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I know what the difference between AM (Amplitude Modulation) and FM (Frequency Modulation) radios is. However, I noticed that when I drive under a bridge while listening to a station on AM, it just gets really quiet, while if I'm listening to an FM station is gets fuzzy and static-y. Does anyone know why this is? My guess it that AM radio waves can't reach their full amplitude under bridges? And maybe that FM radio waves will oscillate on a different range under a bridge, but I'm really not sure.

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It has nothing to do with the modulation, AM or FM; it is because the wavelength difference. The so-called AM band is between 540kHz and 1600kHz, so its wavelength is about 300m, or so. The FM radio operates in the 88MHz to 108MHz band, or around 3m. The longer 300m EM wave reflects from the gap between the metal in the bridge and the ground (the latter is kind of a conductor at these frequencies). The 3m EM wave passes through the gap.

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  • $\begingroup$ And how does the static happen on FM stations? $\endgroup$
    – jhobbie
    Jul 7 '14 at 0:04
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    $\begingroup$ @jhobbie, most likely due to multipath - the radio waves reflect off the bridge structures and arrive at the car antenna at slightly different times, leading to noise. $\endgroup$
    – paisanco
    Jul 7 '14 at 2:53

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