At 05:50 PM 3/6/01 +0000, Jim Moritz wrote:
[..]
What is required for a practical 2 way LF DX mode, capable of
operating at the extremes of distance and SNR? A while ago,
G4JNT posted an estimate of what might be theoretically possible
using different techniques; another way is to look at what is
needed to serve our purposes. I would suggest the following "wish
list":
1)Be able to complete a minimal QSO (about 50 characters) in one
hour. This would give the lowest rate of signalling capable of using
the propagation lifts to complete a QSO "in one sitting".
2)Be able to transmit/receive all alphanumeric characters and
essential punctuation/procedure signs, in order to be generally
usable by any station without special arrangements.
3)Occupy a bandwidth of less than 10Hz - this is neccesary
because of the very limited spectrum available, and the fact that
several stations will be operating simultaneously.
The QRSS modes easily meet 2 and 3; in order to meet 1, a dot
length of about 7 seconds maximum would be required. With the
best possible conditions, I guess several stations might manage
transatlantic QSOs with these dot lengths. However, it would
probably not be enough to reach the more inland parts of Canada,
or the USA and further afield. By the way, I reckon about 6dB SNR
is needed to see a QRSS signal on a spectrogram under
favourable conditions; if there is much QRN, 10dB is probably
required. It is possible to see a trace of signal with 0dB or less
SNR. All this is fairly subjective, however.
[..]
Then there are the "digital" modes, specifically BPSK. Currently,
most effort has been expended on the MS100, 10 bits per second
variety of BPSK. This easily meets conditions 1 and 2. However,
for the same signal levels, QRSS seems to do better with
acceptable, if much slower, speed. Also, the bandwidth occupied is
roughly 40Hz, too wide for condition 3. But with the 16 bits per
character coding scheme normally used for BPSK, 2250
characters per hour can be transmitted, far higher than is actually
required. So the bit rate could be greatly reduced, and/or the
coding altered to a greatly increased number of bits per character,
hopefully improving the readability of the signal. Reducing the
overall speed by a factor as much as 45 would still meet condition
1. To fit into a 10Hz bandwidth, the bit rate would have to be 2.5
bits/sec (MS400) or less, so you could encode each character with
up to 180 bits if you wanted to. Or, sticking with 16 bit codes, 0.22
bits/second (MS4500) would still be OK. What we want is the best
trade off between bit rate and encoding for very poor signal to
noise ratio. I don't know a great deal about this subject, but I
expect some readers of this reflector already know the answer.
Apart from the bandwidth, I believe Stewart Nelson's WOLF system
is the best approach that most closely meets all the requirements.
It shouldn't be difficult to slow down WOLF's 100 msec keying rate
if you really have to reduce the bandwidth. Of course Tx, Rx frequency
stability and accuracy will become increasingly important as with
any slow BPSK mode.
So any suggestions/comments would be welcome - well, almost
any! By the way, I now have BPSK at up to 1200W PEP from my
Decca TX, if anyone would like a sked/tests, etc.
If you'd care to run a test using WOLF, I'd be happy to send you the
results.
For more information on WOLF, see:
www.scgroup.com/ham/wolf.html
Bill VE2IQ
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