[RFI] AM Radio Interference On 75 meters

[Tim Duffy K3LR]

Before resolving (guessing) that any AM station is at fault here - or worse,
getting the FCC involved, you need to make certain that the MIX is not
occurring because of an off site rectification (MIX) problem. 

Most of the 80 meter AM broadcast mixes are NOT the fault of the AM
broadcast stations. They are caused by rectification at the amateur radio
receiver/antenna system or something close to the amateur radio station
location that makes the "mix" happen. 

I have been down this dirty MIX road before. With big signal mixes on 3.510
and 3.730 MHz at K3LR. AM BC transmitters from 570, 790 and 1470 AM - you
can do the math and see how second harmonics can mix with fundamentals. 

Bottom line - the FIX to the MIX was not a problem with the AM station(s). 

Just make sure your "house" is clean before pointing the gun at the AM
station(s). It takes some good investigative work to be sure.

73,
Tim K3LR

-----Original Message-----
From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of K8TB
Sent: Tuesday, September 18, 2012 3:00 PM
To: rfi at contesting.com
Subject: Re: [RFI] AM Radio Interference On 75 meters

I don't think it is KFBK. My guess is KCTC, on 1320 KHz. 3rd harmonic is 
3960, and even cooler, KCTC operates from 2 different sites. The night 
site is 5 kw and is located by Antelope North and Antelope, the towers 
show an address of 7909 Antelope N Rd.

If, the stations is producing a 3 rd harmonic outside of specs, they do 
have to reduce it. But, it only has to be down to a certain level. I 
measured one Am on 1300, omni mode, in its driveway, and you just barely 
hear the 3rd on a good communications receiver. But 3 miles away, the 
3rd was S7. Ham neighbors aluminum downspout.

First thing to do is to get a step attenuator in series with the 
receiver. Drop the signal by 10 db, and see if the S meter goes down by 
10 db. If it does, the 3rd is outside his rx. Could be the 1320 signal, 
could be developed someplace else. If you add 10 db or 20 db of 
attenuation, and the signal drops out of site, well, now its his 
receiver. He is very close to this 1320 signal.

Call KCTC, they should have a field strength meter that can locate the  
source of the signal.

In all cases, you have to let the group know what you find, when you 
find it.

tom bosscher k8tb, 40 odd, and very odd years in broadcasting.


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