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?
Best Answer
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.