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)
