Hi Jim.
I think I could knock up a quick 9kHz
RX here in Luton over the weekend if
that would help. I have been
following this thread with interest, could you
put up some diagrams
etc of your parallel capacitor and other antenna
configurations and
formulae.
73.
Ken
M0KHW
IO91TV
-----Original Message-----
From:
[email protected][mailto:
[email protected]]
On Behalf Of James Moritz
Sent: 24 February 2010 21:14
To:
[email protected]Subject:
LF: Re: VLF_8.79 kHz
Dear LF Group,
A couple of years
ago I did some measurements of Rloss of the 10m high, 40m
long
inv-L antenna at my home QTH, and also a near-identical antenna set up
in the middle of a field away from trees and buildings. The
attached graph
shows Rloss plotted against frequency over the
range 10kHz to 600kHz for
both antennas. At all frequencies, the
loss resistance of the open-field
antenna is much lower than the
home QTH antenna, which is surrounded by
numerous small trees. The
ground in both cases was 4 x 1m ground rods, close
to the feed
point of the antenna. The actual ground around both antennas
was
very similar - a waterlogged clay soil.
At 10kHz,
the open field antenna has Rloss of 50R, against 380R for the
home QTH antenna. Both antennas show a decreasing Rloss with
frequency -
this suggests dielectric losses are dominant (the
antenna voltage increases
at lower frequencies for a given
current) in both cases. The text books say,
for electrically
small antennas, that dielectric losses will dominate at
low
frequencies, while at high frequencies the skin effect will
eventually cause
resistance to start increasing - in the case
of the open field antenna, a
turn-over point might have been
reached at a few hundred kHz.
The 50R figure suggests that
antenna efficiency might actually be higher at
9kHz than people
are expecting - at least in an open field site. A suitable
loading
coil would be a problem. The antenna capacitance was around 350pF -
in these experiments I used a ferrite-cored coil of around 0.7H
with a Q of
about 150, but this had a loss resistance of about
300R. If you tolerated
loosing half the TX power in the loading
coil, a similar inductance with a Q
of around 1000 would be
needed. Increasing the top-loading capacitance of
the antenna
would definitely be useful...
Cheers, Jim Moritz
73 de
M0BMU
