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To send or receive a radio signal requires an ability to select a particular frequency from the limitless spectrum available. Electronically, the trick boils down to finding circuits that respond, or resonate, to a chosen frequency and not to others. Conceptually, most such designs resemble tank circuits.

For example, in an AM (amplitude modulation) radio signal, a voice or musical source is converted into electrical waveforms. These are rather low frequency, in the 100 to 10,000 Hz range. They are added together with a "carrier" radio wave of single frequency, say 1600 kHz. The result is a very dense train of radio frequency waves whose amplitudes vary over time in a way that mirrors the amplitudes of the much longer, voice wavelengths that have been imposed over them. In a receiver's tank circuit, tuned to the radio frequency, 1600 kHz resonates but other frequencies do not. Once captured the signal's radio component is stripped away—the tank circuit sends 1600 kHz resonances in this case to ground—and only the audio frequencies are left, to be amplified and reconverted into sound by other components in the receiver.

In FM (frequency modulation) radio, sound sources are electrically impressed upon carrier waves whose frequency, not amplitude, is made to change up and down at rates that match the audio signal. Lots of natural and manmade electrical disturbances distort the amplitudes of radio waves—they all add together—but fewer spurious sources alter a signal's frequency. For this reason FM signals come in cleaner, with less static and noise. Naturally, it took pioneers in radio a little longer to arrive at this idea; all early radio transmission consisted of either simple on/off pulses or, later, AM signals.


A radio transmitter's final element, also a "tuned" device, is the antenna. It's length or shape must allow the oscillating currents fed into it the chance to radiate their energy efficiently into the surroundings. At the receiving end, where much weakened waves crossing through a receiver's antenna induce feeble currents, tuning is not usually so specific. The receiver will want to pick up, if they are present, signals of selected frequencies within a whole range, or broadcast band.

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