Dear VLF,
In my 9th experiment, my signal was well received at 4X1RF in 2873 km
distance. Chris has recorded the whole procedure and sent me 4.5 GB on
a DVD as a registered letter (many thanks Chris!). I could successfully
extract the 8970 Hz part in a 880 MB file. Then i re-processed this
file several times in SpecLab, adjusting the FFT / Noiseblanker /
Clipper / Filter -settings. This recording is optimal for re-processing
and looking for optimal settings since the SNR is relatively high at
the beginning (up to 12 dB) and becomes lower (due to rising QRN) at
the end. There is a longer section on 8970.00000 Hz and later the
message "CU" that is partly visible and partly lost in the QRN. So the
goal was to make the whole message visible or at least as much as
possible.
The final improvement is rather marginal, probably not much more than 2
dB. The pessimists will say its a pity that the SNR improvement is not
more and the optimists will say its good that we obviously used almost
perfect settings in the last tests ;-)
So, let me tell you about the results:
The left half of the attached picture shows the transmission as
received by using the current distributed USR files/settings (4.5 mHz
FFT, 60 s/pixel, 3 kHz band filter BW, 1 kHz filter slope, Clipper
threshold = 6 dB above avrg., Noise blanker set to 2 ms / 9 dB, 0.05
sec.). The right part shows the best SNR optimisation i achieved. Of
course this is all a subjective decision, there is no strong difference
but i find that the "CU", at least the "C" can be better identified.
Reducing the FFT bandwidth should actually improve the SNR but the
readability decreases! There is not really an improvement between 4.5
mHz and 2.8 mHz, at least in this DFCW-600 transmission.
The SL settings of the best achievement are: 4.5 mHz FFT, 60 s/pixel,
Clipper threshold = 0 dB, Filter BW = 3 kHZ, Filter slope = 1 kHz,
Noiseblanker at 6 dB / 0.0003 sec ramp time, max. pulse width 0.05 sec.
Radio amateurs who are not operating in QRSS/DFCW mode may say there is
no difference between the results ;-)
On 6470 Hz there was no significant improvement, even in longer FFT
times. But my tests on that part are still not finished.
BTW, without using the SpecLab internal bandfilter, noiseblanker and
Clipper that signal would have been TOTALLY LOST!! There wouldn't even
be the faintest trace of my signal!!
As a conclusion i would say that the current SL settings are well
suitable but a few dB SNR improvement could be achieved. Of course this
depends on the kind of QRN/QRM and the antenna that is used. The RX
stations who can receive my signal at 15 dB SNR or better will not see
a significant difference but the new settings may be interesting for
stations above 1000 km or below. Those who can set up the SL paramaters
may try this. I will provide a new USR file for some RX stations in
high distance before my next VLF experiment.
Any comments/hints are welcome.
73, Stefan/DK7FC
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DK7FC@4X1RF_improved.jpg
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