My six-year-old son Obie was riding in the car with me Saturday night and asked a question that brought about the topic for this week. He said, “Hey, Dad, I know that FM is the radio stations, and Aux lets us listen to music from your phone, but what is the AM button for?”
Photo by Ivan Akira
FM radio carries the audio signal by
modifying the frequency of the
carrier wave proportionally to the
audio signal’s amplitude.
So just in case there are others out there that want to know what the AM button on your radio is for, here is a lesson on radio signals. Every radio station in the world operates on one of two different broadcast technologies. They either use amplitude modulation (AM) or frequency modulation (FM). They both use electrical current passed through a broadcast antenna to send an electrical signal through the air, but they carry, or modulate the sound wave in very different ways.
To begin to understand radio broadcast, it is first important to understand that electricity can travel through the air just like sound and light travels through the air using waves. There are two main properties to a wave. The first is the wavelength, which is inversely related to the frequency and tells the amount of distance between individual peaks in the wave. The frequency tells us how many peaks will reach us in a second. It can be thought of as how high or low a sound is, or actually determines the color of light. The second is the amplitude, or height of the wave, it can be thought of as how loud a sound is or how bright a light appears.
FM, which is the most well known radio signal, carries the sound over the air by modification of the frequency of the radio wave. As the sound wave you are broadcasting changes in pitch, the frequency of the wave changes within a given range. FM operates in a frequency range of 88-108 Megahertz (MHz), which means between 88 million and 108 million peaks of a wave hit your antenna every second. Each FM station is assigned a range of roughly 100 kilohertz (kHz), meaning the signal varies by 100 thousand waves per second. You can think of it as carrying the sound by changing the length of the wave. The FM signal will not travel as far as AM signals because of atmospheric effects on the signal.
Photo by Ivan Akira: AM radio carries
the audio signal by modifying the amplitude
of the carrier wave proportionally
to the audio signal’s amplitude.
AM, which is less well known, also happens to be less expensive to operate and the signals can travel much longer distances. This has to due with the longer wavelengths, which cause the signal to bounce off of the upper atmosphere, whereas FM signals pass through it. AM signals carry the sound wave by varying the amplitude, or height of the wave, in proportion to the height of the sound wave being broadcast. They operate on a much lower frequency than FM, around 540-1600 kHz which means between 540 thousand and 1.6 million waves hit your antenna in a second, roughly 100 times fewer than FM. AM stations are assigned a given frequency that never changes during their broadcast. This allows a receiver to work with a much weaker signal since it does not change, making it possible on a clear night to listen to AM radio stations as far away as northern Canada and southern Mexico.
Due to the higher sound quality of FM over AM broadcast signals, most FM stations are used to broadcast music and most AM stations are used to broadcast talk radio. So now you know what the AM button on your radio means.
Another really neat fact about AM radio is that if you tune to a weak AM station during a thunderstorm, you can hear the lightning on the radio nearly the same time as you see the flash, and it gives you an audio warning of the pending clap of thunder.