Return-Path: Received: (qmail 19155 invoked from network); 6 Mar 2001 17:51:57 -0000 Received: from unknown (HELO murphys-inbound.servers.plus.net) (212.159.14.225) by 10.226.25.101 with SMTP; 6 Mar 2001 17:51:57 -0000 Received: (qmail 22039 invoked from network); 6 Mar 2001 17:51:56 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by murphys with SMTP; 6 Mar 2001 17:51:56 -0000 X-Priority: 3 Received: from majordom by post.thorcom.com with local (Exim 3.16 #2) id 14aLXp-0004Uw-00 for rsgb_lf_group-outgoing@blacksheep.org; Tue, 06 Mar 2001 17:46:45 +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 14aLXn-0004Ur-00 for rsgb_lf_group@blacksheep.org; Tue, 06 Mar 2001 17:46:43 +0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from [147.197.200.44] (helo=gemini) by hestia.herts.ac.uk with esmtp (Exim 3.16 #4) id 14aLXT-0000SW-00 for rsgb_lf_group@blacksheep.org; Tue, 06 Mar 2001 17:46:23 +0000 Message-ID: <14313.200103061746@gemini> From: "James Moritz" Organization: University of Hertfordshire To: rsgb_lf_group@blacksheep.org Date: Tue, 6 Mar 2001 17:50:56 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 8bit Subject: LF: Transatlantic 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, The winter has seen a fair amount of success in the transatlantic tests, and now that the "season" is probably near to an end, it is worth thinking about what the next steps could be, in good time for next winter. The following ramblings were the result of thinking what I could do next, in the way of technical development of my LF station, in particular regarding what would be the best mode to work on: ------------------------------------------------------------------------------------- So far, all reported trans-atlantic signals have used some type of QRSS. The variable parameter is the dot length - longer dots mean better signal-to-noise ratio, with the penalty of slower message transmission. Recognisable callsigns have been produced with 60s dots or longer, which requires more than an hour to transmit one callsign. ON7YD's DFCW roughly halves the time required, at the expense of doubling the bandwidth occupied by the transmitted signal and being slightly more complex to implement, which seems to be a good trade-off. The maximum radiated power available to several stations has reached the 1W ERP level in Europe, and somewhat more than this for the stations active in Canada. The ERP that is feasible depends mostly on how big an antenna can be put up, but in any case is limited by regulation to 1W in Europe, which eliminates further improvements. ERPs of a few 100 milliwatts have been sufficient to cross the Atlantic on several occasions. Under optimum conditions, with favorably sited and equipped stations, 3s per dot QRSS signals have been passed both ways across the Atlantic, but this is the exception rather than the rule. The problem with longer duration dots is that any kind of 2-way contact takes an excessively long time, running into several hours. This problem is compounded by the fact that propagation "lifts" on LF only last an hour or so. So it would seem we are reaching the practical limits with QRSS. 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. One way to use longer dot lengths without increasing overall QSO time is to use multiple frequencies - for example, DFCW, VA3LK's 7-tone scheme, and VK2ZTO's one-tone-per character VFSKCW. Taking this to it's logical extreme, it would be possible to assign different frequencies to all possible combinations of callsigns and signal reports, so each over of a QSO would just consist of a single tone. However, it would then be difficult to meet condition 2. I guess there must be an optimum trade off between number of tones, difficulty of encoding and decoding, redundancy and so on. I suspect it might be 2 tone DFCW, but I don't know. 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. Beacon signals are a bit different; here, the only requirement is to positively identify the signal, and make some estimate of the signal level. An on-off keyed carrier with a simple repeating pattern and a well defined frequency is easily identified with simple equipment, and has the advantage of flexibility at the receiving end - you can make the bandwidth arbitrarily narrow, or perhaps take advantage of the coherent nature of the signal, to improve detection capability. You can also monitor several signals at once. 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. Cheers, Jim Moritz 73 de M0BMU