Putting Information on Electromagnetic Waves

Where are AM and FM broadcasts found in the electromagnetic spectrum?

In the United States commercial AM stations broadcast between 550 kHz and 1600 kHz. Commercial FM stations broadcast between 88 MHz and 108 MHz. In the AM band the frequencies of stations are spaced by only 10 kHz. As a result, the broadcasts are limited to sound frequencies up to only 4kHz, while the ear can detect frequencies as high as 15 kHz. Why are broadcast frequencies so limited? Suppose the analog signal is a 440 Hz tone, the A above middle C on a piano. A typical AM transmitter would have a carrier wave frequency of 1 megahertz (MHz). The result is a signal with three different frequencies: 1 MHz, 1MHz 440 Hz and 1MHz - 440 Hz. Thus the total signal requires a set of frequencies 880 Hz wide. Sounds with a frequency of 4kHz require a range of frequencies, called the bandwidth, 8 kHz wide, just about the spacing between adjacent stations.

FM stations, on the other hand, were developed to transmit sounds more faithfully, which means that sounds up to 15kHz must be accommodated. That means that the bandwidth of the broadcasts is 30kHz wide. At the very high frequencies used by FM stations there is more bandwidth available and stations are spaced by 200kHz.

The electromagnetic spectrum has many other users. Police and firefighters usually use AM while aircraft, where noise reduction is important, use FM. Television stations used to use AM for the picture and FM for the sound, but as of June 12, 2009, they now all use digital signals. Other users are the military, marine ship-to-shore services, weather broadcasts, commercial mobile phones, the citizen band, and amateur radio operators. They all share the HF, VHF, UHF, and SHF bands.


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