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AM/FM Radio Waves

This week's experiment comes from a question that I received about which is better, AM or FM radio. As we shall see, the answer depends on what properties you are basing your answer on. As we will see, each has advantages and disadvantages.

To try this, you will need:

  • an AM/FM radio. This can be either a portable radio or the radio in your car. Do not use internet radio, as that will not work for this experiment.
  • a rope about 6-10 feet long

We will begin by listening to some radio stations. Set your radio to the AM setting, and slowly scan through the stations. Listen to each station, to see how clear the reception is, how much static you can hear, and the radio stations call letters. The call letters will be four letters, such as WJRK. Write down the information for each station you can find.

Then do the same thing with the FM stations, again taking notes on reception, static, and call letters. When you are finished, go online, and look up the radio stations that you heard. I did a search for "Jacksonville radio stations" and found most of the stations that I picked up. For each station that you found, write down what city it is located in.

Compare the two lists. Do you see a difference? You probably found that the AM stations tended to have more static, but that you could pick up stations from far away. With FM, the signal was clearer, but all of the stations were fairly local.

The next step in our experiment requires you to move around a bit. Switch your radio back to AM, and tune it to a station that you like. If you are using the radio in your car, drive around a bit. Notice what happens to your radio reception when you drive into a parking garage, under an overpass, or when you cross a bridge that has metal supports. You will probably find that the reception becomes much worse, and the station may disappear until you exit the structure. If you are using a hand held radio, put on your ear phones, and try walking around your local shopping mall or grocery store. You should notice the same thing, with your signal varying a LOT, depending on where you are.

Switch back to the FM setting, and find a station that you like. Then try listening in the same places (parking garage, shopping mall, etc.) Try finding a dead spot for the AM radio, and then switching to an FM station while you are still in the same spot.

You will probably find that dead spots are fairly common with AM stations, while FM stations are usually clear pretty much where ever you go.

OK, so why the difference? To understand that, we need to know what AM and FM stand for. In both, the "M" stands for modulation. Modulation is a series of changes that you use to send a signal. For example, if you were using a flash light to send Morse code signals, you would turn the light on and off in a pattern of dots and dashes. This is known as PM, which stands for pulse modulation. You are sending the signal by using patterns of pulses.

AM stands for amplitude modulation. That word amplitude sounds an awful lot like the word amplifier, and with good reason. Amplifiers make a signal stronger, for example, a musical amplifier can make your radio louder. For AM, the radio signal varies in how strong the waves are.

Since we can't see the radio waves, lets use the rope to understand. Tie one end of the rope to a door knob. Hold the other end of the rope, and move away from the door until the rope is fairly straight, but still a little slack. Start moving your hand up and down to form waves in the rope. Notice that you can move your hand gently to produce small waves, and you can move your hand more forcefully to produce tall waves. You are modulating the amplitude of the waves.

FM stands for frequency modulation. With radio waves, as with other waves, frequency tells us how frequently the waves are passing. We can illustrate that with the rope too. This time, instead of changing the amount of force you use to shake the rope, change how quickly you shake the rope. You will find that you can shake it slowly to make waves that are far apart, or you can shake it quickly to make waves that are close together. You are modulating the frequency of the waves in the rope.

The strength of a signal tends to vary by distance and location. Changes in strength cause changes in amplitude (AM), but not changes in frequency (FM.) This causes AM stations to have more static and interference than FM stations.

Another big difference between AM and FM is the length of the waves. For FM, the waves are fairly short, usually around nine to twelve feet. Short waves are much better at getting into buildings, bridges, and other metal structures, but they are easily blocked by large objects, such as mountains, and they do not travel very far.

For AM, the waves are much longer, often more than a mile long. These long waves are easily blocked by metal structures, making it difficult to get good reception in bridges, stores, and parking garages. On the other hand, the long waves are much better at moving around mountains, and they can travel hundreds of miles, letting you hear stations in distant cities.

So which is better? Well, that depends on what you want. For listening to local stations, FM tends to give a much better signal, with good reception in most locations. On the other hand, if you are on a long trip, FM stations tend to fade quickly as you drive along, while you may be able to listen to the same AM station for hours before you lose the signal. If you are on a camping trip, or somewhere away from cities, AM stations may be the only stations that you can pick up.

So get out your radio, and do some testing. Just remember that this is a test. This is only a test. In the event of a real emergency, you would be directed to the nearest ice cream shop for some emergency rations.

Have a wonder-filled week.