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Date: Thu, 15 Jul 2010 10:39:53 +0200
From: =?ISO-8859-1?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: RE: DLF passive received with earth antenna
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Hello Jim, VLF,

Thank you for your helpful suggestions!
Es tnx for calculating the FS at the earth antennas QTH! This was what i 
didn't know how to do ;-)

Yes, i have a low pass filter attached at the input, a simple RC lowpass 
(100kOhm, 47pF).

So, if the effective antenna area is about 18000m^2 and the area above 
the ground is just 1000m^2, it seems that this kind of antenna could 
work quite fine if i can reduce the ground electrode losses! :-)

It will be interesting if i can increase the DHO38 signal by 6 dB when 
doubling the distance between the electrodes!

The background noise of my netbooks soundcard in 3 Hz is -105 dB. To 
make the arrangement easier to handle, i choose the USBs 5V supply for 
the circuit. Maybe this causes the noise i thought today. Thus, i choose 
a 9V block battery today as the power supply. The noise decreased to 
-100 dB (with short cut at the two antenna input cables), so not much 
more than the soundcard noise. If i cannot see DHO38 now i will decrease 
the FFT time, e.g. to 0,3 Hz.

Maybe your calcutated -94 dB level can be confirmed in the further 
tests. I hope it is even lower of course since that means that the 
effective area is even higher ;-)

Will report here...

73, Stefan/DK7FC

Am 15.07.2010 01:40, schrieb James Moritz:
> Dear Stefan, LF Group,
>
> Taking a guess at ERP of DHO38 as 100kW, the FS at 411km distance 
> would be about 5mV/m (if ERP were only 10kW, FS would be 1.7mV/m, if 
> it were 1MW, 17mV/m, so 5mV is probably not too many dB wrong). The 
> EMF induced in a 1-turn, 1m^2 loop would be about 2.5uV.  From your 
> description yesterday, this should give a level on the SpecLab display 
> of -9dB -(44mV/2.5uV)dB which is -94dB, so below your background noise 
> level, as you discovered.
>
> The noise voltage of a TLC271 is typically 25nV/sqrtHz - this is a 
> poor match for a low impedance source such as a single-turn loop, but 
> even so the signal level should be well above the noise. With 3Hz FFT 
> resolution, op-amp noise might be 50 or 60nV, which would be about 
> -127dB on the SpecLab scale according to your calibration. Therefore 
> the op-amp is probably not the limiting factor here, it is more likely 
> the sound card input is determining the noise floor. So you need to 
> increase the gain in front of the sound card by 30dB  - 40dB to get a 
> good SNR.
>
> You could do this by:
>
> -Increasing the number of turns in the loop - e.g. a 30 turn loop like 
> I used for 9kHz reception, which would increase EMF by nearly 30dB (it 
> would also increase Z of the loop by a factor of about 900, but at VLF 
> this would still be small compared to the op-amp Zin)
>
> -Put a step-up transformer between loop and op-amp - a 1:30 ratio 
> would achieve a similar result to having a 30 turn loop.
>
> -Increase the gain of the op-amp stage. To get 30dB gain at 23.4kHz, a 
> TLC271 has marginal gain-bandwidth product; an OP-27 would be better 
> in this respect as well as lower noise.
>
> Or some combination of these things. But remember this will also 
> increase the level of unwanted signals at the soundcard input - if you 
> have any nearby broadcast stations, you might need to include low-pass 
> filtering.
>
> BTW, if my guess of 5mV/m is correct, and the loop EMF is 44mV, the 
> effective loop area would be about 18000m^2...
>
> Cheers, Jim Moritz
> 73 de M0BMU