Re: Simple but Fundamental Antenna Question

From: John Popelish (jpopelish_at_rica.net)
Date: 07/24/04 

Date: 24 Jul 2004 07:59:18 -0700

j2israel@hotmail.com (JLD) wrote in message news:<aecbba1f.0407221413.1520a904@posting.google.com>...
> Hello all.
>
> I have never studied RF communications. With that said I have a
> question about how antennas work, and I am talking about simple
> antennas (simple straight wires) like on a car.
> How can an AC current exist in a wire that is connected to some
> circuitry at one end but is unconnected, open, floating at the other
> end? How can the current alternate back and forth in an open wire?
(snip)

You have run up against one of the limitations of viewing electric
circuits as consisting of lumped components (resistors, capacitors,
inductors, etc.), only. Since electric energy gets from place to
place as a wave, traveling at the speed of light, an understanding of
things like transmission lines, antennas and free traveling waves have
to take this property of space, matter and time into effect if it is
going to make any sense at all. A whip antenna may be visualized from
the energy's point of view (and as a mental transition from lumped
circuits) as a long chain of little inductors with small capacitors
connected, all along the line between inductors into nearby space
(essentially one plate capacitors). If the antenna were surrounded by
a tube of conductor, it would be easier to picture where the far end
of those capacitors connected, but that would be a tansmission line,
rather than anatenna.
The property that makes a whip an antenna instead of a transmission
line is that the magnetic fields of those tiny inductances and the
electric fields of those tiny capacitances do not terminate into a
conductive container, but into waves of energy that propagate away
from the antenna, always leaving the space next to the antenna
available to soak up magnetic and electric fields from the next part
of the cycle. Those waves consist of globs of electric and magnetic
fields that build each other in the direction they are moving and
extinguish each other where they have been.

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