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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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