VLF: RE: DLF passive received with earth antenna

[Markus Vester <markusvester@aol.com>]

Dear Stefan, Jim, LF,

 

last weekend, I attempted a fieldstrength measurent of DHO38.  I used a small nonresonant loop (6 turns, 41 cm diameter, 0.792 m^2), directly connected to the soundcard, without a preamp or buffer. With the "Mic" slider fully up, full scale was 18.8 mV rms. The noise level (-98dB in 17.6 Hz = 56.4 nV/sqrtHz) was low enough to measure the four strongest MSK signals.

 

As Jim mentioned, the correction factor for thze MSK bandwidth depends on bandwidth, and I spend some time calibrating this. Direct integration of the spectral density across the 200 bd MSK spectrum resulted in an effective bandwidth of 105 Hz.  For Stefan's 2.93 Hz FFT (ie. 4.4 Hz noise bandwidth due to Hann window), the bandwidth factor should theoretically be  20.2 - 6.4 = 14.8 dB.

 

For simplicity, I read the levels from SpecLab's waterfall cursor. Long averaging (> 10 s scroll with Optimum Waterfall averaging) is required to minimize the remaining raggedness of the spectrum. One should try to avoid occasional times with apparent periodic patterns in the transmitted data, leading to significant spectral peaks.

 

An additional offset resulted from the parabolic peak estimation, which is optimized for carriers, but tends to be 1.5 dB high for flat noise. So in effect, Stefan would have to add  12.3 dB (rather than 14.8 or 11 dB) to the cursor readout. For my measurements, I selected an 11.7 Hz FFT and added 6.3 dB.

 

BTW There are probably better ways to read MSK levels from SpecLab, either by using the new "pam().ampl" function, or simply selecting a large enough resolution bandwidth (e.g. 187 Hz) to follow the signal modulation.

 

My results for DHO38 at 475.3 km range were
 -79.5 dB = 4.11 uV (Saturday night 23:30)
 -82.1 dB = 3.05 uV (Sunday daytime 10:15)

 

With the effective height of the small loop
 heff = A / (lambda/2pi) = 0.388 mm at 23.4 kHz
(equivalent to Jim's formula), I get a fieldstrength of
 10.6 mV/m = 80.5 dBuV/m (night),
 7.86 mV/m = 77.9 dBuV/m (day).

 

Here is a table containing also the fieldstrength measurements of three other MSK stations:
kHz      dBuV/m      Call     km
           night   day
45.9   68.3   56.1  (NSY)  1390,
23.4   80.5   77.9  (DHO)  475,
22.1   73.0 ~65.5 (GQD) 1155,
21.75 77.0   71.1 (HWU)  796.

 

On the German blog
http://www.mysnip.de/forum-archiv/thema/8773/109722/Superpulse+auf+19_6+kHz.html
someone who worked at DHO38 states that they have eight 100 kW transmitters, feeding an array of eight antennas. With 80% combined antenna efficiency, they can radiate 640 kW (EMRP). This would theoretically result in
 E = 84.1 dBuV/m
here in Nuernberg. The difference to the measured values might be attibuted to an ionospheric component, or ground loss, or simply to the fact that they may not be regularly operating at full power.

 

To apply my DHO measurement to Stefan's ground loop experiment in Heidelberg, we would have to scale daytime fieldstrength from 475 to to 411 km, giving
 E = 9.1 mV/m
at his site. The voltage at the antenna terminals was
 U = 44 mVpeak - 11dB + 12.3 dB = 36.1 mV rms.
Thus the effective height of the 280 m long earth antenna was
 heff = U/E = 3.97 m at 23.4 kHz,
giving a the ground loop area
 A = heff * lambda/2pi = 8090 m^2,
and an effective return current depth
 d = heff /length = 29 m.

 

This depth appears to be much less than half of the baseline length, so it is probably indeed skin effect limited. Scaling the 23.4 kHz results to 9 kHz would give 1.6 times more skin depth, but 1.6 times less effective height (ie. 2.5 m) due to the larger wavelength to loop length ratio. Thus the transmit efficiency at 9 kHz would be about 26 dB less than that of a 100 m kite antenna with similar loss resistance.

 

Kind regards,
Markus (DF6NM)

-----Ursprüngliche Mitteilung-----
Von: James Moritz <james.moritz@btopenworld.com>
An: rsgb_lf_group@blacksheep.org
Verschickt: Sa., 17. Jul. 2010, 23:43
Thema: LF: RE: DLF passive received with earth antenna

Dear Stefan, LF Group, 
 
It gets confusing when the bandwidth changes! In this case, both measurement bandwidths are much smaller than the modulation bandwidth, and the signal has a continuous spectrum, so it is reasonable to assume the measured signal power will be proportional to the measurement bandwidth. The amplitude of the signal on the earth antenna was -20dB in 3Hz BW. To normalise the measurements to a 3Hz FFT BW, you need to add 6dB to the amplitude of the 1m^2 loop signal in 0.75Hz bandwidth, to give -94dB as you said. The ratio of signal amplitudes is then 74dB, or a factor of about 5000. So the eqiuvalent area of the earth antenna is 5000m^2. 
 
Adding Markus' factor of 11dB to give the total signal amplitude from the 1m^2 loop gives -83dB, 74 dB below your reference level of 44mV = -9dB. Therefore the 1m^2 loop EMF is about 9uV, corresponding to FS of DHO38 being about 18mV/m. If this is correct, it corresponds to about 1.1MW ERP, which is impressive, but since the signal is down in the noise in your system, these figures can only be a very rough estimate. 
 
Cheers, Jim Moritz 
73 de M0BMU 
 
----- Original Message ----- From: "Stefan Schäfer" <schaefer@iup.uni-heidelberg.de> 
To: <rsgb_lf_group@blacksheep.org> 
Sent: Friday, July 16, 2010 11:32 PM 
Subject: Re: LF: RE: DLF passive received with earth antenna 
 
> Ehm, 18000m^2? I have just calculated 5000m^2. It is A = 1m^2 * > 10EXP((-94--20)/20), right? 
> 
> 73, Stefan 
> 
>