Why do AM radio signals increase in amplitude near high-voltage power lines?

When driving under high-voltage power lines, AM radio signals get louder. Is the RF carrier somehow getting amplified by the 60Hz electromagneting field around the lines? Is the modulation depth getting somehow increased? Does the field act as a coupler between my antenna and the lines, in effect making the power lines extension of my antenna? Is the RF carrier being ducted from the transmitter?

3 Answers

  • 1 decade ago
    Favorite Answer

    A 60 Hz power line's EM field would have almost no effect on an AM radio signal (~1 MHz). The only effect it would have on a car stereo is a 60 Hz "hum" if the field coupled with the car.

    What is likely to be happening is that, when you drive near the power lines, part of the signal that is incident upon the power lines is reflected back toward your car. Depending on your location relative to the power lines, this reflected signal will either add to, or subtract from, the energy received by your car's antenna.

    Since it is an AM signal (this doesn't work with FM), if the scattered field "constructively interferes" with the signal at the antenna, the amplitude of the signal at the receiver increases and you get a louder output from the speakers.

    The opposite can also be true. You may notice that sometimes, when you drive under power lines, the signal fades until you are out from under them. This is called "destructive interference" and is a result of the reflected waves subtracting from the signal at your car's antenna.

    Source(s): To many sleepless nights studying for Dr. Kishk's electromagnetics class.
  • 1 decade ago

    As long as the AM radio signal is a harmonic of the 60 Hz there is no reason that the radio signal will not be amplified by the power lines. The reason is this:

    The power line acts like an amplified. When the EM-radiation interacts with the cables, the photons become coupled with the oscillating electric field produced by the AC current. Coupled fields can exchange energy, nonlinearly, so the modulation in amplitude would cause a nonlinear amplification of the signal.

    Look up coupled waves and three-wave mixing.

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