9:30 PMHow the Radio Spectrum Works
Introduction to How the Radio Spectrum Works
You've probably heard about "AM radio" and "FM radio," "VHF" and "UHF" television, "citizens band radio," "short wave radio" and so on. Have you ever wondered what all of those different names really mean? What's the difference between them?
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. See How Radio Works for more details on radio waves.
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 an 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 (frequency modulated) 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, 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 (amplitude modulated) 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:
What is funny is that every wireless technology you can imagine has its own little band. There are hundreds of them! For example:
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 a man named 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 radiocan 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:
In order to use a scanner, you need to have good frequency tables so you know where the action is. For more information, check out the links on the next page.
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