[TowerTalk] AM Broadcast Harmonic Interference to 75 Meters

john at kk9a.com john at kk9a.com
Tue Dec 8 16:20:09 EST 2020

I concur.  I had the same 80m broadband broadcast mix when I lived  
near Chicago.

John KK9A

Earl Morse n8ss wrote:

I think the previous  posts have covered most of the possibilities.  I
would hazard to guess that the crud you are hearing is the main culprit and
the 3rd harmonic of the AM station is getting the bad rap for it.  Had a
similar issue when I lived in the Detroit area with 3 AM stations mixing to
create a birdie in the 80M band.  They were all a couple miles from my
house.   A single birdie I can live with but broadband noise takes out the
whole band so focus on that.  Remove any preamplification and lower the
gains of any RX settings or antenna preamps to see if that makes it go
away.   After I was content that the noise wasn't being created by overload
in my equipment, then I would move on to see if it was being generated by
mixing with another very local noise source or from the dissimilar joints
mentioned.   I would look for the local noise source.

You will probably spend a lot of time flipping breakers or even driving in
the mobile to see if it changes to prove out whether it is a widespread
area problem or just at your house.  Not sure if you live in a neighborhood
or out in the country which would affect the number of noise sources you
would come into contact with.

It is interesting that the crud goes 3600-3960 which is 90 kHz above and
270 kHz below the 3870 kHz frequency.  If it was related to the 1290 kHz
3rd harmonic I would expect it to be more balanced around the carrier
frequency.  So my money says there are two noise sources and the wideband
hash is being generated nearby and the AM station is getting blamed.

Good luck.  Let us know how the investigation goes.


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