DAY THREE - RADIO TRANSMISSIONS

This day starts with a similar evaluation of the FM Radio Challenge as for the AM Radio Challenge.  Two things should be noticed:  Daytime vs. nighttime doesn't matter as much for FM and FM stations can not be picked up nearly as far away, leading to the question:  What is different between AM and FM radio waves?

Prep Work:

  1. Test FM reception in classroom.  It should be better than AM reception, but it still may be necessary to place the radio by the window.

Materials Required:

  1. Radio
  2. Calculator (for calculation of wavelengths and frequencies)

Class Outline:

Begin class by repeating the previous day's activity, except have the students share their FM stations.

Two questions can then be discussed:

Can they receive all of the FM stations during the day as well as they can during the evening?

Is the furthest nighttime FM reception as far as the furthest nighttime AM reception?

Students should write answers to both of these questions in their notebooks and then share their responses with the class.

 

After this discussion you can start to provide answers to the question:

What's different about AM and FM signals and how they travel?

You should note that FM waves can only travel as ground waves, and, therefore, the distance you can receive an FM signal from is limit by your (our, more specifically, your radio's) ability to "see" the radio transmitter tower.  Once the tower is below your horizon, you lose the station.

However, AM waves can "bounce" off of the ionosphere, and therefore you can "see" them over the horizon!

There is a very simple discussion of this in a web-site maintained by the Louisville Courier-Journal.

There is a wonderful image in the Windows on the Universe discussion of radio wave bands showing the difference between ground waves (FM, TV, and AM) and sky waves (AM).

 

So, some radio waves bounce off of the ionosphere and others do not.  Why?   What is different about the radio waves?

To answer this question, you first need to discuss relationship between wavelength and frequency:

Therefore......frequency up = wavelength down!

AM and FM bands are defined by their ranges of frequencies.

(As an aside, AM bands are considered medium wave lengths and FM bands are considered very short wave lengths.)

(The unit of a Hertz (Hz) is 1/second.  kHz are kilohertz, or thousands of hertz and MHz are megahertz, or millions of hertz.)

Students (as describe in the Homework assignment below) will need these frequencies and the speed of light (3.00 x 108 m/s) to calculate the wavelengths of the AM and FM radio waves.

 

After the wavelength-frequency discussion, students can then be asked:

What is different about the ionosphere?  Why do radio wave bounce off of it at all?

This last question should be discussed and students ideas shared in preparation for the next days class.

Homework assignment:

Students should calculate the range of wavelengths found in both AM and FM radio waves.   These calculations can be performed using the frequency ranges given above and relationships discussed between wavelength and frequency.