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Today between 7:30 and 14:00, Lubos OK2BVG and
I successfully completed a two-way contact on 8.97 kHz. We believe that this is
the first international QSO ever on VLF.
The distance between Breclav (JN88KS) and Nuernberg
(JN59NJ 69es) is approximately 424 km. The weekend before, Lubos and I had found
that we could see each other's VLF transmissions from our small home antennas in
a slow spectrogram, using about 0.45 millihertz FFT resolution ("DFCW-6000").
This long integration requires about 2000 seconds of continuous non-interrupted
carrier to reach full sensitivity.
We decided to use absolute frequency encoding of
characters ("MFSK-37" mode), which has a simple structure but is significantly
more efficient than two-frequency DFCW. The software signal generators in
SpecLab were used to create 30 minute dashes by editing the frequency in 1 mHz
steps every half hour. This can be automated by opening a textfile which
is read by the "periodic actions" function. Between transmissions,
additional half-hour gaps were inserted to let the FFT ring down, and allow for
TX-RX reconnections.
Characters are identified by reading the
frequencies above 8970.000 Hz, with 8970.000 to .009 assigned to the numbers,
.010 idle or space, and .011 to .036 the letters A-Z. Lubos used a Rubidium
standard to lock his transmit samplerate, while both receivers and my TX were
synchronized by military MSK signals from GQD (22.1 kHz) and DHO (23.4 kHz).
With about 80 watts from an audio amplifier, I
could get up to 0.38 A antenna current into my top-loaded vertical 9 m above the
roof, radiating on the order of 10 to 15 microwatts. I believe that Lubos is
using similar equipment, perhaps a couple of dB stronger. For receive, my
soundcard was connected straight to the loading coil and antenna, whereas Lubos
has an active probe at a quiet site remote from his TX
location.
We wanted to go for a "full QSO" format with
reports and confirmations, beyond the rudimentary three-dash "micro-QSO" format
which was used in June 2009 between DJ2LF and myself. With single-letter
suffixes, we ended up proceeding as follows:
07:30-08:30 "NB" ;Lubos'
call: df6Nm de ok2Bvg
09:00-10:30 "BNM" ;my reply and report: Bvg Nm
M
11:00-12:00 "RO" ;his confirmation and
report
12:30-14:00 "RTU" ;my confirmation and thank you
Although today the noise was not as low as it had
been before, we managed to exchange the essential information in 5.5 hours until
13 UT, after which QRN from lightning in southeast Europe became strong enough
to obliterate further copy.
Attached image contains captures from Lubos'
grabber at Apollons temple (top) and my receiver (bottom), with screenshots from
the DK7FC and OE3GHB grabbers pasted inbetween. The original captures are
at
http://df6nm.bplaced.net/VLF/ok2bvg_df6nm_120325/
All
spectrograms were stretched to the same timescale (5 min/pixel). On the left
hand side, you can see a MFSK pre-test from Lubos, sending his call to hs own
grabber at very low power. There was also a long dash from OE3GHB on 8970.030,
and a carrier from DJ8WX on 8970.022 is visible at DK7FC.
The rightmost part of the captures shows the
essential eight dashes from our QSO. You can see that I had almost lost
Lubos' "B" when my noise blanker triggered heavily on local QRM, inducing me to
give an "M" rather than "O" report. But hovering the cursor to 8:15 UT does show
the peak at 8970.012 Hz in the spectrum graph. My final "R" is just barely
visible at .028, whereas the following "TU" at .030 and .031 ended up drowning
in the increasing noise.
Thanks again to Lubos for the effort and
patience! Although exchanging half-hour symbols is tedious and may sound
boring to some, both of us enjoyed it and actually found it quite exciting.
Best 73,
Markus (DF6NM)
Sent: Friday, March 16, 2012 11:46 PM
Subject: VLF: MFSK-37 tests above 8970 today
To explore the possibility of further two-way VLF contacts from my
home antenna, I have conducted a test in MFSK-37 mode this morning. I was hoping
to reach 0.42 mHz ("6000") grabbers using UT-synchronized half-hour dashes.
Similar to an earlier kite experiment, I used absolute frequency
encoding of characters in 1 mHz steps (0 = 8970.000, 1 = 8970.001, ... 9 =
8970.009, idle = 8970.010, A = 8970.011, .. Z = 8970.036 Hz).
The plan was to transmit my complete callsign in 2.5 hours between 8:00 and
10:30. Unfortunately DHO locking failed during the first two characters,
which came out some 9 mHz low. Attached image shows what was actually
sent according to my "monitor", which is a high harmonic of the transmitted
VLF signal accidentally aliased into the LF TA window.
I was pleased to find all the dashes showing up clearly as bright
dots on the OK2BVG "6000" grabber, and even left visible traces in his
"600" window. As I had also received Lubos' transmission last weekend
with a good margin in 0.48 mHz, a two-way QSO between us would probably be
feasible within a few hours.
Paul Nicholson's 0.278 mHz super-sensitive spectrogram also shows the
transmission at 120 degrees azimuth, even though the dashes were really too
short to reach full SNR in this bandwidth.
Results on the DK7C grabber were not quite as good, with only a single
clear dot on .006 Hz. This may partly have been due to the previously
experienced daytime minimum at our distance (180 km). Nothing distictive
was visible at OE3GHB, who is at similar distance as OK2BVG, but seems to
have suffered from a bit of local QRM at the time.
Again we find that a great deal is possible with very modest means...
Best 73,
Markus (DF6NM)
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