|MadSci Network: Physics|
It's actually a very complex process by which radio waves propagate from one point to another point on the earth. When you first think about it, it seems like it might be a relatively straight-forward process. However, there are so many factors that influence the process that it quickly becomes overwhelming. I will give a very simple view of what happens but I would encourage you to search the web and your local library. There are many sites and books that have very detailed explanations of the phenomenon. A good place to start on the web would be at: http://www.radio-electronics.com/info/propagation/index.php Or http://tpub.com/neets/book10/40.htm ===================================== Radio waves are a form of electromagnetic radiation just as like visible, UV, infrared and X-rays. What makes it very different and somewhat peculiar in its behavior is the frequency or wavelength of the radiation. All the other forms of radiation that I mentioned have wavelengths much much smaller than common objects we would know like a book, a house, a car, the window of your room, etc. In many ways this makes them relatively easy to describe in terms of propagation. Radio waves on the other hand occur in a very large range of wavelengths (millimeters to thousands of meters). The wavelength of FM radio waves is on the order of 1-10 meters and AM radio waves are on the order of 100-1000 meters. Therefore they occur in a range of wavelengths about equal to common everyday objects. There are two main ways radio waves interact with objects. One is because of what I described above; the wavelength is about the same as ordinary object. Radio waves tend to exhibit diffraction, that is they can bend around objects that are near the same size as their wavelength. So, radio waves can go around objects in general. The other way radio waves interact with objects is by reflection. Radio waves can bounce off objects. Often when radio waves hit an object like a building for example it will do both, refract around the building and reflect off of it as well. Now finally to your question. The main thing that makes radio waves go through an object or reflect off of it is the electromagnetic nature of object itself. Most common building materials like wood, plaster, glass, brick, stones, cement, etc are very poor conductors of electricity. These materials are referred to as dielectric materials. Materials like metal, solutions of ions (like the ocean waters) have the ability disperse an electrical charge and so are good electrical conductors. Radio waves will reflect off of good conductors but can penetrate to varying degrees dielectric materials. This is why radio waves can go through your house but not through metal doors or walls. This is the straight-forward part but what about buildings with steel frames or large amounts of metal reinforcement bars? Everyone knows that it is relatively easy to get a radio signal in your house but not in a large office building. The reason is that the large metal interior skeleton of the building acts as a metal cage with distance between conducting elements (the steel) about the same or smaller than the wavelength of the radio waves. This is where refraction comes into play again. Each of the metal structural elements refracts the waves but because of the close the proximity of the elements to each other in the building skeleton and because the wavelength is just right, the waves refracting around the steel beams destructively interfere with each other. This means the signal is either completely attenuates or substantially weakened to the point that reception is horrible. I will stop here even though there are more details that could be explained regarding the different kinds of radio waves and reflection from other objects like the ground, the oceans and lakes, and interaction with the atmosphere (primarily the charged layer called the ionosphere), but I think this is enough to get you started. The rest I will leave up to you to explore on your own.
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