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> Education & Outreach > Pre-College Resources > AM Lesson > Day 4
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:
Materials Required:
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:
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:
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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