How AM/FM Radio Works

Radio may seem like magic, but it's actually pretty easy to understand

Figures 3 & 4. Gary Altunian

Every so often it occurs to me that radio reception is pure magic. When you turn on the radio you hear music, voice or other audio entertainment that is broadcast from hundreds or thousands of miles away. It's really not magic. Radio is actually easy to understand and this article will help de-mystify how radio waves are created and broadcast.

What Are Radio Waves?

Both AM and FM radio programs are transmitted over the air via radio waves, which are part of a broad range of electromagnetic waves that include visible light, X-rays, gamma rays and others.

Electromagnetic waves are all around us in different frequencies. Radio waves are similar to light waves but are at a frequency our eyes are not sensitive to.

Electromagnetic waves are generated by alternating current (AC), the electrical power used to run every appliance in our homes from washing machines to televisions. Alternating current in the US is 120 volts at 60Hz, which means that the current alternates or changes direction in the wire 60 times per second. Other countries use 50Hz as the standard. Both 50 and 60Hz are relatively low frequencies, but even 60Hz alternating current generates some level of electromagnetic radiation, meaning that some of the electricity escapes the wire and is transmitted into the air. The higher the frequency of the electricity, the more electricity escapes the wire and into space. Thus, electromagnetic radiation can be described as 'electricity in the air'.

The Concept of Modulation

Electricity in the air is nothing but random noise. To be turned into useful signals that transmit information (music or voice) it must be modulated, and modulation is the basis for AM and FM radio signals. In fact, AM stands for amplitude modulation and FM stands for frequency modulation.

Another word for modulation is change. The electromagnetic radiation must be modulated or changed to be useful as a radio transmission. Without modulation, no information is carried in a radio signal. Modulation is an easy concept to understand and is all around us. Our sense of vision is a good example of how modulation works. A blank piece of paper is useless unless it is modulated or changed. Someone must write or draw on the paper for it to communicate useful information. Hearing is another example; still air must be modulated or changed with music or a voice to be useful. In radio broadcasts, the electromagnetic radiation or electricity in the air must be modulated.

AM Radio Broadcasts

AM radio uses amplitude modulation and is the simplest form of radio broadcast. To understand amplitude modulation, consider a steady signal broadcasting at 1000kHz on the AM band. The amplitude or height of the constant signal is unchanged or un-modulated, thus no useful information. The steady signal produces only noise until it is modulated with a voice or music as shown in figures 1 & 2. Notice how the amplitude or loudness of the signal increases or decreases to produce useful sound or information.

AM radio in the Americas operates in a range of frequencies from 520kHz to 1710kHz. Other countries and regions have a different frequency range. The specific frequency is called the carrier frequency, the vehicle by which the signal is carried from the broadcast antenna to the receiving tuner.

AM radio has the advantage of transmitting over greater differences because AM signals bounce off the upper atmosphere but suffers from more noise and interference than FM, especially during thunderstorms. The electricity generated by lightning produces noise spikes picked up by an AM tuner. AM radio also has a very limited audio range, from 200Hz to 5kHz, which limits its usefulness to talk radio and less for music.

FM Radio Broadcasts

FM radio uses frequency modulation, which changes or modulates the frequency of the unmodulated signal while keeping the amplitude of the signal constant. When the frequency is modulated, music or talk is transmitted via the carrier frequency (see figures 3 & 4).

FM radio operates in the range of 87.5MHz to 108.0MHz, a much higher range of frequencies than AM radio.

The distance range for FM transmissions are more limited than AM, usually less than 100 miles, but are better suited for music because the frequency range of FM is from 30Hz to 15kHz. FM broadcasts are also commonly in stereo, although a few AM stations also broadcast stereo signals.

Although FM signals can be subject to noise from lightning like AM signals, FM broadcasters use a limiter function that clip-off the noise spikes to produce a relatively noise-free signal.

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