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#11
August 9th 05, 02:02 AM
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Mike Coslo wrote:
wrote: Mike Coslo wrote: wrote: Mike Coslo wrote: an old friend wrote: Dave Heil wrote: Mike Coslo wrote: Dave Heil wrote: Of course the patience of Job will be needed for all the error correction needed Slow...error prone...not used by other services... Where'd I hear that before? ;^) HF will never be the place for high speed digital transmission. There is too much noise and signals are subject to the vagaries of wave propagation phenomena. A lot depends on what you mean by "high speed", and what resources you can use. If you're used to 45 baud RTTY, 1200 baud is high speed, isn't it? For example, if you're allowed to use wide-enough bandwidths, all sorts of stuff is possible. Sure. I think some people are believing that I am saying this is impossible. It is not impossible. But not very practical. I'd have to work it out, but I think there are some ham segments where we'd need more bandwidth than is alloted The difference is that unlike, say, the US Army in 1952, there is no High Command that determines who gets what frequencies for what path at what time. If you're allowed to use very high power and high gain antennas, all sorts of stuff is possible. A strong signal mode? 8^) Sure - that's predictable by Shannon's. It's not hard to understand and explain, if you really understand what Shannon is all about. (Some folks make a lot of noise about it, acting as if it's some big deal, but it's really quite easy to understand.) Consider this situation: Plain old on-off keying lets you send one unit of information per unit time. Either a 1 or a 0, on or off. Digital and all that. But suppose we have a system (transmitter, receiver and connection between them)that has four states - 100% on, 66% on, 33% on, and off (0% on). We can then apply a meaning to those four states - say, 11, 10, 01, and 00, respectively - and send two units of information per unit time in the *same* bandwidth. We've just doubled the data rate without changing the modulation method, bandwidth, or basic keying rate. But for it to work, the system must have good enough signal-to-noise ratio to determine the four states reliably. That principle can be continued as far as our signal-to-noise ratio allows. For example, a system of 128 states could be done, if the system S/N is good enough, allowing the transmission of seven units of information per unit time in the same bandwidth as was previously used for one unit. IIRC we had this conversation before, only it was about a PSK system with many more states, rather than an AMK system. And there's no reason multiple carriers can't be used, as well as multiple ways of modulating the same carrier - say phase and amplitude modulation at the same time. Of course the entire system has to have adequate s/n to deal with the tiny variations of phase and amplitude. Telephone line modems make use of the predictable stability and characteristics of the telephone lines. HF radio is a bit less stable. Of course if the taxpayers are footing the bill, all sorts of things can be done. Sure! Look at what the military folks did. Big rhombic and curtain arrays, etc. Find a suitable site, take it over as necessary for the military purpose, put up whatever is needed - on the taxpayer's dollars. If you need more power, just get it! Receivers like the R-390, costing thousands of 1950s dollars? How many racks of them are needed? Etc. Completely different from what most hams deal with. If you can separate the transmit and receive sites and/or frequencies so that full duplex is achieved, all sorts of things are possible. OY! Very common military and commercial practice. If your setup has adaptive features so that it evaluates the path characteristics and adapts the modulation and frequencies used to conditions, all sorts of things are possible. Doubly Oy! It's what ALE is all about. Just not too applicable for our purposes. My Elecraft K2 has programmable memories that can be set for each band and mode. Other rigs have similar features. It would not be difficult to have it step through them (using the RS-232 port and computer control) comparing each frequency with the others. With suitable time synchronization between, say, you and I, the rigs could step through the various preprogrammed QRGs, looking for the spot with the best propagation and no QRM. Of course we'd have to set it all up beforehand so we'd - or rather the rigs - would know precisely where to send and listen. The K2's ATU also remembers antenna tuner settings per memory. Of course most of the above is simply not practical for the average ham, and/or is incompatible with current US regulations. really? gee it falls into the same catagory as when it was said that we ham had been banished to "useless frequencies" everything above 200M Hams were never banished to everything above 200 meters. What happened was that amateur stations were required to use only wavelengths of 200 meters and below. But every station had a specified wavelength. If a ham wanted to use, say, 159 meters, s/he needed a station license that said "159 meters". In 1912 there wasn't much known about how HF propagation actually worked. The 'useless' idea came from extrapolation of what happened on longer wavelengths. The ionosphere's role was not even guessed at by "professionals in radio". There are some pretty darn good reasons why high-speed digital HF won't work well. And they aren't related to early "knowledge" that caused hams to be relegated to those higher frequencies at the time. Radio is a fairly mature field, and digital is getting there. Many people have a pretty good idea what will likely work, based on education and experience. And HF is an unruly beast, given to noisy and incredibly variable conditions. We don't have to be rocket scientists to gain that knowledge. Just as an exercise, how much information can be carried by a 1.8 MHz signal? As much as the S/N allows! See above. How much error correction will be needed during the summer, and how much during the winter? Why is there a difference? Why would a wireless digital transmission system use UHF and above for data transmission? All very good questions! when was that Jim A long, long time ago. When almost nothing was known about propagation. Jim might note that they do some bandwidth tricks in similar manner as he proposed per our conversation in here earlier. Not exact, but along the same lines Hopefully we will see an article from those who know the right way to HF digital soon. 8^) I don't have my CQ handy, but it took them a fair amount of time (measured in minutes IIRC, to transmit some heavily compressed (beyond maximum jpeg compression), and therefore really poor quality (by almost everyones standard) pictures. Didn't I make a challenge with some of the HF high-speed digital believers in here to do a sked? I think the "answer" was that I was going to steal the technology. You can't steal vaporware. Not that that is likely, but how about say some of the believers among themselves, do a proof of performance of the technology? Or is this just one of those Wondertenna type ideas that crop up from time to time, only to be found lacking when introduced into the real world? Heck, Mike, you want *practical* stuff? You betchya! I have to note that we speak of actual systems, real world devices - some of which are yet to be made of course, but practicalities (granted you much more than myself - but I am an RF neophyte, having come from the digital world) Others seem to be more in the vein of "so there! or someone is going to come along and prove you naysayers wrong!" mode. Bingo. Faith based electronics. Yes - those who dare to even like modes such as Morse Code are cursed as infidels who do not understand the Word that "newer is always better" and "the PROFESSIONALS know best" I wonder how that tremendous antenna from the UofD is coming along? You know, the one that is going to revolutionize radio? HF antennas a few feet long that outperform anything we have today. The one that was so "efficient" that it melted when the inventor powered it with 100 watts. And I'm not the one who used the word efficient, *they* did. It *is* efficient, Mike! It's very efficient at turning HF RF energy into heat. Don't expect it from "John Smith", Len, "b.b.", or even "an old friend". You won't get it. And I'm starting to think that some of them might be duplicates anyhow. Quitefine and Darkguard (nee Blackguard) indeed! Doesn't matter - they have nothing practical to offer. They're not about actually *doing* ham radio, just arguing about it. Too bad they cant do a better job. Indeed 73 de Jim, N2EY |
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