Dear LF Group,
Some useful points raised on the subject of Transcontinental
modes - here are some responses:
The simplest way to improve the capabilities of existing methods is
undoubtedly what G3LDO suggests - tailoring the QRSS dot length
to the prevailing conditions. As W4DEX observes, there are often
times when faster keying could be used to advantage. But to do
this requires feedback from receiving to transmitting station so
that dot lengths at both ends can be adjusted, to acheive any
benefit. There is no point in the receiving station reducing
bandwidth without the transmitting station increasing dot length,
and vice versa. This could be done with the aid of a phone call, HF
talkback, etc, but to me this is missing the point a bit - it's much
more impressive if an LF contact can take place without any
"external assistance". One way to do it might be to change the
signal reporting system from the TMO good, bad or indifferent, to a
single letter system where the letters represent "OK for 3s dots",
"OK for 10s dots" etc. That way, both stations could start with long
dot lengths, and adjust speed accordingly as the contact
progressed. If this could be made to work efficiently, I think it would
be a great help.
I used the figure of 1 hour for a QSO because in my experience,
this is a typical duration for a propagation "lift". Occasionally, I
have seen good signals for up to 2 hours continuously. I don't think
I have ever seen a 4 hour lift as reported by CT1DRP - maybe
propagation is different at his more southerly QTH?
I set a bandwidth of 10Hz based on the experience of the last few
months. The total LF band is 2.1kHz, but a large chunk of this is the
long standing "CW Only" band segment, and various other parts of
the band are not really usable at any particular QTH because of
various types of QRM; CFH for example. As I recall, the use of
135.9 - 136 was a sort of consensus amongst North American
stations as to what were usable receiving frequencies for weak
signals. There are big practical advantages in having 2 segments
with some separation in frequency. Weak DX stations can then be
received without the RX being de-sensitised by the local stations
transmitting to them. All this means that the spectrum available for
a group of transmitting stations participating in "transatlantic tests"
is probably only 100Hz. This segment is likely to include assorted
other narrow-band modes, so there would only be room for one or
at most 2 MS100, 40Hz bandwidth BPSK signals. In the short term
this is not a big problem, but with increased activity on this mode it
would soon become one. Unless a high bit rate proves absolutely
neccessary, it would seem to make sense in the long term to
design systems that only require small bandwidths, so that several
stations can transmit for extended periods without mutual
interference. For general QSO use with reasonable signal
strengths, a wider bandwidth would be perfectly reasonable in the
digi-modes band segment, with the advantage of quicker QSOs.
As far as frequency stability requirements go, the recent use of
very slow QRSS has seen the sudden improvement of amateur LF
frequency control to within small fractions of 1Hz. It could be made
better still fairly easily.
For operation of the Decca TX at 1200W PEP in BPSK mode,
amplitude modulation of the PA DC supply was essential to get
reasonable sideband levels. It will work quite happily with no
amplitude control, but the sidebands go on for ever... The
modulator circuit is quite big of course, but not unreasonably so.
The unwanted sidebands are 40dB or more down on the main lobe
of the signal. The overall efficiency is about 75 - 80%, so still much
better than a linear amplifier. The modulator circuit only needs the
logic level phase keying signal and the 136kHz carrier as input. At
the moment, it is hard wired CMOS logic and analogue bits, but
much of it could be put on a PIC or similar. This eliminates the
need for a soundcard and it's attendant wrinkles, so excellent
frequency accuracy and stability are easily achieved. For receive,
I am using G4JNT's very simple PIC implementation of the VE2IQ
interface - it is also easy to set up the frequency very accurately
with this.
To the software developers, I would say that soundcards are
probably best avoided for BPSK or similar modes. They work fine
for QRSS, but this mode is relatively not fussy about frequency
accuracy. I suppose the benefit of a sound card is that it provides
a ready made A/D, D/A converter module that can be plugged into
the MIC input of a ready made HF tranceiver to generate an RF
output. But an HF rig does not make a very good LF transmitter by
itself, and the combined frequency errors of rig and soundcard,
plus the "compatability" issues that come with soundcards, have
already caused plenty of hair loss, both for software writers and
the people using them. A transmitter that generates the BPSK
waveform directly at the RF frequency is simpler overall, and
probably capable of better performance and efficiency. Any LF
transmitter requires quite a lot of home construction, and dedicated
interfaces can be quite simple, so I would suggest that this is the
better way. The VE2IQ/G4JNT interface already exists for receive,
and only requires a serial port on the computer, so I guess it would
be easier to program too.
It was funny that the postings about the "WOLF" mode crossed
mine - I have not looked at it thoroughly yet, but I would certainly
be interested to try some tests.
Cheers, Jim Moritz
73 de M0BMU
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