### Re: Which materials block radio waves the most (and why)?

Date: Tue Feb 26 22:01:14 2002
Posted By: Adrian Popa, Director Emeritus, Hughes Research Laboratories
Area of science: Engineering
ID: 1014075960.Eg
Message:

Greetings:

Your project sounds very interesting. I have an experiment that you
might perform to demonstrate the attenuation (the scientific word for

Radio waves are electromagnetic waves and travel at the speed of light
which is 186, 280 miles per second (983,558,400 feet per second). The
voltage in a radio wave alternates back and forth between plus and minus
many times per second and we call this the frequency of the radio wave
in cycles per second. Scientists have named frequency Hertz
(abbreviated Hz) after Heinrich Hertz, a German scientist, who succeeded
in transmitting the first radio waves across a room in 1888. Thus when
you hear a radio wave has a frequency of one megahertz ( 1 MHz), it
means one million (1,000,000) cycles per second. This frequency is
usually marked 10 or 100 on the middle of AM (amplitude modulation)

A radio frequency of 100 megahertz (100 MHz) means 100 million
(100,000,000) cycles per second. This is usually marked 100 on the
middle of FM (frequency modulation) radio dials.

To understand my answer to your question you must know the wavelength at

Wavelength equals the speed of light divided by the frequency.

For a frequency of 1 MHz in the AM radio band the wavelength is:

Wavelength at 1 MHz = 983,558,400/1,000,000 = 983.5 feet

For a frequency of 100 MHz in the FM radio band the wavelength is:

Wavelength at100 MHz = 983,558,400/100,000,000 = 9.84 feet = 118 inches.

There are two general types of matter (substances) in the universe that
affect electromagnetic waves, conductors and insulators
which are called dielectrics by scientists. Most, but not all,
conductors are metals, such as copper, aluminum, silver and gold. However,
salt water is also a rather poor conductor! Most, but not all, dielectrics
are non metals. Examples of dielectrics are paper, plastic, Teflon, glass,
ceramic and dry wood. Pure water is a good dielectric substance!

Reflection, Transmission and Absorption of Radio Waves

Light waves are also electromagnetic waves and I will use them for
examples; however, not all materials behave the same way at both light
frequencies and radio frequencies. For example cardboard is transparent
to radio waves and is opaque (blocks) to light waves. Light waves have a
frequency around 500 trillion cycles per second (500 terahertz or 500 THz).

When a radio wave hits a material some of the power is reflected at the
surface and some of the power is transmitted into and possibly through
the material. If the material is metal, almost all of the radio power is
reflected within the first few atoms of the material. A small amount of
power is absorbed by the silver atoms and converted to heat.

Example: a silvered mirror reflects about 95 % of light power and about

If the material is a dielectric, some of the power is reflected at the
surface and some of the power travels through the material

Example: Some light reflects from the surface of clear glass and some light
travels through the glass. The same is true for clear glass and radio waves.

As the radio wave travels through the dielectric material some of the power
is absorbed generating heat and some of the power travels through and comes
out of the other side.

Example: Light traveling through sun glasses has a few percent reflected
at the surface and between 10% and 90% of the light power absorbed in heating
inside the glass and a few percent of the power coms out the other side. Depending on the absorbing material in the glass, the same is true
for radio waves. However, the light absorbing material in the glass is usually
different than radio wave absorbing material .

This power absorption in a dielectric is called the Attenuation Coefficient
of the material. How much power travels through a dielectric depends on
both the thickness of the material and its attenuation coefficient.

Dielectrics such as cardboard, paper, clear glass, Teflon, some plastics,
pure water and many building materials have low attenuation coefficients
and radio waves reflect from them and also easily pass through them.

plaster, wall board, cement etc.. Buildings made of metal or metal coated
glasses, or steel reinforced concrete, reflect most of the radio energy

Earth contains many different materials that absorb radio waves and so you
do not receive radio waves inside of long tunnels. However, some long tunnels
have wires placed through the tunnel to transmit radio waves, so that
drivers and emergency vehicles can still hear their radios while driving
through them.

Experiment Number 1

Now let me discuss metal wire grids such as screens, chicken wire, chain
link fences etc. Grids are mostly space with a small amount of wire in
them. What happens to radio waves that they hit a metal grid? The answer
depends on the wavelength of the radio wave. If the holes in the wire
mesh are greater than one tenth of a wavelength across, most of the radio
power passes through them and a small amount is reflected. If the holes
in the mesh are one hundredth of a wavelength across or less. Most of the
radio power is reflected and almost zero is transmitted through the grid.
At sizes of holes between 1/10 and 1/100 wavelength, different amounts of
transmitters and receivers in the laboratory we often work inside of screened
rooms. This way we block out external radio signals and keep in the radio
waves that we are working on. Thus we can have hot or cooled air
circulating within the screen room to keep it comfortable and to let the
heat generated by the radio equipment escape.

For 1 MHz AM radio 1/100 of a wave length is 118 inches
For 100MHz FM radio 1/100 of a wave length is 1.18 inches.

Experiment Setup:

Make 3 boxes or cylinders about 2 feet long and 2 feet wide, one made of
chicken wire with very large holes, one made of copper or aluminum screen
with small holes and one made of cardboard covered with several layers of
aluminum foil with no holes in it. Place an AM/FM transistor radio on a
wooden platform in the middle of each box and compare the strength of
several strong and weak AM and FM radio stations in open air with the
strength of the same stations inside the boxes.

Expected Results:

Because of the difference in wavelengths you should be able to hear the FM
stations and weak or no AM signals inside the chicken wire box.

The screen box will probably block most FM signals and all AM stations by
reflection.

The foil box will block all AM & FM signals by reflection.

Experiment Number 2

You could also compare AM & FM signals in a cardboard
box, a wooden box and perhaps a glass box (fish tank with a metal lid). The
AM and FM radio signals strength should be the same inside and outside of
the boxes. You should not be able to receive radio signals inside of
an aluminum foil covered box.

NOTE: Radio waves are very sneaky and can get through the smallest slits
in metal boxes so be sure that you overlap all openings very well or make
a double layer covering the seams of the boxes. Also the radios must be
battery powered for radio waves can come into the boxes along the power
cable.