On 9/14/2015 11:14 AM, gareth wrote:
Taking one's cue from the Alford Slot Antenna, let us propose the use of the
fields in
the air gap of a parallel plate capacitor as the source of RF radiation.

To do this, we will terminate our 50 ohm coax with a series LC circuit of 50
ohm
resistive impedances (so one will be 25+jX and the other 25-jX) at the
operating
frequency.

I think your intent and your values don't correspond. The +jX and -jX
terms are correct for the imaginary parts of the impedances, but you
don't want the capacitor and inductor to have 25 ohms each of
dissipative, real resistance do you? That would leave nothing for the
transmission resistance if you were to radiate any energy. So either
you would be designing an antenna at other than 50 ohms impedance, or
you are trying to design an antenna which doesn't radiate.


Very little of the energy supplied to our antenna will be radiated, and most
will be stored
as energy in the resonant LC circuit, until such time as the stored energy
outdoes the incoming energy,and
the LC circuit will now act as an energy source, sending it back down the
coax.

Not sure what you are trying to say with this. Why would the LC circuit
ever send power back down the feed line?


So, we'll have a short antenna, matched to its feeder, that is not radiating
all the power fed to it, and
is returning some of that power back down the coax.

The antenna you describe will be absorbing and dissipating nearly all
the energy it receives from the feed line as heat in the capacitor and
inductor.


(An even simpler case, although it would give a reactiv match, would be the
capacitor alone, but think
for one minute, why do you get the 90 degrees phase relationship between the
volts and current
in a capacitor? Because it acts like an energy source in its own right!

Not so much when it has a resistance of 25 ohms dissipating a huge
amount of power on each cycle.

--

Rick