio amateur to
operate on 2 metres ,be it with moderate power.

Frank GM0CSZ / KN6WH