Hello Stefan,
your calculations seems very resaonable to me. But if I compare it to the "classic" antenna with loading coil at ground level, I am not sure that it is worth the effort.
Using MMana-Gal a 300m vertical gives almost identical values as EZNEC (8-j604 Ohm).
The wire loss (1mm Cu wire) it about 2 Ohm, so the overall resistance rises to 10 Ohm.
Assuming you use a good loading coil (Q = 350) the coil loss is 1 ohm and the antenna impedance is 11 Ohm.
Plus 25 Ohm ground loss = 36 Ohm overall loss.
Overall efficiency = 8/36 = 22%
Adding a (lossless) 1mH inductance at 100m height gives 13-j20 Ohm (no wire loss).
When including wire loss the resistance rises to 15 Ohm.
Further assuming that the elevated coil has a Q= 100 it's loss is about 8 Ohm and the overall loss is 23 Ohm.
Plus 25 Ohm ground loss the resistive part rises to about 48 Ohm (very convenient), the small variometer (about 23uH) will hardly contribute to the overall loss (at Q = 350 the loss is less than 0.1 Ohm).
So the overall efficiency is 13/48 = 27%.
The "flying coil" will improve the antenna by 27/22 = 1.23 or 0.9dB. Maybe the extra sag will cause more loss than that.
But at the other hand, I have good experience with elevated coils, that can not be explained by improved current distribution (larger effective height).
So if it is not too much trouble making the elevated coil I would say: try it and you will know.
73, Rik ON7YD - OR7T
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
