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)
