On Sun, 01 Oct 2006 01:23:51 +0300, Ceriel Nosforit
wrote:

A few quick questions; the efficiency at higher frequencies does only
increse logarithmically, correct? Around where is the 'knee' where
increment of frequency no longer provideas a significant increase in
efficiency? -

You can model the antenna system as a series connection of a loss
resistance and the radiation resistance. The loss resistance will
convert the RF power created with your expensive gear into heat :-),
while the power "dissipated" by the radiation resistance will actually
disappear as EM fields.

For full sized antennas, the loss resistance is small compared to the
radiation resistance and most of the RF power generated will actually
radiate. However, even with 1/4 wave vertical antennas, the ground
losses can be significant, if not ground planes are used, but the
antenna is grounded with grounding electrodes.

When the antenna size is below perhaps 1/10 wavelength, the radiation
resistance drops by the square of frequency, so the radiation
resistance can quite easily be well below 1 ohm at LF and below. The
loss resistance (include grounding and loading coil losses) can be
several ohms, thus the majority of the generator power is dissipated
in the losses and only a very small part is actually radiated by the
very small radiation resistance.

While the metallic conductor skin effect losses are proportional to
the square_root_ of frequency, the ground losses are harder to
predict, so the total efficiency at VLF frequencies is almost
proportional to the square of frequency (not logarithmic). The 'knee'
would be at 1/4 wavelength for a vertical antenna. At even higher
frequencies, the radiation resistance would vary cyclically.

Paul OH3LWR