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AM, FM Waves and Sound

Radio Lecture 3
by

Drew Hamilton

on 7 March 2011

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Transcript of AM, FM Waves and Sound

AM, FM Waves and Sound
Radio Waves
FM and AM waves work very differently
AM stands for Amplitude Modulation
AM frequency band ranges from 535 to 1705 kHz…those are the numbers you see on the AM dial
AM radio is toward the bottom of the electromagnetic spectrum
AM radio doesn’t sound as crisp and nice as FM, because it has that lower frequency
AM radio also isn’t in stereo
Electromagnetic Spectrum
AM Band
Stations can theoretically be placed every 10kHz, along the AM band. This means that there are a total of 117 different channels available for AM radio stations in a market area.
However, you can’t have stations that are geographically close to each with frequencies that are very similar. So, you can’t have two AM 600 stations unless they are far enough apart; or they will interfere with each other.
The good news is that since the signals of stations tend to be limited in their range, you can use some of the frequencies many times — as long as the stations are far enough apart geographically.
This is why the United States can have nearly 5,000 AM radio stations on only 117 different frequencies.
How AM Waves Travel
5 things play a factor in how far AM waves will travel
The station’s frequency or channel (if other stations with similar channels are close, interference can occur)
The power of its transmitter (measured in Watts)
The quality of the transmitting antenna
The conductivity of the soil where the antenna is placed (damp soil is good; rocky, sandy soil isn’t)
Ionospheric refraction. The ionosphere is a layer of heavily charged ion molecules above the earth's atmosphere. (You’ll see a picture in a minute)
The Ionosphere and AM
Ionospheric refraction can be both good and bad.
The Good: Ionospheric refraction can carry AM signals hundreds or even THOUSANDS of miles away from their source; making long distance communication possible
The Bad: Ionospheric refraction carrying those AM signals thousands of miles can interfere with local AM signals on the same frequency (EX: AM 600 in Seattle, Washington interfering with AM 600 in Memphis)
AM antennas send out 2 types of radio waves: a Ground wave and a Sky wave
The ground wave simply follows the curvature of the Earth and doesn’t go far
The sky wave goes into the atmosphere (and the Ionosphere)
-The Sky Wave hits the Ionosphere and get reflected back to Earth, sending it miles away from the source
The Ionosphere reflects more at night, which is why a lot of AM stations at sunset have to either: reduce power, go off the air until morning, or directionalize their signal (split the signal in different directions)
That is why AM stations are often harder to hear at night
FM and TV Waves
FM stands for frequency modulated
FM waves and TV waves are virtually the same
FM (frequency modulated) radio and TV waves don't act in the same way as AM radio waves
For starters, they are on a higher frequency in the RF spectrum
The FM radio band goes from 88 to 108 MHz (megahertz, or millions of cycles per second)
If you notice, all FM radio stations are between 88 and 108 on the radio dial
FM and TV Waves
FM stations must be 200kHz apart at these frequencies, which means that there's room for 200 FM stations on the FM band.
Because they’re at a much higher frequency, FM stations don’t get nice round numbers like AM stations. Which is why you see specific numbers like 99.7 FM vs 560 for AM
You may have noticed that FM stations don't reduce power or sign off the air at sunset. Because of their higher frequency ionospheric refraction doesn't appreciably affect FM or TV signals.
FM and TV Waves
FM and TV waves are called Line-of-Sight waves
That means they go in a straight line from their antenna, and don’t curve with the Earth…after 50 miles, they shoot into space
So, the farther away from the FM or TV station you are, the higher you have to have an antenna to receive the FM or TV signal.
FM and TV waves can be blocked by mountains or tall buildings…this creates that swishing sound you hear on FM stations when you go in a tunnel, car wash or next to a large building
This is why FM and TV transmitters are placed very high (on mountains or tall antennas); AM antennas don’t have to do that
All FM and TV waves eventually fly into space
AM & FM Differences
To understand the difference between AM & FM we need to explore the parts of a wave
Amplitude refers to the height of a wave
Frequency refers to the number of waves
The term modulation refers to how sound is encoded on a radio wave called a carrier wave; or, more accurately, how the sound affects the carrier wave so that the original sound can later be detected by a radio receiver.
not modulated by any sound.
There would be silence on your radio receiver.
Sound transmitted by an AM radio station affects the carrier wave by changing the amplitude (height) of the carrier wave
Sound transmitted by an FM radio station affects the carrier wave by changing the frequency of the carrier wave
Other Differences
AM waves are subject to static interference from such things as household appliances — and especially from lightning storms. FM waves are largely unaffect by static interference
FM has a greater dynamic range, and it can handle sounds of higher and lower frequencies. This is why music, with its much greater frequency range than the human voice, sounds better on FM radio.
Another difference between AM and FM is the loudness and softness of sound that can be transmitted
FM stations can cover a louder range of volume, which is why music sounds so much better on FM than on AM
The reason AM stations sound a little dull is because their range of volume is so small, everything tends to sound the same
Full transcript