Hi Luis,
Well, some PA designs using ferrite input transformers will have
problems at that frequency. But i built that H bridge for the guide
rail antenna experiments in a way that it can work down to ELF, i.e.
< 30 Hz. It simply uses IR2110 half bridge drivers. I'm also using
this PA in this experiment. So the output voltage is a rectangle. The
transformer would handle a rectangle but quite a high input voltage is
needed. However, if you use a capacitor in series to the primary
winding you series-resonate the complete RLC network where the antenna
is one part. This is what i have done. With the 470 pF on the output of
the transformer i need about 20 uF for that. Then the output voltage is
a very well looking sine wave. And, just 8 V DC (and abt 1.2 A) is
needed for the bridge supply voltage to reach 5 kV rms at the output.
Oh BTW regarding crazy, last night was very cold (< 0 C), clear, the
moon was fully shining. I took the scope on the top of the building.
With a 1:1000 HV probe i measured the voltage directly across the
antenna. It is all a great fun and moments to think back in some time.
Let me attach another small png file showing the voltage across the
wire :-)
These oil ignition transformers are very fine and become rare! I had 3
of them but meanwhile they are all damaged :-( Tom DK1IS seems to
operate with them at the moment but he will report later i think.
However these types of transformers are designed to be run just for a
minute or so, then you have to let them cool down. I don't think they
will handle the high voltage 24/7, even if nearly no output power is
token.
Just measure the input current during normal operation (with a
continous arc on the secondary side) and then without an arc. I would
assume that the difference is only a factor 2 or so and the cos phi
will be quite resistive, i.e. it will heat up after less than a hour i
guess. But maybe i'm wrong.
You have that remote RX location in 7 km, right? That may be ideal for
such experiments.
73, Stefan
Am 23.11.2018 14:06, schrieb VIGILANT Luis Fernández:
Hi
XLF
Great stuff Stefan. I save it to show to a neighbour,
just in case they say I’m crazy ;-)
I assume you are not using a resonant antenna at 80Hz,
right ?
So, no coil. Just the inverted L element and ground at
the HV transformer secondary ?
Mmmm. I have a similar device around which was the spark
generator of an oil heating boiler
Wait for you at DC
73 de Luis
EA5DOM
Dear SLF friends,
A new step forward towards DC: Since yesterday 22:30 UTC i'm TXing a
carrier at
80.005 Hz. That's the 3750 km band, where the far field
begins at a distance of 598 km.
The antenna voltage is just 5 kV and i'm getting 1.2 mA antenna
current, giving an
ERP of 20 fW (2E-14 W).
The voltage is even higher than in my recent experiment on 270 Hz,
anyway the S/N is lower, indicating that my RX lacks of sensitivity.
Indeed this experiment helps to estimate the lack of sensitivity. Looks
like i'm missing about 10 dB on that frequency. Anyway the signal is
making the path of 3.5 km to my tree grabber, as usual shown at
http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber2.html.
Not a big distance but still well beyond most garden fences!
The signal leaves a barely visible trace in the ZEVS window which is
running at 3.8 mHz FFT bin witdh. Unfortunately ZEVS is off the air
since a week or so. actually i wanted to transmit side by side with
ZEVS, of course a bit deeper even.
>From a quiet period at night, i calculated the SNR of a 1 hour carrier
period using vlfrx-tools: carrier
S/N: 16.83 dB in 277.8 uHz, -18.73 dB in 1Hz
For the crazy homebrewers i'd like to share that website where i
ordered the HV-transformer,
http://www.hosin.de/Specials/HV-Trafo/hv-trafo.html
A nightmare for all safety-fetishists! For the completeness it must be
mentioned together with that page:
http://kurts-werkstatt.de/hv-trafo.htm
73, Stefan
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