Re: Simple but Fundamental Antenna Question

From: John Jardine (john_at_jjdesigns.fsnet.co.uk)
Date: 07/25/04 

Date: Sun, 25 Jul 2004 02:05:58 +0100

John Popelish <jpopelish@rica.net> wrote in message
news:855a1a14.0407240659.79813146@posting.google.com...
> 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.

What I've never really been able to rationalise, is that we know with
certainty that real energy is being radiated from the aerial yet we're told
that E=MC^2 no longer applies, as the EM 'energy' travels at light speed
therefore can't have mass. Photon thingies are then invoked. I'd feel easier
if I knew the power supply was losing mass all the time the transmitter was
switched on. (like Newtonian mechanics and powering a rocket by throwing
mass out of the back).
regards
john

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