Return-Path: Received: (qmail 21504 invoked from network); 7 Mar 2001 17:27:50 -0000 Received: from unknown (HELO warrior-inbound.servers.plus.net) (212.159.14.227) by 10.226.25.101 with SMTP; 7 Mar 2001 17:27:50 -0000 Received: (qmail 28608 invoked from network); 7 Mar 2001 17:27:50 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by warrior with SMTP; 7 Mar 2001 17:27:50 -0000 X-Priority: 3 Received: from majordom by post.thorcom.com with local (Exim 3.16 #2) id 14ahbU-0004OR-00 for rsgb_lf_group-outgoing@blacksheep.org; Wed, 07 Mar 2001 17:20:00 +0000 X-MSMail-Priority: Normal Received: from hestia.herts.ac.uk ([147.197.200.9]) by post.thorcom.com with esmtp (Exim 3.16 #2) id 14ahbS-0004OD-00 for rsgb_lf_group@blacksheep.org; Wed, 07 Mar 2001 17:19:58 +0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from gemini ([147.197.200.44]) by hestia.herts.ac.uk with esmtp (Exim 3.16 #4) id 14ahb5-0003IY-00 for rsgb_lf_group@blacksheep.org; Wed, 07 Mar 2001 17:19:35 +0000 Message-ID: <18142.200103071719@gemini> From: "James Moritz" Organization: University of Hertfordshire To: rsgb_lf_group@blacksheep.org Date: Wed, 7 Mar 2001 17:24:08 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 8bit Subject: LF: Re: Transcontinental modes - what next? X-Mailer: Pegasus Mail for Win32 (v3.11) Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: 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