Stefan,
Congratulations on this latest milestone. 57.6 km at 470 Hz at an SNR that would have supported 6 bits of information with 90% of battery energy unused is impressive; nice work.
Using r^2.5 extinction it seems like another 13 dB would reach the far field.
I wonder if some fine-tuning of distance could be helpful in that case. I’m not sure how to easily calculate modal null separation distances at wavelengths greater than ionospheric height, but by extrapolation from 3 kHz it would seem that at 470 Hz, ten nulls (from modes 1 through 5) might be fairly closely spaced near a 110 km target location, such that a small increase or decrease in receiver distance from transmitter could move the receiver toward a peak in the interference pattern.
I’m not sure how you changed receiver locations in the past so perhaps in this case you can’t easily move the receiver.
Your 2-hour spectral plot is a welcome sight.
73,
Jim AA5BW
Hi ULF friends,
Last Friday i run another carrier transmission on my 900m long ground loop antenna. It was the second experiment at 470.1 Hz. The last experiment failed because just a few 1/10 dB were missed to reach the 14 dB SNR level which is the criterium for a valid detection.
During the last week i managed to reduce the background QRM on the E field significantly (at least 10 dB) in that frequency range by changing the ground connection on the RX system, so it was an EMC issue.
In the recent experiment the DC measurement showed 1 A at 58 V, so the loop loss was just 58 Ohm, which is one of the best values so far.
Conecting the PA to the LiFePo4 accu produced 620 W RF power and 3 A antenna current (!) in the first moment. But the DC/DC converter became a bit to warm and i (thought) had enough time so i reduced the input voltage. The power dropped to 460 W and so the antenna current during the experiment was 2.75 A. The carrier started at 14:50 UTC and was on the air for 2.5 hours!
It was a quiet day and so i decided spontaneously to do the experiment. But i've been at work in the morning and so i was late, a bit to late actually, since the band already opened and the QRN increased during the experiment. Who knows what happened to the phase over that path. The only RX station detecting the signal was my own tree in 57.6 km distance. Due to the rising QRN the last 30 minutes did not help to rise the SNR of the postprocessed spectrum peak. But 2 hours were fine to get a clear copy!
Usually i have to ways to determine the final SNR: One is to read the output of the EbNaut decoder decoding a '*' message. The ouput, if using the -f15 -f16 -M'*****' options, then shows the carrier S/N directly.
The other method is a script that calculates the SNR dB value based on the spectrum peak and 0 dB noise level. This 0 dB noise level calculation is done according to the description in SpectumLab's help sites. Here is the quote:
"The definition of noise levels is not easy. Here is the basic algorithm of the 'noise' function:
- An array of amplitudes (usually dB values) from the last FFT calculation is sorted into order of increasing amplitude.
- The amplitude of the lower quartile value (for example bin number 256 in a sorted set of 1024 points) PLUS 3dB is then returned as an estimate of the mean noise level.
This technique automatically throws away very high values (strong signals) that would otherwise affect the result."
This technique is used in my script. I find the result adequate, the 0 dB level is visually in the center of the noise. But sometimes the SNR statements are a bit different between my script and the result of the EbNaut program. Maybe it is due to the fact that i am calculating the mean level based on just 1000 bins arround the spectrum peak, and EbNaut watches a higher bandwidth, which may contain more QRM peaks that rise the mean noise baseline?
That may explain why my script whows an SNR of 18.2 dB whereas the EbNaut decoder says it is an SNR of 16.17 dB. Anyway, well above 14 dB.
The signal was strong enough, it could have carried a 6 character EbNaut message.
Attached is a 424 uHz spectrogram showing a short carrier trace just before the high QRN sets in. Also attached, a single spectrum peak detection and the result for an EbNaut message (showing the carrier S/N).
So, it is the first amateur radio signal on the 638 km band detected over such a distance i think.
73, Stefan
