A new step forward: Now i'm running 2 kV on 2970 Hz.
Further ERP calculations were slightly wrong. With 470 pF antenna
capacity i'm getting Z = 1 kOHm -j114 kOHm, so at 2 kV the antenna
current is now 17.5 mA and the ERP is 39 nW.
I'm testing this voltage level now and observe the signal in 3.5 km
distance.
Once the 3 kV level is reached i'm planning to do a portable RX test
with an E field antenna in a nice location which is 7.9 km distant.
That would still be in the near field for this ultra long wavelength
but i find it would be a distance that can be interpreted as
respectable: 0.078 Lambda :-) The far field starts at 16 km distance...
73, Stefan
Am 10.06.2016 18:01, schrieb DK7FC:
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 101 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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