Hi ULF,
In the past days i wound the rest of the available enameled cu wire
onto the coil, now that's 4 kg of wire. This reduced the resonance
frequency (when using a capacitor which has a C that is equivalent to
the antennas C) again, from 3.667 kHz down to 3.537 kHz (so L must be
4.05 H).
Today i took the coil on the roof and switched it in series to the
'small' coil that i use for 6.47 kHz (must be 1.2 H). I had clearly
expected that this will reduce the resonance frequency to 2.97 kHz and
even below. But obviously i am not a friend of calculations :-)
I just thought "Oh well it's just a decrease of 567 Hz that i need. The
coil that is fine for 6.47 kHz will do that easily". But then i found
that the resonance frequency just dropped to 3.185 kHz. So the series L
seems to be 5 H, which is still to low :-/
Now the coil is back in the office, further wire is ordered. The next
experiment will be delayed by two weeks or so. More time for 6.47 kHz
tests :-)
A needless effort one can think but it was interesting to check the
actual resonance and i enjoyed the nice wx during the afternoon.
And BTW i saw that i can easily run > 200 mA on that frequency. 200
mA at 3185 Hz and 5 H means 20 kV which is an ERP increase of about 12
dB relative to my 31 km distance ULF experiment :-) The ERP at 20 kV
would be 4 uW then.
Soon more on that project...
73, Stefan
PS: Of course i know that i can couple the two coils and will get more
L then. But this would be a mechanically and electrically instable
construction.
Am 08.10.2016 01:30, schrieb DK7FC:
Hi ULF,
This night i have completed the 4th stack of my large ULF loading
coil.
An image: https://dl.dropboxusercontent.com/u/19882028/ULF/20161008_ULF_TX_coil.jpg
The exciting question was, what is the new resonance frequency together
with my antenna. In the lab i put an equivalent C in parallel and found
the resonance peak at 3.67 kHz. So the L seems to be 3.8 H
now. This is still not enough. The l/d of the coil dimension becomes
larger now, so i need more and more wire to reach the final resonance
frequency of 2.97 kHz at about 500 pF.
I will do a next test on the actual resonance frequency in some days,
when the WX permits. I will also check where the resonance can be found
when switching the small coil in series that i use for 6.47 kHz now. It
could be quite close to 2.97 kHz then! So maybe this allows a next QRO
step on 2.97 kHz: 100 mA antenna current must be reached for the
planned experiment in 65 km distance.
BTW the 4 stack coil is wound out of almost 4 kg of enameled Cu wire
with 0.4 mm diameter. The layers are isolated by Kapton tape. My
assumption is that it will hold 20 kV now.
Ah and the DC resistance is 445 Ohm now. When connecting 275 V
AC / 50 Hz, i can measure 269 mA. So the actual inductance is 2.93 H
(beeing farer away from the self resonance frequency)
I can tell you it's quite fascinating to operate down on these low
frequencies :-) To be continued...
73, Stefan/DK7FC
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