Hi John and group!
Thanks a lot for your coments. It is a very interesting subject for me
because I am not a HAM only but a physicist also. I have do some
theoretical investigation of power absorption by ground when there is
horisontal wire upon the ground. And I have find that the ground losses
is about the same then losses in coper! Pointly ground loss and coper loss
are the same when wire diameter is 7 millimeters. If wire diameter is
less then 7 mm the coper losses is main! I was very suprised because I
know that ground losses is 30 - 100 Ohm... I thought that the reason is
the simlification of my theoretical model. It may be so, but now I
undestand then it is not the only reason.
It should be very interesting to carry out a "pure" experiment when no
enviroment losses is present. I do not know when I can do that. But I hope
to do that this summer time when being out of city.
Now I have understand that 30 - 100 Ohm is not really ground losses but
an enviroment one! So I have a subject to think how to describe it
theoreticaly. It seems to me that any attempt to measure the ground and
enviroment losses separately (unfortunely I do not know how to do that :-(
) are wery worth...
Now my 136 kHz TX is only 50 W out and antenna is about 15 meters up.
Unfortunely I am so far from Europe and there is no LF funs in nearly
region yet...
Also by my own investigation I get some sucses in Q-factor calculation
of loadin coil. It is very usefull when desining a coil. Experimental
mesurement has confirm my theoretical estimation. If this is interesting
for anybody I can send some info on this matter...
On Sat, 23 Mar 2002, john sexton wrote:
Hi Sasha,<br />
The common view often reported on this reflector is that there is no acceptable
theoretical model. In practice, ground resistance can vary from as low as 30 ohms up
to as high as 300 ohms, but typically it is about 100 ohms. It appears to be more
dependent on the environment of the antenna rather than on the ground itself. At
least that is certainly true of amateur antennas. Professional antennas can be
engineered to be clear of environmental obstructions and can have a very large earth
mat.<br />
<br />
In my experience it varies very considerably (10s of ohms) when I move the loading
coil away from nearby objects, even a few metres makes a big difference. Trees
(especially when in leaf), buildings, etc. absorb much of the radiation and this
appears as a high resistance when the antenna is tuned to resonance. It would be
difficult if not impossible to model such losses.<br />
<br />
A top loading coil or division of the loading coil into two parts, one at ground
level and another high up has been found to help a lot, but this requires top loading
wires to provide capacity above the coil and is mechanically difficult to
construct.<br />
<br />
You will find a lot of information on amateur web pages. A good place to start is
ON7YD's page at http://www.qsl.net/on7yd/136ant.htm.<br />
<br />
If you do find a model that works, please let us know.<br />
73 and Bcevo Xopowevo,<br />
John, G4CNN<br />
[email protected]<br />
http://www.g4cnn.f2s.com<br />
<br />
<br />
73 de RA9MB/Alex
http://www.qsl.net/ra9mb
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