At 15:19 23/03/2002 +0000, you wrote:
Hello, lowfers!
I wonder is any methods to estimate theoretically groung loss resistance
of transmitting LF antenna (T-antenna with wire conterpoints mainly but
others antennas also).
I have good expirience in theoretical physics but I do not know, how solve
the problem correctly...:-( My attempt to use EZNEC-2 was unsucsesfull.
Dear Alexander, LF Group,
I am not aware of any reliable way of calculating ground loss resistance -
The problem with using EZNEC or similar is that it does not provide a
realistic way of modelling a buried ground system. Some old antenna books
provide some info about ground losses, but this data is concerned with big,
professional LF antennas, and does not seem to be representative of the
performance of much smaller amateur antennas. From a theoretical point of
view, a small LF antenna could probably be modelled as a lossy capacitor -
but this requires a knowledge of the dielectric properties of the soil
under the antenna, and the field distribution around the antenna, which are
usually not known with any accuracy.
Practical figures for ground loss of amateur top-loaded vertical antennas
at 136kHz vary from around 10-20 ohms to a few hundred ohms at the extremes
- typically, tens of ohms can be expected - the larger or higher the
antenna, the lower resistance gets. The loss resistance of a particular
antenna is frequency dependent, very approximately proportional to 1/f.
Environmental effects are significant - if there are many trees or
buildings near the antenna, loss will increase. However, even in an "ideal"
location in an open field, the loss resistance is not greatly reduced. On
the whole, people have been disappointed with the effects of improving
ground systems using radials, counterpoises, and earth rods - the actual
resistance of the ground connection seems to be only a minor factor in the
overall losses.
Will be interested to hear about any of your investigations on this
subject. Also, I'm sure many of us would be keen to try and receive your
136kHz sigs - using QRSS or similar modes, you can go a long way with low
power and quite small antennas.
Cheers, Jim Moritz
73 de M0BMU
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