Hi all,
I've added a small air gap to my transformer and increased the number of
turns on the transformer (which meanwhile just acts as a loading coil).
Now there are 7 layers and about 1400 turns. The new resonance is 2.87
kHz. I put 6.8 nF (and 47kOhm in parallel) in series to the antenna to
move the resonance to 2.97 kHz. It is better to 'sit' on top of the
resonance because there is a risk of a flash over when it starts to rain
(the C of the wet antenna would be higher, so i come closer to the
resonance and so the voltage would rise).
At 1.4 kV i'm now getting about 14 mA antenna current and 25 nW ERP!
The signal is now well visible in my 3.8 mHz window at
http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber2.html
At 5 UTC i'm running a first EbNaut message in 10s symbols, just local,
just for the fun.
The next step would be to wind further 1600 turns on the transformer to
reach its final voltage limit of 3 kV (rms) and 100 nW ERP.
A photo of the transformer
https://dl.dropboxusercontent.com/u/19882028/VLF/ULF%20loading%20coil%206.1H.jpg
73, Stefan
Am 08.06.2016 11:15, schrieb DK7FC:
Hi all,
I've got a first result with my ULF project: One of the ideas to
generate and radiate a signal on the 100 km band to use a ferrite
power transformer with using two E cores out of N27 material. These
are available at https://www.buerklin.com/de/e-kern/p/83d970
At 2970 Hz i can apply 1 V / turn without saturating the core, maybe
even more. I think i can generate 3 kV (rms) with one transformer
without a risk of a flashover. And i can set an air gap between the
two E cores.
For 2970 Hz i need L = 6.1 H! Now without an air gap i need about 800
turns (hand wound!)to resonate the antenna. So i can apply 800 V now.
On 2970 Hz, my antenna radiation resistance is 70 uOhm. With 800V on
the wire it radiates 6 nW!
The distance to my remote site is 3.5 km or 0.035 lambda.
I'm TXing since last night and got a trace of up to 15 dB SNR in 424
uHz. Quite weak but it's a first step.
The signal was radiated with an E field antenna and received with a H
field antenna, so there could be a better result in the near field.
I' now planning to wind more turns on the transformer and add an air
gap to keep the inductance and rise the voltage limit.
73, Stefan
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