Tammy Moerman, age 9, of Sioux City, Iowa, for her question:
WHAT IS THE DIFFERENCE BETWEEN RADIO AM AND FM?
All radio communication depends on a number of logical steps. First, the communication signal, whether it's a song or the spoken word, must be changed into radio waves. Next it must be transmitted or sent out over radio airwaves. And finally it must be picked up by a receiving radio and changed back into a form that can be understood.
Transmission of a radio program can be made in one of two ways, depending on the way the carrier wave and program signal are combined. The two methods are AM, or amplitude modulation, and FM, which stands for frequency modulation.
In an AM transmission, the amplitude or strength of the arriving waves varies to match changes in the electric waves coming from the radio station.
In an FM transmission, the amplitude of the carrier waves remains constant but the frequency of the waves meaning the number of times they vibrate each second changes to match the electric waves sent from the studio.
AM radio waves leaving an antenna spread out horizontally and travel through the air along the earth's surface and follow the curve of the earth. They also move in sky waves and are reflected back down to earth when they reach a layer of atmosphere called the ionosphere.
FM waves, on the other hand, are not reflected as they go skyward. Also, they travel horizontally in what we call a line of sight, so they do not reach beyond the curve of the earth.
Although FM's range is limited since it will not carry as far as AM, FM is not affected by static and the transmission also produces a much truer reproduction of sound than does AM.
Radio frequency is measured in units called kilohertz and megahertz, meaning vibrations per second. One kilohertz equals 1,000 hertz, and one megahertz equals 1 million hertz. AM stations broadcast on frequencies between 535 and 1,605 kilohertz, while the FM broadcast band extends from 88 to 108 megahertz.
AM stations operate on between 250 and 50,000 watts of power. Power in FM stations can vary between 100 watts, which will carry a signal about 15 miles, to 100,000 watts, which can take the broadcast about 65 miles.
A program carried by radio waves travels at the speed of light, which is 186,282 miles per second. As a contrast, sound waves move through the air at about one fifth of a mile per second. This means that in a live radio broadcast of a concert in New York City, the music could reach radio listeners in California a fraction of a second before it would reach the audience sitting in the back of the concert auditorium.