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Sometimes while listening to radio on a car I notice the tuning will periodically go off tune (you hear just noise) and back in. Apparently it depends on the car's position, i.e. if I stop on a "in tune region" it stays this way and vice-versa.

Is it "shadow" caused by buildings? But then why periodic? Is it some kind of interference with other radio waves? How does it work? Why don't we notice a similar phenomenon with light? Or is it something particular to FM encoding?

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  • $\begingroup$ I've always attributed it to handoff between repeaters or something $\endgroup$
    – CoilKid
    Sep 4, 2015 at 22:12
  • $\begingroup$ Multipath, topography, ... $\endgroup$
    – Jon Custer
    Sep 4, 2015 at 22:13
  • $\begingroup$ I've always noticed this effect to be persistent in specific spots. My favorite parking space at work happens to be one of them, though mostly I notice it near intersections where many power lines converge. This may be an effect of confirmation bias though, since I took a guess at the cause before I started paying any attention to the data. $\endgroup$
    – Asher
    Sep 4, 2015 at 22:54
  • $\begingroup$ notice the nulls: en.wikipedia.org/wiki/Interference_(wave_propagation)#/media/… $\endgroup$
    – hyportnex
    Sep 5, 2015 at 1:59

1 Answer 1

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I assume you're talking about FM radio, which (at least in the US) is broadcast in the 87.5 to 108.0 MHz band, with wavelength of about 3 meters. "FM" means "frequency modulation"; the audio information is encoded as variations of the carrier frequency.

Although FM radio is fairly resistant to interference, it is vulnerable to something called multipath inteference, which happens when the radio signal takes multiple paths from the transmitter to the receiver. Depending on the lengths of the two paths, the signals can interfere, where the peak of one signal coincides with the trough of the other signal, canceling each other out.

Classic multipath interference is heard while driving a car, where motion changes the lengths of the two (or more) paths, periodically canceling and reinforcing the signal. In fringe reception areas you can even get this effect from an airplane overhead, where one path bounces off the airplane and interferes with the direct path, fading in and out even though you may not be moving.

Presumably, the parking spot with no reception mentioned in the comments is where the multiple paths (probably one bouncing off a building) exactly cancel. The solution would be to move the car, the building, or the transmitter. Or, choose a new favorite station.

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  • $\begingroup$ Exactly. It is like you are driving through a standing wave pattern - reception is OK near the antinode but poor near the node. $\endgroup$
    – Floris
    Sep 5, 2015 at 23:26

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