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Re: ULF: Crossed 14.4 km on 970 Hz / 309 km band

To: [email protected]
Subject: Re: ULF: Crossed 14.4 km on 970 Hz / 309 km band
From: DK7FC <[email protected]>
Date: Sat, 22 Apr 2017 16:01:19 +0200
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PS: Another image, showing the whole recording from 0...5 kHz, without filters and blankers...

73, Stefan

Am 22.04.2017 15:59, schrieb DK7FC:
Dear friends of the Ultra Low Frequencies,

Yesterday a new experiment was done on the 309 km band. With the setup bescribed below, still running just 5 kV / 15 mA on 970 Hz, i crossed a distance of 14.4 km which is twice the distance of the last experiment. A plain carrier was sent on 970.005 Hz, at least this was the plan.
This is the location: http://no.nonsense.ee/qth/map.html?qth=JN49KL09AB
And this is the path: http://no.nonsense.ee/qth/map.html?qth=JN49KL09AB&from=jn49ik00wd
I recorded 196 minutes at 24000 kS/s starting 15:33:03 UTC.

As expected in the near field, the signal is much weaker than just 6 dB, rather 16 dB! But it is still there! A weak but doubtless trace in 3.8 mHz. The signal was peaking 18 dB SNR in 424 uHz.

There were some strange problems on the transmitter side. For the first recording hour there was no carrier. I had to walk 45 minutes to reach the next village to get some mobile internet to remotely restart the transmission. After 80 minutes the carrier disappeared again and came back 30 minutes later on 970.000 Hz. That was not intended but it acts a bit as a keyed carrier, a better ID which is well visible in 3.8 mHz.

It is very interesting just to listen to the recording (using a 6 kHz low pass filter). I like the sound of the sferics which are propagating well below 500 Hz, not only down to 4 kHz. It all helps to better understand propagation from DC to VLF. BTW there were many whistlers that evening and i catched many of them. It is a quiet location but i still think that the antenna has not the maximum possible sensitivity arround 1000 Hz.

Some pictures in the attachment (i still need to work out a comfortable dropbox alternative).

Now, that was the next step, 29 % of the far field border is crossed.
I'm planning a next experiment in 25 km distance, which would be 50% of the far field distance. Maybe i can do some QRO or i need to do a very long recording. The the current signal strength there should still be no problem in 47 uHz. This time i'm planning to use a larger E field antenna.

On the end of the 3.8 mHz spectrogram the signal became significantly stronger. Can there be some QSB, i.e. groundwave-skywave interaction even in 14.4 km distance? Or was it just constructive QRM/N? It will be interesting in the next (longer taking) experiment to see if there are some diurnal signal level changes. Maybe the band turns out to work much better during a certain time period...


73, Stefan





Am 03.04.2017 13:55, schrieb DK7FC:
Hi ULF friends,

Saturday night i build an active E field receiver optimised for ULF. It is using a BF862 front-end as a source follower and a LT1028 with 20 dB gain. There are 3 RC filter stages cutting off at 10 kHz. Furthermore there are two isolation transformers in series, 4:1=>1:10. In the center they are parallel resonated. This gives a further good low pass filtering and some additional gain below 3 kHz. It was a quick construction without thinking to much, soldered at night, 02 AM local time. The antenna probe is a 1m long steel rod (for welding) with 2 mm diameter. The antenna height above ground was just 2m. There were no trees in a radius of 20 m.

For transmitting i'm again using the modified 5 kV mains transformer. At 970 Hz, the antenna impedance is 342 kOhm! So i can just run 15 mA antenna current which means 3 nW ERP. The new ALC build inside SpecLab holds the 15 mA accurately and protects the transformer that way! SpecLab is a very well usable tool for transmitting on VLF/ULF, thanks to DL4YHF!!
The transmit frequency was 970.005 Hz.

I didn't expect much, thought that this distance may be to optimistic. The last signals were very weak on my tree grabber in 3.5 km distance. But that tree grabber is using loop antennas and they are not sensitive in that frequency range. So there was a certain chance to see a trace, maybe in 212 uHz???
I drove to JN49JL00EB and built up the receiver there because it is a quiet location, a nice region for a walk and, there is a good restaurant not to far!!!!!! So it was easy to spend some time there and let the Raspi (using a GPS module on the right soundcard channel, with PPS+NMEA) record for nearly 3 hours at 24 kS/s.
This is the path between TX and RX: http://no.nonsense.ee/qth/map.html?qth=JN49JL00EB&from=jn49ik00wd  A distance of 7.2 km, or 0.023 wavelengths or 14% of the distance to the far field border. So it is still a near field experiment.
It is about twice the distance i've managed in the last test.

Now i'm back in the shack, analysing the recording and to my surprise i can see a strong trace of 30 dB SNR in 424 uHz!!! See attachments in 424 uHz and 3.8 mHz.

All this makes me much more optimistic to reach farer distances. I tell you i will crack the far field border on that 309 km band! That would be a distance of 49.3 km.

There must have been some local thunderstorms not to far away, because there was QRN in the observed spectrum. So the SNR can be improved a bit by doing the next test in the late morning hours. Also the resonance of the transformers seem to be a bit to low, so maybe i can reach a bit more sensitivity when optimising that resonance. It could further help to rise the effective height of the antenna. Flat fields rather than the deep forest is the region to select now....

73, Stefan



Am 10.01.2017 22:43, schrieb DK7FC:
Hi ULF,

Since a few hours i'm running 15 mA antenna current on 970 Hz, the 309 km band. This requires to apply 5 kV to the antenna. You can see a very faint trace on the lower image at http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber2.html

Just about 10 dB SNR in 424 uHz in 3.5 km distance, or in 0.011 lambda distance. The receive antenna is a H field antenna that is not even pointing to the transmitter. Also the preamp noise is dominating the background noise on that frequency. So the RX is deaf on that band. Anyway, there is something.

The ALC into SpecLab does a very good job, it holds the antenna current stable during all the changes and working point drifts. The plot can be seen at http://www.iup.uni-heidelberg.de/schaefer_vlf/VLF/TX.png
15 mA results in an ERP of  3 nW.

My new preamp circuit is waiting for a first test together with the large loop. I hope to pick up the signal in at least 5 km distance with that preamp which is really low noise down to the lower Hz range.
An E field reeiver would be a better choise for the reception from that E field Tx antenna, at least in the lower near field. Maybe that will give another test then.

With 30 kV i could reach 0.3 uW. Not sure where this could be detected? And who knows the advantages of this part of the spectrum for our purposes!?!

Since 21:20 UTC, a 2 character EbNaut message is running. It will take 2h, 2min, 40s. Hopefully the tree grabber is available until the message ends. It will shut down in a few hours due to lack of solar energy in these days (an improvement of this system has already been prepared and waits for the installation).


73, Stefan



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