How the Radio Spectrum Works

You've probably heard about AM radio and FM radio, VHF and UHF television, citizens band radio, shortwave radio and so on. Have you ever wondered what all of those different names really mean? What's the difference between them?

A radio wave is an electromagnetic wave propagated by an antenna. Radio waves have different frequencies, and by tuning a radio receiver to a specific frequency you can pick up a specific signal.

In the United States, the FCC (Federal Communications Commission) decides who is able to use which frequencies for which purposes, and it issues licenses to stations for specific frequencies.

When you listen to a radio station and the announcer says, "You are listening to 91.5 FM WRKX The Rock!," what the announcer means is that you are listening to a radio station broadcasting a frequency modulated (FM) radio signal at a frequency of 91.5 megahertz, with FCC-assigned call letters of WRKX. Megahertz means "millions of cycles per second," so "91.5 megahertz" means that the transmitter at the radio station is oscillating at a frequency of 91,500,000 cycles per second. Your FM radio can tune in to that specific frequency and give you clear reception of that station. All FM radio stations transmit in a band of frequencies between 88 megahertz and 108 megahertz. This band of the radio spectrum is used for no other purpose but FM radio broadcasts.

In the same way, amplitude modulated (AM) radio is confined to a band from 535 kilohertz to 1,700 kilohertz (kilo meaning "thousands," so 535,000 to 1,700,000 cycles per second). So an AM radio station that says, "This is AM 680 WPTF" means that the radio station is broadcasting an AM radio signal at 680 kilohertz and its FCC-assigned call letters are WPTF.

On the next page, learn more about about frequency bands and the frequencies that common gadgets use.

Radio Frequency List

Common radio frequency bands include the following:

AM radio: 535 kilohertz to 1.7 megahertz
Shortwave radio: bands from 5.95 megahertz to 26.10 megahertz
Citizens band (CB) radio: 26.965 megahertz to 27.405 megahertz
Television stations: 54 to 88 megahertz for channels 2 through 6
FM radio: 88 megahertz to 108 megahertz
Television stations: 174 to 220 megahertz for channels 7 through 13

What is funny is that every wireless technology you can imagine has its own little band. There are hundreds of them! For example:

Garage door openers and alarm systems: Around 40 megahertz
Cordless phones: Bands from 43 to 50 megahertz, 900 megahertz, 1.9 gigahertz, 2.4 gigahertz, and 5.8 gigahertz
Cell phones: 824 to 849 megahertz
Baby monitors: 49 megahertz/\r\n/
Radio controlled airplanes: Around 72 megahertz/\r\n/
Radio controlled cars: Around 75 megahertz/\r\n/
Wildlife tracking collars: 215 to 220 megahertz/\r\n/
Air traffic control radar: 960 to 1,215 megahertz/\r\n/
Global Positioning System: 1,227 and 1,575 megahertz
Deep space radio communications: 2290 megahertz to 2300 megahertz/\r\n/

Why is AM radio in a band at 550 kilohertz to 1,700 kilohertz, while FM radio is in a band at 88 to 108 megahertz? It is all completely arbitrary, and a lot of it has to do with history.

AM radio has been around a lot longer than FM radio. The first radio broadcasts occurred in 1906 or so, and frequency allocation for AM radio occurred during the 1920s. (The predecessor to the FCC was established by Congress in 1927.) In the 1920s, radio and electronic capabilities were fairly limited, hence the relatively low frequencies for AM radio.

Television stations were pretty much non-existent until 1946 or so, which is when the FCC allocated commercial broadcast bands for TV. By 1949, a million people owned TV sets, and by 1951 there were 10 million TVs in America.

FM radio was invented by Edwin Armstrong in order to make high-fidelity (and static-free) music broadcasting possible. He built the first station in 1939, but FM did not become really popular until the 1960s. Hence the higher frequencies for FM radio.

Radio Frequency Scanners

Most radios that you see in your everyday life are single-purpose radios. For example, an AM radio can listen to any AM radio station in the frequency band from 535 kilohertz to 1.7 megahertz, but nothing else. An FM radio can listen to any FM radio station in the band from 88 to 108 megahertz and nothing else. A CB radio can listen to the 40 channels devoted to citizens band radio and nothing else. Scanners are different.

Scanners are radio receivers that have extremely wide frequency ranges so you can listen to all kinds of radio signals. Typically, scanners are used to tune in to police, fire and emergency radio in the local area (so scanners are often called "police scanners"), but you can use a scanner to listen to all kinds of conversations. Generally, you will either:

Set a scanner up to scan (switch between) a whole range of frequencies and then stop scanning when it detects a signal on any of the frequencies it is scanning. If you're interested in learning what the police are doing, you can scan the police radio frequencies in your local area. When a patrol car calls in to report a problem, the scanner will stop on that frequency and let you hear the conversation.
Set a scanner to a specific frequency and listen to that channel. For example, say you want to listen to the transmissions between the control tower and airplanes at the local airport—you can do this by listening to the specific frequency used at the airport. Because a scanner can receive a huge range of frequencies, you can set it to receive nearly anything on the air.

In order to use a scanner, it's ideal to have good frequency tables so you know where the action is. For more information, check out the links on the next page.

How the Radio Spectrum Works

Radio Frequency List

Radio Frequency Scanners

Lots More Information

Related HowStuffWorks Articles

More Great Links