MadSci Network: Physics

Re: Why do radio waves pass through some materials and not others?

Date: Sat Feb 3 09:28:37 2007
Posted By: Tom Hancewicz, Staff, Advanced Imaging and Measurement, Unilever Research & Development
Area of science: Physics
ID: 1169422873.Ph

  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:



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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