Message-ID: <18142.200103071719@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Wed, 7 Mar 2001 17:24:08 +0000
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Subject: LF: <TECH>Re: Transcontinental modes - what next?
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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