Dear LF Group,
After the long "K"s, DK7FC's signal was visible up until 1630, but not
readable. I'm not sure what transmission speed Stefan was using - I was
using settings for 120s dots, but am not sure, looking at the spectrogram
(see attachment), if they were actually longer. I think if they had been
DFCW240, they probably would have been just readable with the correct FFT
length.
I made a rough estimate of field strength of DK7FC's signal as 1.4uV/m, or
4.7fT if you prefer. The distance was about 650km. Assuming the SNR equates
to about 6dB, with a noise BW of about 7mHz this would make the apparent
noise level about 8uV/m per sqrtHz, or 28fT/sqrtHz.
I used basically the same preamp and antenna as before. By replacing the
loop antenna with a dummy antenna (a small choke of about the same
resistance and inductance) I could compare the band noise and RX system
noise floor. The RX noise was at least 10dB below the band noise. However, I
also added a 7.8 - 10.5kHz bandpass filter to the preamp, which allowed the
use of clipping, without cross-modulation from the strong VLF utilities at
higher frequency. I used the Spectrum Lab Hard Limiter, with "clipping level
B" set to "0dB above average level", so fairly heavy clipping I guess.
Clipping like this has proved effective in the past with high QRN levels
when receiving QRSS on 136kHz, and has been mentioned recently in connection
with VLF also. In this case, it had the effect of reducing the apparent
noise level by roughly 10dB, while having little if any effect on the
apparent signal level. So this was rather effective - I doubt I would have
seen the signal otherwise.
The wide-band noise floor at the portable QTH (IO91WR I think) was actually
about the same as my home QTH. There were quite strong 50Hz harmonics - the
field I was in is criss-crossed by 11kV overhead lines, although these seem
to be no worse than domestic mains wiring for noise. But there were many
fewer narrow-band spectral lines at the /P location - at home these drift
into the signal bandwidth quite often.
So a very interesting and quite successful afternoon - thanks to all
involved in the tests!
Cheers, Jim Moritz
73 de M0BMU
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