# Learning Lesson: AM in the PM

OBJECTIVE Demonstrate that AM radio signals can travel many 100's of miles at night. The student will listen to as many radio stations as possible obtaining the call signs and places of origin during the nighttime hours 30 minutes to two hours during the evening or early in the morning before sunrise. Radio with an AM bandPen/pencil Radio Station Reception Form Supply the students with a list of the local AM radio stations. NOAA Weather Radio

Background

Radio signals bounce off the ionosphere all the time. However, we often do not notice it except at night when we listen to AM radio. These signals can travel 1000's of miles.

Procedure
1. Most homes should have some sort of portable AM radio. If not, automobile radios will often work well. (Have the student to ask a parent or guardian to help supervise them while listening to AM radio stations in an automobile. Students without a valid driver's license are not to be left alone with keys to the vehicle.)

2. Search for a radio signal that is not from a local station. (Most will be faint but the reception is usually clear enough to understand.) Stations broadcasting sporting events are easy to identify.

3. Listen for the station identifier "call sign". The call sign is a three or four letter identifier beginning with the letter "W" or "K".

4. Log the call sign and location (city) of the transmission. Also note the quality of the signal. Was it loud? Soft? Fade in and out? etc, and the what was broadcast; news, sports, etc., Was there a lot of static?

5. Search for another signal and repeat.

6. You can compare the students results with the list of clear channel stations. It is entirely possible that students hear broadcasts that are not local and not one of the powerful nighttime radio stations.
Discussion

During the daytime, the distance the AM radio signal travels is the distance the ground wave travels based upon the power of the transmitter. The signal also reaches the ionosphere.

The D-Layer of the ionosphere plays an interesting role. While there are no radio signals reflected off this layer it does absorb AM radio signals. Because of the absorption of the signal, there are also more radio stations transmitting during the daytime and these stations can often transmit at higher power.

At night, the D-Layer disappears and the transmitted signal can then bounce off the ionosphere and return back to the earth. As a result, some low power stations must cease transmission at sunset while others reduce their transmitted power to reduce interference.

However, there are high power clear channel stations that can broadcast all night. It is these stations the students will most likely find.

Fast Facts
Stations are assigned a call sign beginning with "K", if they are west of the Mississippi River, and beginning with "W" if they are east of the river.

However, there are some stations with only a 3-letter call sign, or stations with a "K" east of the Mississippi, or "W" on the west side. These stations have been "grandfathered" meaning they are allowed to keep their current sign until the station's ownership changes hands.

Before 1922, three-letter call signs were arbitrary. In the 1920's, the FCC issued three-letter call signs based on slogans, such as WGN, which is owned by the Chicago Tribune, the "World's Greatest Newspaper", and WLS, then owned by Sears, Roebuck, the "World's Largest Store".

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