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Mon Oct 14 20:58:09 EDT 2013

What is an antenna


> What does something like an antenna do? How does it know what to do?
> Is it mechanical or electrical or both or something else?


Antennas exploit pure electro-magnetic (EM) phenomena which are
captured by Maxwell's laws[1].

They have no moving parts (not mechanical).

The way they operate is quite cool and antenna design is a deep black
art.  The creative act is to come up with original configurations of
metal in space.


I had it re-explained early this year by an old-time EE / HAM friend.
I thought it would be interesting to try to explain to see if I get it.
Here's an attempt:

An antenna combines these two phenomena:
(1) standing EM waves in a conductor
(2) EM radiation in vacuum / air

- Apply an alternating current (AC) of single frequency to a piece of
  metal.

( Note that most radio signals are "small band" which means for the
  sake of antennas they look like pure sine wave oscillations.  Small
  band here means that the carrier frequency is much higher than the
  modulation applied to it == information content. )


- If the dimensions of the metal are tuned to the particular frequency
  of the alternating current, an oscillating standing wave pattern is
  created inside the metal[2].

( Think of it as the EM waves being "trapped" inside the metal by the
  metal/air border: they reflect off of the edges of the metal. The
  standing waves are pretty much the same as for other physical
  vibrations, i.e. for mechanical tension/displacement vibrations in a
  bowed string, or air pressure vibrations a flute. The reason why
  this happens for an antenna is the same as for sound: electrical
  wave propagation speed is finite. )


- In vacuum (or air), any oscillating or otherwise accelerating
  electrical charge "radiates" a tiny bit of energy that propagates
  through space.

( One way to think of it is that an accelerating / oscillating
  electron changes energy, and that energy needs to go to / come from
  somewhere.  This can only be done by emitting or absorbing photons.
  For a well-explained entertaining read on the theory of electrons
  and light, see [3].)


- If you put a put a bunch of these oscillating charges next to each
  other (as in the manually crafted standing wave pattern in an
  antenna) you can "aim" the energy by using constructive/destructive
  interference: in some points in space contributions add up, in other
  they cancel out.

You want to lay out the metal in such a way that when you apply an
alternating current to the metal, most of the energy radiates away in
the direction you desire.



[1] http://en.wikipedia.org/wiki/Maxwell%27s_equations
[2] http://en.wikipedia.org/wiki/Standing_wave
[3] http://en.wikipedia.org/wiki/QED:_The_Strange_Theory_of_Light_and_Matter



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