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Noise on am radio

recently my 2002 ford windstar am radio has a buzzing/annoying sound (fm side no noise). When car is turned off the radio still plays (until door is opened) and the noise continues without engine on. What could cause this?

AM radio signals are vertically polarized, as is most sources of static and noise. FM is horizontally polarized so it isn’t as sensitive to noise and static. The noise is probably caused by a deteriorating electrical connection somewhere. Could be a corroded light bulb socket, but it would have to be a light that stays on until the door is opened. Could also be a relay that controls the lights and radio after the engine is shut off.

Actually Keith, the polarization of the signals has nothing to do with it. AM means amplitude modulated, and interference noises are also amplitude modulated, so you can hear then on an AM radio.

FM means frequency modulated, and since interference is usually amplitude modulated it will not produce sound on the output of an FM radio.

Retired senior electronics technician here.

Not that the difference is earth shaking, but thought I’d give a summary.

Correct me if I error,I equate “modulation” with information,when a carrier wave is modulated it has information on it. A AM signals information is dependant upon the amplitude of the signal,a FM signals information is dependant upon the frequency of the signal.

When you mix signals (like the carrier and the information) you get the sum, the difference and the original two signals (the process known as “superhetrodyning”)

What I am getting at is the reason interference (of the type generated by the secondary ignition system) heard on the AM band dependant upon the frequency of the interference signal or is it dependant upon the detection technique used. Maybe with FM signals you can limit the necssary amplitude of the desired signal to effectivly exclude the interference while still getting the information you need.

Just a thought.