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Blocking Radio Waves
Name: Tomas
Status: student
Grade: 6-8
Location: FL
Question:
I am doing an experiment on what materials can block radio
waves, and I am having some trouble attempting to quantify the
materials I will be testing. I need to know if the amount of
material I test will have anything to do with if the radio wave will
be blocked or not. Also: what common, easily attainable objects are
emitters of radio waves (excluding things like cell phone towers)?
Replies:
Tomas,
A bit of advice on the first part of your question, radio waves usually
tend to bend around many objects quite well, reducing their apparent
ability to block them. The more fully encased your receiver/detector
can be, the more reliable your results. In most cases, the thickness of
the material can have some effect, but there are many materials which if
they are thick enough to work with, will be completely effective against
the power levels you will likely be working with.
As for a good test source, if you can find a walkie talkie, or other two
way radio that you can transmit on fairly continuously, you will have a
decent source. However, it is important for your measurements that its
distance from your receptor be kept at a constant level.
Ryan Belscamper
Hi Tomas,
I think you may not be clear on how radio waves are shielded. Radio
frequencies pass though any non conductive material, regardless of
thickness. Radio frequencies are affected by conductive materials such
as metals, but you cannot effectively "block" radio frequencies merely
by putting a metal plate (for example) between the transmitter and the
receiver. In fact in order to have any significant reduction in
signal, such a plate must have a width and height of at least several
wavelengths, and even then the difference would only be small. What is
more of a problem, is that radio wavelengths are not small: FM radio
waves for example have a wavelength of about 10 feet, so you would
need a huge metal plate in order to notice any effect at all. At cell
phone frequencies, the wavelength is smaller, but still in the range
of a foot or more.
The only effective way to shield an object from radio frequencies, is
to place it in a shielded box. That is, a metal box that completely
encloses the object. This is why radio reception in a concrete
building is often poor: The steel rebar in that reinforces the
concrete, acts as a metal box. The concrete itself does nothing.
Generally, shielded enclosures made of higher conductivity metals like
copper or aluminum work better than those made of lower conductivity
metals such as brass, steel, or stainless steel. The higher a metal's
conductivity, the thinner it can be and still be effective.
Regards,
Bob Wilson.
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Update: June 2012
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