Thanks Paul, for devising this experiment in the first place, and encouraging me to take
part in it. It looks like we are now getting real close to the ultimate Shannon limit, making best use of your
superb receive capabilities.
The 8270 Hz carrier comes from a
Rubidium-referenced decadic synthesizer, with frequency calibrated to
Loran-C to within about 1e-10. For the test, an encoded bit sequence for the five-character message
was obtained beforehand from Paul's script on his
website. SpecLab's digimode terminal (configured similar as previously
for Opera, and initiated by "scheduled actions") is used to generate a
keyed sidetone, with bit timing controlled by NTP to within a second.
The tone is being rectified at the soundcard
output and drives an ancient and sensitive telephone relay, commuting
the carrier between two complimentary outputs of a small pot core
transformer.
The phase-keyed signal is then amplified
by the usual "Nitro" audio PA to about 60 Watts (28 V
rms). A large ferrite transformer (in the brown chocolate
box) bought the voltage up to 200 V rms, feeding 0.3 A
antenna current to the venerable seven-bucket coil which is sitting in
the blue rubber bins outside the window.
To keep the antenna on resonance, the phase
detector from my LF automatic tuner (basically an XOR comparing the phase
of voltage and current, with sensors modified for the lower frequency)
was inserted between the amp and the transformer. It drives a little DC motor, pulling on a nylon string, which then
rotates a 10x10x0.5 cm^3 ferrite plate inside the coil.
This effectively mitigates current reductions and phase variations
caused by wind, temperature and humidity. During
operation, one can hear the whistle of the coil and transformers,
occasionally interrupted by a short "krk" sound when the relay is bouncing
during phase changes at full or half minute.
All the best,
73, Markus (DF6NM)
Sent: Sunday, May 11, 2014 8:36 AM
Subject: VLF: Coherent BPSK at 8270 from DF6NM
I
am pleased to report reception of two test transmissions from
DF6NM on
8270.000 Hz which took place on Saturday 2014-05-10
morning.
Markus
was sending coherent BPSK with UT synchronous symbols
using a symbol period
of 30 seconds. The FEC is a terminated rate
1/4 convolutional code with
constraint length 21, cascaded
with an outer error detecting code using a 16
bit CRC.
Two transmissions of 46 bits were made, each lasting
132
minutes with a 20 minute carrier test in between. ERP
was
probably around 5 or 10 uW and the range is 1028 km.
Eb/N0 was
about -0.5dB in the first test and about -1.5 dB
in the second, which is
below -7dB in the symbol bandwidth
of 33.3 mHz.
Both messages were
decoded with some margin to spare.
The decoder is a list Viterbi decoder
using the tree trellis
algorithm with a list length of 2000 and stack size
20000.
Both messages decoded at the top of the list so the list
decoding
wasn't actually necessary for this strength of signal.
The transmitter
uses a rubidium source and the receiver is
GPS timed. A reference
phase at the receiver is obtained by
averaging the phase of the squared
signal but at such low signal
strengths the resulting reference is unreliable
and the decoder
makes a search for the correct phase and phase drift
rate.
The signal is completely invisible in any spectrogram at
the
receiver. In a spectrogram running at the symbol
bandwidth
the signal is too far below noise and the bandwidth of
the
transmission is such that, in a resolution capable of seeing
the
signal above noise, the signal is too wide.
For example, this spectrogram
uses the symbol bandwidth,
http://abelian.org/vlf/tmp/df6nm_140510a.png
Maybe
someone can see some very faint line at 8270.000 ?
The first test ran
07:02 to 09:12 and the second
from 09:32 to 11:44. The 20 minute
carrier in between is
also below noise at this resolution.
Following
the second test, an hour of carrier is visible in
the 278 uHz spectrogram
at
http://abelian.org/vlf/fbins.shtml#p=1399780800&b=110&s=sp
More
information on the FEC codes, trials, and search for
good polynomials can be
found at
http://abelian.org/fec
Stronger codes
with constraint length up to 25 are available
and hopefully further tests
will be made soon.
--
Paul Nicholson
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