LF,
I've just done some simulations in EZNEC. The model is my 300m vertical
that will be used on 137 kHz, probably this weekend.
When choosing a lossless gound and no wire losses i get the radiation
resistance of that antenna. It is 8.37 Ohm at 137.7 kHz. The reactance
is -j588 Ohm, so 680 uH is needed to compensate that. Assuming 25 Ohm
earth losses and 5 Ohm coil losses, the antenna efficiency is about 17
% (!).
If i choose copper losses for the wire and keep the ground lossless
AND add a L of 1 mH (plus 3 Ohm assumed coil losses) in a height of
100m (where i add the 100m wire to the 200m vertical wire, so no cut is
needed), i am getting R(rad+wire)=22 Ohm and a reactance of j17 Ohm.
Since the ground will add some capacity in this series circuit the
actual reactance will be slightly capacitive again i assume. Assuming
25 Ohm earth losses will achieve about R=50 Ohm :-) and maybe almost no
reactive part, so no coil is needed at the ground and the antenna can
directly be connected to the RG58 cable (i would spend 10 kOhm against
ground due to static voltages). The radiation resistance would be 13
Ohm (calculated by assuming lossless coil/wire/ground). So the antenna
efficiency would be 26%.
Probably the effective height would be reduced due to stronger sagging
of the antenna wire. This will reduce the efficiency a bit, depending
on the wind.
Questions: Is my calculation reasonably useful? Would YOU add
that coil in a height of 100m above GND?
I tend to let it be since the improvement isn't that much and the
effort is high. Furthermore the coil losses could be higher since i
have to use a thin wire to keep the weight small...
About the weekend: WX forecast says there will be strong wind but rain
as well (not such a problem on LF).
I already have the permission of the German air traffic control for
that 300m antenna... :-)
73, Stefan/DK7FC
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