LF: Re: DK7FC's 7th VLF kite experiment

["James Moritz" <james.moritz@btopenworld.com>]

Dear Stefan, LF Group,

This is the screen shot of today's signal from DK7FC at my home QTH (IO91vr) 
from about 1020 until 1300 utc. The time markers are at 1 hour intervals. 
The QRN level seemed to be quite low. The horizontal streaks are local 
mains-related QRM. My laptop decided to switch itself off at about the time 
Stefan's transmission started, but fortunately I was present at the time and 
only a few minutes of signal were lost. The SNR reached about 15dB with 3 
millihertz FFT resolution - I think if the maximum ERP could be maintained, 
copy of QRSS60 or QRSS30 signals would be possible under these conditions.
A spectrogram with this resolution begins to show the limitations of using 
the laptop's internal sound card. The start of the trace shows the warm-up 
drift of the sound card, which isn't too bad. But you can see that the 
signal trace is spread over several FFT bins, apparently due to some 
short-term instability of the sampling rate. I think this is due to the 
cooling fan in the laptop cycling on and off every minute or two, frequency 
modulating the soundcard clock  by several millihertz due to the changing 
temperature. I think if this instability could be eliminated, the SNR would 
be improved by some dBs. Probably an external sound card would be a good 
idea.
Thanks to Stefan for another successful VLF test,

Cheers, Jim Moritz
73 de M0BMU

dk7fc_2_10.jpg
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