Blocking radio waves
Ken
Here is a followup to our conversation about how radio waves are
blocked. Corby raised the question about what would happen to radio
waves if the walls in the room were framed with metal studs rather
than
wood.
My understanding is explained in this MadSci Network posting:
http://www.madsci.org/posts/archives/2002-03/1015162213.Eg.r.html.
It's
all worth reading, but the relevant section is under the heading
"Experiment Number 1". When a metallic mesh is smaller than 1/100th of
the wavelength, radio waves primarily reflect.
In this case we could consider the metal studs as forming a
reflector for signals that are greater than 16" * 100 wavelength.
Converting to metric, we get (16" / 39) * 100 ~= 41 m. Converting to
frequency, we get 3 * 108 / 41 = 7.3 MHz. So frequencies less than
this
will be reflected. Signals less than 16" * 10 wavelength or at least
73 MHz mostly will pass through.
I would expect electrical wiring in the wall also to attenuate the
signal, and the wall material itself should have less impact on the
signal. There's probably a lot of detail concerning particular
materials and wavelengths that I'm glossing over, similar to Krishna's
example of extremely low frequencies for deep marine submarine use.
In my research last fall, I tested use of a 1/4" wire mesh screen as a
reflector for a standard 433 Mhz whip antenna. The relationship here
is that 100 * 1/4" equates roughly to 433 MHz. Just holding the screen
by hand a few inches behind the antenna increased received signal
strength by 4-5 dBm.
Ken