Re: Communications
- From: dubious@xxxxxxxxxxxxxxxxxxxxxxxxxxxx (Bilge)
- Date: Sun, 09 Jul 2006 21:45:30 GMT
Joe:
Hello everyone,
I have a few questions:
Why are typical communication systems (mobile, satellite) done in the
radio frequency range and not, for example, infrared, visible, UV or
x-ray?
The choice of carrier frequency depends upon a number of factors,
like the ease of modulating the carrier, the bandwidth it provides
and so on. Communications systems have typically used rf frequencies
because rf is easy to implement in transmitters/recievers, is very
economical and is not highly directional (unless one wishes to make
it so, in which case that is also easy to accomplish by carefully
designing an array of antennas). Light is being used more frequently
now that fiber optics have become a well developed and economically
viable alternative to wires. You can think of an optical fiber as
a waveguide for light which, unlike rf waveguides (with the exception
of coax and other transmission lines for low frequencies), is flexbile.
Couldn't you use visible light as a carrier and mdulate that?
Sure, and if you purchase a laser pointer, a photodiode (or
phototransistor), and few other odds and ends (op-amps, resistors,
etc.), you could build your own transmitter and receiver for
about $20.00. The major expense is the laser pointer from which
you would obtain the laser diode. (You could use an LED as is
done for infrared remote controls, but the range would be limited).
Why does visible electromagnetic radiation require line of sight but RF
does not?
The best way to understand this is look up ``Huygen's Principle.''
A few pictures are going to be a lot more illuminating than a lengthy
explanation would be.
Why does IR radiation feel "warm" and x-ray does not, but x-ray is
far more energetic?
The intensity of the radiation and the stopping power upon which the
radiation is incident determine the power absorbed by any given material.
For visible and infrared light, the stopping power of your skin is pretty
good, as you can see by virtue of the fact that light doesn't pass through
you. If a large amount of power is absorbed over a small area of your
skin, you will feel that as heat.
X-rays, are not readily stopped by your skin, which is why medical
X-rays are useful. (The stopping power is a function of the density and
Z of the material, so bone which is much denser than skin, stops X-rays
much better than skin does.) With a sufficiently intense X-ray source,
you would certainly feel your skin get hot for the same amount of power
absorbed.
.
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- From: Joe
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