Stefan,
Congratulations! A new record of 30 dB SNR in
424 uHz at 7.2 km, 970 Hz is great news!
Your 10.01.2017 22:43 message did not reach my
e-mail server so I am glad that you attached it below; you mentioned:
“And who knows the advantages of this part of
the spectrum for our purposes?”
I imagine that your experiments will help to
answer that intriguing question.
Some likely advantages:
1)
Propagation should be much more
predictable at 970 Hz than at 2970 Hz*, especially at ranges less than
2000 km, perhaps making experiments easier to design with reasonable
confidence
2)
Propagation should be significantly more
stable over minutes, hours, and day-to-day at 970 Hz than at 2.9 kHz -
30 kHz, especially at ranges less than 2000 km**
3)
Path loss at 970 Hz may be ~ 2/3 of path
loss at 2970 Hz (in dB/m); still high compared to 9 kHz (but natural
noise attenuation is correspondingly high)
4)
Rock, fresh water and even seawater
penetration at 970 Hz is good compared to 2970 Hz and very good
compared to 10kHz. At 970Hz: roughly 10dB loss at 10 meters in sea
water, 10dB loss at 100 meters in some common types of rock and fresh
water. The rock and water examples are more interesting when
considering that natural and some cultural noises attenuate about as
much as the desired signal at depth, so at depth a very sensitive
receiver could be fully effective even with little or no electronic
cancellation of natural and cultural noises. With a 1 fT/rt-Hz
receiver, a signal barely detectable on the surface without natural
noise cancellation might still be detectable at 100 to 200 meters
penetration (or depth) in caverns, valleys, canyons etc.
*2970 Hz is theoretically in a less-complicated,
more stable, below-high-order-cutoff part of the spectrum; but
practically speaking 2970 Hz is close enough to cutoff to be highly
sensitive to modal interference effects with changes in the ionosphere
for paths 100km – 1000 km. 970 Hz is well outside this region of
sensitivity to changes in the ionosphere for paths 100km – 1000 km.
** At 970 Hz basically no modal interference
effects when ionospheric disturbances occur.
73,
Jim AA5BW
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
