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
inv_L_Rloss.jpg
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