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Re: VLF: New experiments on 17.47 kHz planned...

To: [email protected]
Subject: Re: VLF: New experiments on 17.47 kHz planned...
From: DK7FC <[email protected]>
Date: Mon, 27 Aug 2018 15:09:53 +0200
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After transmitting carriers for the last 5 days, there are 3 good days to stack, the others were noisy.
The resulting SNR via start time plot for different carrier length is attached. With some uncertainty we can say that these days the ideal start time for the path from DL to VK7 is 17:50 UTC and the message duration should be between 2.5...3 hours.

This evening i'm going to start an attempt for a 2 character EbNaut message. With some luck, just 2 days, i.e. 2 stacks will be enough to get a clear decode.

f = 17470.1000 Hz
Start time: 27.Aug.2018  17:50:00 UTC (daily)
Symbol period: 12 s
Characters: 2
CRC bits: 23
Coding 16K21A
Duration: 02:56 [hh:mm]
Antenna current: 1.2 A

73, Stefan

Am 24.08.2018 14:02, schrieb DK7FC:
Hi Edgar,

Thanks for the spectrogram showing JXN nicely! Here i'm watching NWC but the SNR is much lower, probably due to the strong QRN we still have in late August...
I'm going to calculate the weighting factors for the stacking based on the noise amplitude from 18...21 UTC. And obviously it is not necessary to transmit until 0 UTC, so i will stop an hour earlier from now on.

Your SpecLab settings are fine as they are. There is no advantage from a shorter and wider FFT. The observed spectrum width currently allows to display more than 2000 FFT bins centered on the TX frequency, allowing to display enough noise arround the peak to demonstrate that the peak is most unlikely a by-chance peak...

Thanks for last nights txt file. We can produce a first stack from the two days. Last night is weighted by -1.3 dB. The result is attached. We can already see a peak of 11.3 dB SNR, not unrealistic but still not significant enough. The spectrum peak produced from the suggestion of the Excel plot (Start 17:50 UTC, duration 2 hours) is attached as well. It shows a higher SNR, which depends on the noise floor calculation (0 dB line). I need to work on this but anyway the peak looks good relative to the noise already.

Another carrier will follow tonite.

73, Stefan

Am 23.08.2018 23:40, schrieb Edgar:
Hi Stefan,

Based on the past two days signal from JXN, would it improve the chance of detecting the carrier if the "FFT window time (length)" matched the time when the signal is at maximum?

The FFT length is now 8.847 hours. The period when JXN signal is white is about 1.5 hours.

The FFT bin width would be wider, and a larger percentage of bins would have a stronger signal.

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