Harmonics

[jackb911]

I am located about 3 miles from the local 5,000 watt radio station. I am getting very strong 1st, 2nd, 3rd and 4th harmonics and a weaker but audible 5th harmonic signal from this station. Is this considered normal and allowable under FCC rules?

 

[rfburns]

I am located about 3 miles from the local 5,000 watt radio station. I am getting very strong 1st, 2nd, 3rd and 4th harmonics and a weaker but audible 5th harmonic signal from this station. Is this considered normal and allowable under FCC rules?

Depends on what you are using to listen with. I worked for a 50kW AM for several years and would receive complaints from local ham operators about harmonics that fell in the HF band. I was more than happy to prove to them that my transmitter met FCC specs for harmonics. The problem was the receiver / antenna combination they were using was so sensitive, it would receive my AM harmonics anyway. To accurately determine AM harmonic measurements, a spectrum analyzer is needed. If they play by the rules, they will have annual NRSC measurements on file to verify they are operating correctly.

RFB

 

[rfry]

It might be a normal condition for a receiver unable to reject the very strong fundamental signal of that broadcast station when tuned to its 2nd (and higher) harmonics.*

In such cases, a radio receiver can produce harmonics of that signal internally.

So your experience doesn't necessarily mean that such interfering signals were produced and radiated by that broadcast station, and/or that they exceeded FCC Rules.

You might need to add a reject filter for that AM broadcast signal in your antenna/receive system.

*there is no "1st harmonic"

 

[jackb911]

Thanks for the replies! Still not sure if its my radio or the station and don't have a spectrum analyzer to check. I don't know how to use it anyway .
Sorry about the terminology error RFRY, I am not a pro. Add 1 to each of my listings.
Here is some more info on my setup:
The radio is a Grundig Sat 750. I originally heard the station harmonics using the built in whip antenna, Now I have an external longwire (~80 feet) with the end pointing in the general direction of the station which should minimize the signal a bit. The main problem is that the station drowns out legit frequencies on 41m and 49m, the other images are outside of the normal bands so they are not a problem. The signal on these bands is much stronger than most everything else. I do get 2nd harmonics from a few other stations, but not as strong as the local station.

 

[kenglish]

Assuming you are not just getting ingress through the radio's case (what happens with NO antenna connected?), you can try something like this in line with the external antenna:
http://www.kiwa.com/bcb.html

I built a couple of them a million years ago, but I don't know where the plans are. You could build one a bit cheaper, but you might need an analyzer to test it anyway. Google around for some plans for a high-pass filter....some JAVA stuff is often available, to do the math for you as you experiment.

 

[Kelly A]

My experience is Ham's and DX'ers are the WORST to deal with for complaints to a broadcast station, because they think they know everything. No matter what you do with professional equipment, to prove the broadcast station isn't the problem, many can't believe there is something going on in their shack.

 

[frankberry]

Rusty barbed-wire fences in the vicinity of AM towers can generate harmonics. Loose fence connections can arc in the presence of a high RF field.
The arcing produces harmonic energy. The long runs of barbed-wire are efficient radiators of the harmonic energy.