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Re: LF: SL settings for EbNaut FFT recording ?

To: [email protected]
Subject: Re: LF: SL settings for EbNaut FFT recording ?
From: DK7FC <[email protected]>
Date: Tue, 02 May 2017 14:55:52 +0200
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Hi Luis,

Am 02.05.2017 10:49, schrieb VIGILANT Luis Fernández:
I was already running an SL instance showing the first 24KHz in a fast way to get an idea of what's going on with QRM and known signals
It is already included in the grabber at 1 min/pixel. Hour changed to UTC just now
Yes but it starts at 6 kHz, so not the first 24 kHz ;-) From 0...6 kHz there are also informations which are relevant to evaluate the situation of your receiver(site).

 
As you can see, there is a LOT of noise around. What I'm using is your VLF E-Field probe with the first JFet separated in a 50cm aluminium rod probe
Not only the length of the rod, also the height above ground, the effective height, playes an even bigger role. Sourrounded by trees you miss sensitivity, the lower the frequency is. And on the top of a high tower, the signals will be much stronger, with the same rod!
If there is a strong local AM transmitter on MF, you may get some interferences, and so on. Thus it is good to see what happens below 6 kHz too.
When DLF on 153 kHz has been active here, running 500 kW to a 350m high antenna in 40 km distance, i sometimes saw AM components, demodulation stuff on VLF, in one of the first versions. So when the antenna is on a high tower and the signal levels are strong, you may afford a second RC low pass filter in front of the first JFET, giving a cleaner spectrum on the lower VLF. On the other side, with a longer rod (giving more capacity) you may decrease the 10 MOhm resistor. This should reduce the noise of the first JFET a bit.
I just played with all that stuff a week ago when optimising my ULF E field circuit for 970 Hz.

Obviously, no clear trace from your last transmission in the 31mHz spectrogram with this Rx conditions
31 mHz? That's quite optimistic! I would rather try 31 uHz.
Are you using an AF isolation transformer between soundcard and antenna? What type of power supply are you using for the antenna? Is the GND of the antenna separated from the mains earth?

So, 7Km is an outstanding Dx at this moment. But really challenging 
 
I do already have a failed microwave oven to use the transformer. And the audio PA too. So you mean to apply the high voltage to the antenna
without resonating coil, right ?
Yes.
Better start with some measurements in the shack, not on the roof :-)
Measure or estimate the capacity of your antenna. Switch a fixed C of that value parallel to the transformer. Use a sine signal generator connected to the audio PA. Try to find a resonance or check the voltage at 8270 Hz. Maybe a normal mains transformer 12V -> 230V will give better results! Maybe you need to add your own primary winding to the transformer. Try to get 1 kV or so, that should work well. With a mains transformer, 500V may work for a few hours. If you have a toroidal transformer, use that!
You can also switch an external C parallel to the transformer if that helps to rise the voltage on the desired frequency. Or switch it in series to the primary winding. Ah and a resistor, maybe 2 Ohm or so may help to protect the PA from transients and short cuts (resonances).



Nice therm. Don't know if the pipe running down to ground would have any issue with this. At the moment 80w at MF didn't show to be harmful
But there can be some noise at 8KHz and this would really be an annoying side effect if that happens :(
The antenna currents will be very low. There will be no unwanted effects, except maybe some local E field coupling on the roof.

 
The resonating coil for VLF is in the "To-Do" list. Actually found a huge plastic (resin) garden flowerpots which are 60cm diameter by 35cm high
This can be stacked upside down leaving airgap between them for ventilation. Just adding one final on top with no coil serves as rain protection
of the full arrange.
Maybe it's a good idea to start in small steps. With your own monitor you will have some first results. And maybe some of the local EA amateurs can help to reach the next distance step :-)
It is all not easy! I would say it is harder than EME ;-)

73, Stefan

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