Understanding How AM/FM Radio Works

Radio may seem like magic, but it's a fairly simple process

AM/FM radio can feel like pure magic. Switch on the radio and listen to music, talk shows, or any other audio entertainment being broadcast from a source located hundreds, if not thousands, of miles away. But radio isn't magic; it's a straightforward if fascinating process. Here's a look at how radio waves are created and broadcast.

Stylized image of radio waves against a radio tower
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What Are Radio Waves?

We're all familiar with AM, which stands for Amplitude Modulation, and FM, which stands for Frequency Modulation. 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 gamma rays, x-rays, ultraviolet rays, visible light, infrared, and microwave.

Electromagnetic waves are all around us, everywhere, at different frequencies. Radio waves have properties similar to light waves (e.g., reflection, polarization, diffraction, and refraction), but they exist at a frequency our eyes aren't sensitive to.

Electromagnetic waves are generated by alternating current (AC), the electrical power used to run most of the technology in our homes and lives, from washing machines to televisions to our mobile devices. In the United States, alternating current operates at 120 volts at 60 Hz. This means that the current alternates (changes direction) in the wire 60 times per second. Other countries use 50 Hz as the standard.

Although both 50 and 60 Hz are considered relatively low frequencies, the alternating currents still generate a basic level of electromagnetic radiation (EMR). This means some of the electric energy escapes the wire and is transmitted into the air.

The higher the electricity's frequency, the more energy that manages to escape the wire out into open space. This is why electromagnetic radiation is sometimes loosely 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), electricity must first be modulated. Therefore, modulation is the basis for AM and FM radio signals.

That's how the terms AM and FM originated, since AM stands for amplitude modulation and FM stands for frequency modulation.

Another word for modulation is change. Electromagnetic radiation must be modulated or changed to be useful as a radio transmission. Without modulation, a radio signal couldn't carry any information.

When it comes to radio broadcasts, the electromagnetic radiation (electricity in the air) must be modulated with the information we want to be sent.

Modulation is all around us. To better understand the concept, think about vision. A blank piece of paper is useless until it becomes modulated or changed in some meaningful way. Someone has to write or draw on the paper to communicate useful information.

AM Radio Broadcasts

AM radio uses amplitude modulation; it's the simplest radio broadcast form. To understand amplitude modulation, think about a steady signal (or wave) broadcasting at 1,000 kHz on the AM band. The constant signal's amplitude (or height) is unchanged, or "unmodulated," so it doesn't carry any useful information.

This steady signal produces only noise until it's modulated with information, such as voice or music. This modulation results in a change to the steady signal's amplitude strength, which increases and decreases in direct proportion to the information. Only the amplitude changes; the frequency remains constant.

AM radio in the Americas operates in a range of frequencies from 520 kHz to 1,710 kHz. Other countries and regions have different frequency ranges. The specific frequency is known as the carrier frequency, which is the vehicle by which the actual signal is carried from a broadcast antenna to a receiving tuner.

AM radio can transmit over greater distances. It has more stations in a given frequency range and can be picked up easily by receivers. But AM signals are more susceptible to noise and static interference, such as during a thunderstorm. The electricity generated by lightning produces noise spikes that AM tuners pick up.

AM radio also has a very limited audio range, from 200 Hz to 5 kHz, making it better for talk radio than music. For music, AM signals are of lower sound quality than FM. 

FM Radio Broadcasts

FM radio uses frequency modulation. To understand frequency modulation, consider a signal with a steady frequency and amplitude. The signal's frequency is unchanged or "unmodulated," so there's no useful information contained.

But once you introduce information to this signal, there's a change to the frequency directly proportional to the information. When the frequency is modulated between low and high, the carrier frequency is transmitting music or voice. Only the frequency changes as a result; the amplitude remains constant the entire time. 

FM radio operates in the 87.5 MHz-to-108.0 MHz range, a much higher frequency range than AM radio. The distance range for FM transmissions is more limited than AM, usually less than 100 miles. But FM radio is better suited for music. The higher bandwidth range of 30 Hz to 15 kHz produces the sound quality we enjoy and prefer. To have a greater coverage area, FM transmissions need additional stations to carry signals further.

FM broadcasts are also commonly done in stereo (a few AM stations can also broadcast stereo signals.) And although FM signals are less susceptible to noise and interference, physical barriers, such as buildings and hills, can limit them, which impacts overall reception.

This is why you can pick up certain radio stations more easily in some places than others, or why you "lose stations" as you drive through different areas.

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