### 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
Message:
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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:

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

```

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