Antennas affect the quality of the sound you get from a radio in indirect ways. An antenna's job is to turn radio signals into electric currents, and an efficient antenna yields clear-sounding broadcasts with low noise, even with stations that transmit weak signals. The type of antenna -- whether it's used for AM, FM or satellite signals -- also strongly influences radio sound quality.
Description and Function
Antennas are generally made of long, thin pieces of metal, carefully designed to receive a particular range of radio frequencies. Radio waves, passing through the metal, move electric charges in the conducting material, producing signals that contain radio broadcast frequencies. A radio amplifies these signals and tunes into one particular frequency corresponding to the station you want to hear. Without an antenna, the radio picks up very little signal and cannot tune in a station.
The shape and layout of an antenna determines its gain, or how strong a signal it delivers to a radio; an efficient antenna is well-matched to the signal's frequency. Good signal strength reduces noise and allows the radio to lock into a station, while a weak signal causes a station to drift in and out, making you miss parts of a broadcast. Gain is important in areas where radio signals are weak, while you can get by with a low-gain antenna if stations are close to your home.
Stereo FM Reception
To hear stereo music on an FM station, the radio must receive a strong signal. The station encodes an inaudible channel separation signal and pilot tone as part of the radio transmission; on the receiving end, the radio decodes stereo only if it receives all the components in the FM broadcast. Most FM radios cope with a weak signal from a marginal antenna by automatically disabling the stereo part of the broadcast and reverting to a single-channel mode, which means reduced sound quality. If the signal improves, the radio enables the stereo mode again, but listening to a broadcast that's erratically switching in and out of stereo mode can be annoying.
Each type of radio broadcast -- AM, FM and satellite -- uses its own kind of antenna. A typical AM antenna is a compact design consisting of a short iron rod wound with wire. Because AM signals have the lowest frequencies -- 530 to 1605 kHz -- audio quality suffers and broadcasts are more susceptible to noise. FM antennas range from a simple shielded wire pair a few feet long to highly-tuned outdoor aerials. FM has a frequency range of 88.1 to 107.9 MHz, giving you better fidelity with less noise. Satellite radio frequencies are in the very high 2.32- to 2.345-GHz range, and broadcasts are digitally encoded with CD-quality sound. The antennas used with satellite radio are a few inches in size and shaped in a variety of forms; small antennas are well-suited to high-frequency satellite signals.
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