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?”
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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.
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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.
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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.
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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.
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