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DAY FOUR - RADIO SCREENS

The main objective in this experiment is to show how a mesh can block radio waves.   This will help answer the question "Why is a radar dish not solid?" and help provide an analogy for the reflectivity of the ionosphere.

You can compare mesh size and mesh material (aluminum, steel, fiberglass)

Prep Work:

  1. Obtain screens.  This experiment can be successfully done with replacement screens purchased at a home supply store like the Home Depot or HQ.  Both aluminum and fiberglass screens can be used.
  2. Meshes of different sizes can also be obtain, in an effort to demonstrate the dependence on mesh size to the ability of a screen to block radio waves of different frequencies (i.e., AM vs. FM).  However, a successful experiment of this type has not yet developed for this lesson plan.  If you have any ideas or are able to construct such a cage, PLEASE let us know!

Materials Required:

  1. AM/FM Radio
  2. Screens and Meshes - Aluminum & Fiberglass

Class Outline:

Have you ever notice that, as you drive over a suspension bridge, FM radio reception is OK, but AM radio reception can be messed up.  The cables of the bridge are of a spacing that can block AM waves but not FM waves.  (The cables are spaced too far apart.)  This experiment will at least show that a screen can block radio waves.

Part of this exercise is to demonstrate both the difference in the size of the wavelengths of AM and FM radio waves and to show how the incoherent scatter radar dishes do not need to be solid.

With your radio and screen in hand, do the following:

  1. Get a small AM/FM radio and go outside.  (This can be done inside, but it is always nice to eliminate the students doubts about the effects of tuning a radio inside.)
  2. Tune in an AM station.
  3. Place an aluminum screen over the radio.  What happens to the signal?
  4. Tune in and FM station.
  5. Place an aluminum screen over the radio.  What happens to the signal?
  6. This can be repeated with a fiberglass screen.....which won't block the radio waves.

Ask the students if they have ever traveled over a cable-suspension bridge (like the George Washington Bridge in New York.)  What happens to AM radio reception as you cross the bridge?  Is FM reception effected the same way?

It may or may not be desirable to get very quantitative with your class, depending on the level of the students involved.  If you don't think it will scare them off you can tell them that a mess of a size on the order of one tenth of the wave length of a radio wave will essentially act as a solid object to the radio wave.  Am radio waves are on the order of 300 m wavelengths (at 1000 kHz).  FM radio waves have wavelengths of 3 m (at 100 MHz).

The Millstone 440 MHz signal has a wavelength of about 70 cm.  Therefore, a 7 cm mesh would seem solid and 7 cm imperfections in the shape of the dish are inconsequential.

CONCLUSION:  The size of the mesh that "blocks" a radio signal is directly related to the wavelength of the radio signal.

(A qualitative statement that says:  "As we've shown, a non-solid object can block radio waves" can be just as effective.  This can be expanded to explain specifically why the Millstone radar can be a mesh and still work.  The point should be made that wavelength DOES matter.)

This wavelength dependency is also found in the ionosphere and is why AM waves are reflected and FM waves are not.

 

Homework assignment:

In preparation for the final day of the lesson (when, hopefully, everything will be tied together!) students should be encouraged to explore the MIT-Haystack web-site, the Window on the Universe, the Space Environment Center, or any of the other links provided in the Background Information page of this web-site.  The types of questions that students should now be able to look up information for and answer are:

  • What can help you determine when AM reception will be better?
  • What types of effects can fluctuations in the ionosphere have on the Earth's surface and the activities of humans?

Special thanks go to Rock Woodland of the MIT-Millstone facility for coming up with the original idea of blocking radio signals with different sized screens and continued discussions that lead to this activity.

 

 
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