Your spectrogram is more consistent with FDTD than with the NAVELEX 0101, 113 Plot that you sent on February 2nd. This is an encouraging finding even though your experiments are possibly more reliable than FDTD in this application. The NAVELEX 0101, 113 Plot is based on a simplified algebraic approximation, and does not consider all modes as FDTD does*, so FDTD is likely to be more accurate than the NAVELEX approximation especially at 970 Hz < f < 2970 Hz, and your spectrogram is a rather close match to FDTD. This type of assessment would be more meaningful with your extracted carrier.
Maxwell and Stone showed spectral noise nulls between 4 kHz and 2.5 kHz but for a very limited set of conditions. Sferics from strong local (< 200km) lightning tend toward a much (perhaps 2x) lower null frequency than seasonally-averaged natural noise, so your strong-local-storm conditions may be consistent with your spectrogram null below 2 KHz.
* Despite the nominal modal cutoff frequency, tests 970Hz – 2970 Hz (especially for lightning or narrowband signals at ranges less than 200km) incorporate a significant likelihood of multimodality, which FDTD is perhaps best equipped to model.
The attached FDTD plot was not based on the ionosphere for your recent test which may account for some differences between your spectrogram and the attached FDTD plot.
So far the recent 970 Hz experiment went very well:
-The transmissions were done as planned. I slightly modified the transmission plan and sent 2 EbNaut messages. The TX was running almost 4 days without interruptions, despite the heavy rain and local thunderstorms.
-When coming back to the RX site, everything was in its right place. The 4x 7 Ah lead gel accus were discharged to 12.1 V only. The recording process was still running.
-I have now a 30 GB .dat file on the USB stick. This file will be analysed in the coming days. I hope to extract a clear carrier and maybe even decode the EbNaut messages?
A first image showing the 0...3 kHz spectrum without filters and blanker: https://www.dropbox.com/s/26bbkfb8q0hs20h/First%20two%20days%20from%20the%20recording.%20No%20filters%20and%20blankers.png?dl=0 (there is a bug on the timescale)
Does this look normal for the experts? Any comments/ideas?
Unfortunately there was a microphonic effect on the E field antenne. I can hear bumblebees from time to time!! ;-)
Am 17.05.2017 12:07, schrieb DK7FC:
Yesterday it was a perfect day here, sunny all the time, about 26 °C, the forests have a fresh green :-) ...and i put a ULF E-field RX somewhere into JN49LN :-) It will record about 4 days at 24 kS/s.
It is the next experiment on the 309 km band. Since 16.May.2017 a plain carrier is running on 970.005 Hz. I'm again using the modified mains transformer which generates just 5 kV on the antenna. In SpecLab i'm keeping 15 mA antenna current constant, as well as the phase of the signal. That results in an ERP of about 3 nW.
The active E field probe is a 1m vertical and 3m horizontal wire, lifted about 10m above ground.
The attempt is to reach a 27 km distance! First i searched for a 25 km distant location but when i arrived there, there were loads of electrical fences.
In the new location i took a short recording which can be analysed now. The spectrum contains relatively high levels of mains hum, but all in all it looks fine.
So far the QRN levels are relatively low, which is good.
I made a few adjustments to the receiver, hopefully it performs nicely.
I expect maybe a very faint trace in 424 uHz when QRN is low but it should be a clear trace in 47 uHz.
The transmission plan is to keep the carrier on the air until midnight, i.e. 33 hours for the carrier. Then Thursday 0 UTC an EbNaut message (N=5, CRC=4, 100s, 16K21A , 24:00:00 hh:mm:ss) will start at 970.005 Hz. Friday and Saturday will be for a carrier at 970.0025 Hz.
So far the carrier runs very well. Hopefully it will be a successful experiment again :-)