MadSci Network: Engineering |
Greetings:
Your project sounds very interesting. I have an experiment that you
might perform to demonstrate the attenuation (the scientific
word for
blocking) of radio waves.
Radio Frequency
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)
radio dials.
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.
Radio Wavelength
To understand my answer to your question you must know the wavelength
at
the radio frequency.
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.
Radio Wave Attenuation
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
95% of radio power and absorbs about 5 % of light and radio power.
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.
Example: You can receive radio waves in most houses made of brick,
wood,
plaster, wall board, cement etc.. Buildings made of metal or metal
coated
glasses, or steel reinforced concrete, reflect most of the radio
energy
and you cannot receive radio signals inside of them.
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
radio power are reflected and transmitted. When we work with radio
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.
Good luck with your project.
Your Mad Scientist
Adrian Popa
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