Re: LF: DX VLF experiments in 3HD

[Roger Lapthorn <rogerlapthorn@gmail.com>]

With 420uHz or 47uHz BW could be very readable with Marcus.

73s

Roger G3XBM

On 2 February 2014 14:27, Stefan Schäfer <Stefan.Schaefer@iup.uni-heidelberg.de> wrote:

Markus,

I am 16 dB stronger now!


Am 02.02.2014 14:52, schrieb Markus Vester:

So what should be expected here? Scaling the fieldstrength from 1 km to 180 km (1/r^3 to lambda/2/pi, 1/r thereafter), the signal should be 82 dB weaker here on 5.755, and 63 dB weaker on 17.265 kHz. Neglecting ionospheric contributions (not really correct), and assuming similar noise levels (though 5.5 kHz is much affected by railway QRM here), my SNR in 0.47mHz should theoretically have been 

 35 - 82 + 10 dB = -37 dB on 5 kHz and

 50 - 63 + 10 dB = -3 dB on 17 kHz.

So while a detection on the fundamental seems pretty hopeless, the third harmonic should already be getting close.

Yes, the QRN and QRM is much lower in 3HD :-) Why didn't we get this idea earlier???

 What is the inductance of the loading coil?

Ehm , i didn't really measured it, it's just trial and error.

Would be interesting to see a picture of the scope trace picked up near the antenna ;-)

Looks quite good but could cause misunderstandings...

The amperemeter in the coil shows 350 mA now! There is some kind of selfoscillations, audible. Thus i do not rise the power even more. Have to search, find and fix this problem later. The AC voltage from the vari-transformer is 110V now, so there is room for at least another 6 dB. Power is in the range of 100W now.

I'm going to measure the strength at home now. Hopefully the signal is audible :-) Yesterday it was still weaker than the mains hum, so far away from any interferences!

73, Stefan

 

Best 73,

Markus (DF6NM)

 

From: Stefan Schäfer

Sent: Sunday, February 02, 2014 2:06 PM

To: rsgb_lf_group@blacksheep.org

Subject: Re: LF: DX VLF experiments in 3HD

Markus, VLF,

Am 01.02.2014 20:34, schrieb Markus Vester:

Stefan,

 

running two 478 µHz "6000" windows centered on 5755 Hz (on the grabber, but much QRM here) and also 17265 Hz (offline) which seems more quiet. So hoping for a long carrier...

Well, i hope that 16 hours key down were long enough ;-)

On which of the two frequencies would you expect to radiate more power?

I don't know. But the harmonic signal is very weak and cannot cause interferences for JXN or HWU or SAQ or GBZ...

I've done measurements at home, 950m distance, of that 5.755 kHz transmission and saw that the signal was very weak, see attachment. Just 50 dB SNR in 4.5 mHz at 3HD. The TX power was just 15W at the end. Also the frequency response of the active antenna + soundcard is attached.

The experiments are very interesting here and i like to do the measurements. Despite rain the coil was on the top of the building and on air. It was just covered by a trash bag. The voltages are very small...
I just hang out a 30cm long wire connected to a probe of the oscilloscope and so the voltage waveform of the antenna wire. It nearly looks as expected :-)

Now i got another idea. There is a 2.2 uF WIMA MKP-10 capacitor in series to the output of the H bridge, just to protect it against damage in case of a DC output. Now i've disconnected the antenna and tuned for the series resonance of this C and the L of the primary winding of the loading coil. The resonance is at 1.77 kHz. This allowed me to calculate a much smaller additional series C which helps to attenuate unwanted frequency components, i.e. makes the spectrum cleaner and provides a lower impedance on the wanted frequency :-)

There is really no alternative to that H bride PA which works from 10 Hz to 100 kHz :-)

in a few minutes i will be on air again for a longer time, hopefully with a much stronger signal.

Maybe you can detect something of that 5.755 kHz transmission.

73,GL, Stefan/DK7FC

 

BTW Today I have updated SpecLab from version 2.79 b08 to b10 (Jan 26), and it seems that GPS 1-pps sync is finally working well now, even with 42 µHz resolution.

 

Good luck for the test!

 

73, Markus (DF6NM)

 

 

 

From: Stefan Schäfer

Sent: Saturday, February 01, 2014 5:46 PM

To: rsgb_lf_group@blacksheep.org

Subject: LF: DXl VLF experiments in 3HD

VLF,

This evening and night i'm planning to transmit on 5.755 kHz, using the large antenna, the large coil and the large PA :-)
I don't know what can be expected and how the system works at that frequency. Actually it is just a first test for playing arround and doing some measurements :-)
If some nearby stations want to give it a try, maybe even in 3HD, then don't hesitate :-)

73, Stefan/DK7FC

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