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Author Topic: AM broadcast RFI  (Read 2325 times)
WB4SPT
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Posts: 150




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« on: June 06, 2014, 09:09:08 AM »

My station is being noticed by a neighbor on his AM clock radio.  Of course, his favorite program coincides with my favorite schedule.   Sad
I also see this with my Bose clock radio on AM.   The culprit is my T4X on 20M, with or without the linear.  THe antenna is a dipole up 15 meters with a Balun Designs common mode at the end o the ladder line, after that, LMR400 to a in-shack tuner. 
Any great ideas?   Looks like I head into the guts of AM radio frontends?  Undecided
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WX7G
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Posts: 6136




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« Reply #1 on: June 06, 2014, 09:19:05 AM »

If ferrite core filtering of the neighbor's AM clock radio does not eliminate the problem the fastest and safest approach can be for the neighbor to get a new AM clock radio that is immune to your transmissions. This will be a matter of trial and error and I'm sure he would appreciate you paying for it.
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KF7CG
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Posts: 840




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« Reply #2 on: June 06, 2014, 10:41:40 AM »

Good luck on finding an inexpensive clock radio with enough extraneous signal rejection to handle your transmissions. Even the ancient 5 tube specials of the 50's and 60's were not particularly happy when 100 watts or more of Amateur Radio signal appeared nearby.

Since then the selectivity and extraneous signal rejection has gone down as the number of tuned circuits in the signal path of the IC based receivers has gone down.

Check to see if you interfere with your car radio tuned to the same frequency. If you don't it is probably in the other receivers. Worse it is highly likely that your signal is coming right on the loop antennas of the radios.

There is probably nothing wrong with your entire setup. The problem is probably what is known as front-end overload. Additionally, the 10th harmonic of an AM radio's local oscillator can be on the right frequency to mix with your signal and give the 455 KHz intermediate frequency of most AM radios. If the clock radio design has no appreciable front-end selectivity and a noisy local oscillator this is quite probable. Also, most of the less expensive clock radio designs match those conditions.

KF7CG
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KF7CG
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Posts: 840




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« Reply #3 on: June 06, 2014, 11:04:25 AM »

A little more source identification/interference route checking to help you.

Does your signal "tune"? That is does it become weaker and stronger with the tuning of the clock radio? If so, a harmonic of the clock radio's local oscillator is mixing with your signal to produce the interference. Given the small size and limited external connections of modern clock radios, it is highly unlikely that you will be able to find a way to put in enough front-end rejection, and local oscillator harmonic suppression into the clock radio to suppress your signal.

If it doesn't tune you signal if you are lucky is coming in on the power cord and "normal" powerline filtering will help. If you signal is strong enough to come through the tuning section and the IF and get to the detector there is not much that can be done in a modern clock radio.

Most modern consumer electronics use the minimum circuitry to function adequately and are incompatible with any nearby transmitters. The manufacturers count on there not being enough noise sources around to cause a problem. That is why CFR 47, Part 15 is there.

Again your radio and antenna are most likely in the clear.

KF7CG
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