Regimental Division,
Office Chief of Signal

United States Army Signal Center, Fort Gordon, GA
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Boombox The stereo or radio you listen to today came about partly because of the Signal Corps' work during World War II. Signal soldiers today still work with radios as part of their Army jobs.

How radios work

Radio involves sending or receiving electromagnetic radiation in the radio frequency range. You probably know the more common words for this: radio waves. Radio waves are sent from one place to another without wires.

A radio wave is like two waves in one: the carrier wave and the signal wave. The radio wave carries signals that have information in them. This information may be coded onto the wave by interrupting its transmission at times – this is how telegraph works. Or the information is impressed on the signal by a process called modulation. There are three types of modulation: amplitude modulation, or AM; frequency modulation, or FM; and pulse (or pulse code) modulation.

Radio signals most often contain sound. When sound is transmitted on radio waves, sound is converted into electric signals by a microphone and amplified by an amplifier. The signal is then transmitted on a modulated carrier wave the transmitting machine generates. The modulated carrier wave is also amplified as it goes to a transmitting antenna. The antenna converts the electric signals to radio waves that go into the air at the speed of light. Receiving antennas intercept part of the radio wave, convert it back into electrical signals and feed it to a receiver.

Let’s look farther into modulation, amplifiers and antennas as part of learning how radio waves are sent. Then we’ll see how the antenna in your stereo makes sounds that you hear as your favorite radio station.

Modulating a radio wave is done when some part of the carrier wave is made different to match the information-bearing signal wave. The carrier wave’s amplitude, frequency or other property is made different.

Earlier we said there are three types of modulation: AM, FM and pulse. Amplitude modulation means the carrier wave’s frequency is constant but its amplitude, or intensity, is changed. In frequency modulation, the amplitude is constant but the frequency is changed so that the carrier wave’s frequency is changed to match another signal (the signal wave). In pulse modulation, the carrier wave is a series of pulses all the same amplitude, width and equal spacing. Information is transmitted when pulses in the carrier wave are present or absent.

We’ve mentioned "frequency" several times, so let’s look into that more. Radio waves have a property called wavelength, or frequency. This means the length of the wave is the distance the wave travels from peak to peak as it goes through the air. A higher frequency has a shorter wavelength. A lower frequency has a longer wavelength. Transmitting radio waves involves working with wavelength.

An amplifier controls a signal’s energy flow to produce another signal that is similar but has a larger amplitude. An antenna is a system of wires that transmit or receive radio or other electromagnetic waves. A transmitting antenna produces electric charges that generate radio waves, which are transmitted through space. In a receiving antenna, radio waves create a signal detected by an electric circuit.

The antenna must match the radio wave’s frequency so the signal will be strong.

Once the receiving antenna has separated the signal wave from the carrier wave, your stereo sends those signals to a loudspeaker, where they are converted to sound. That’s why if your loudspeakers aren’t working well, you don’t hear anything, even if everything else has worked right up to now!

Satellites send radio signals. Cars that link into the Global Positioning System receive radio signals telling the driver where he is in case he’s lost. Your parents’ car alarm remote sends signals to unlock or lock the doors with a tiny radio transmitter. To open your garage door, you push a button on the remote, and a radio signal is sent to your garage, to the radio receiver in the garage door opener’s motor housing.

For more information on how the radio spectrum works, link to

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Amplified carrier wave going to antenna
A transmitter sends a modulated carrier wave to the transmitting antenna. The carrier wave is amplified as it goes to the antenna.

Pulse modulated carrier wave
The carrier wave in pulse modulation is a series of pulses all the same amplitude, width and equal spacing. Carrier wave and signal wave are together. In this diagram, the carrier wave is represented by white, the signal wave by black.

Illustration of radio wave frequency
A radio wave's frequency relates to the length of the wave, or the distance the wave is from peak to peak as it travels through the air. Waves have peaks and valleys. In a close-up look at this diagram, you can see the peaks and valleys as the radio wave goes from the transmitting antenna to the receiving antenna.

Last modified on:
April 04, 2012

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