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rmax
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February 14th, 2012, 3:24 pm

QuoteOriginally posted by: Traden4AlphaQuoteOriginally posted by: outrunwhat about laser instead of microwave?Much much worse. I'd expect near zero transmission beyond a few kilometers during any kind of fog, snow, rain, dust, smog, etc.Longer wavelengths are better WRT atmospheric attenuation but worse WRT bandwidth (which means worse latency for a given message length).Yes indeed. I believe the 30 mile hop os LOS as mentioned by T4A. Laser is dreadful - the US/UK etc were very lucky in Gulf War 1 that it took place in a nice warm sunny country, and hence laser sighting worked well, rather than were it was designed to take place (Central Europe against the Russkies) when there is quite often fog, rain, snow etc.
 
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farmer
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February 14th, 2012, 5:02 pm

Whatever. If the Northeast weren't such a hellhole, I would come up there and build it in one hop with $600 worth of junk from the ham radio swap meet just to prove by example...
 
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farmer
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February 16th, 2012, 12:46 pm

I am thinking of crowd-sourcing my antennae. Find the local ham radio club in each city, and then offer $1000 or something as a prize to whoever gets the best signal through. I know these people all have their own transceivers. So I am trying to think of a way to make a nice cheap transceiver, and a way to modulate a signal onto it from a really simple PC program. It don't need no minimum shift keying or anything like that. Getting the received signal off of there is a little harder. Any ideas as to how I can go from a C program over a USB cable to either a standardized budget radio peripheral and their antenna, or even somehow interface to a USB pin to a a variety of transceivers that are not designed for such an interface?At 15 kilohertz, I could go three cycles on, three cycles off, and get 5 bits per millisecond. If we establish a base price or handle over wired Internet, that gives us a 32-tick range around it for the latest price over radio. So it is not totally crazy to contemplate an open-air audio modulation. It would be pretty crappy. But it might be enough to test the antennae, and then bring in the real equipment after the antenna has been selected.
Last edited by farmer on February 15th, 2012, 11:00 pm, edited 1 time in total.
 
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farmer
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February 16th, 2012, 8:23 pm

QuoteOriginally posted by: farmerAt 15 kilohertz, I could go three cycles on, three cycles off, and get 5 bits per millisecond. If we establish a base price or handle over wired Internet, that gives us a 32-tick range around it for the latest price over radio. So it is not totally crazy to contemplate an open-air audio modulation. It would be pretty crappy. But it might be enough to test the antennae, and then bring in the real equipment after the antenna has been selected.I could make a web application that plays the tones and listens to them. And then offer a $5000 prize in a worldwide antenna contest. Anybody have any experience on whether acoustic coupling to a shortwave radio from a PC mic and speaker has any chance of transmitting 5kbps?
 
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farmer
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February 17th, 2012, 7:11 pm

I am maybe going to try the acoustic coupling with a Galaxy Saturn Turbo. If anyone can think of why this would be really stupid, please tell me before I waste my time.
 
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farmer
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February 17th, 2012, 10:14 pm

QuoteOriginally posted by: outrunCan't you connect AUX out with AUX in, instead of using a speaker and a microphone?My PC has an AUX out? Be more specific. I am trying to make something that anyone with an HF radio can participate in propagation experiments.
 
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farmer
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February 18th, 2012, 10:55 am

QuoteOriginally posted by: outrunI'm certain your pc has a plug somewhere that'll allow you to connect external speakers or a headphone. Some older pc's have audocards with lots and lots of plugs...I can see it now...farmer: I'm not getting anything. Did you plug the other end of the cord into your radio?ham radio expert: Yes.farmer: Is your radio set to 29 megahertz?ham radio expert: No, what? It is set to 29.00 these are frequencies, what is megahertz?ham radio expert: Hold on a minute. My PC was not turned on.
 
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farmer
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February 19th, 2012, 7:44 pm

I am still not sure how well sloppy acoustic coupling will work for the home user. It may need to be done with hardware designed to place speakers and microphones in tight positions. Here is an example of two bits high at 20 cycles per bit, 4khz triangle waves drawn at 16khz, sampled at 32khz:The first one has a pretty clean definition where the high-amplitude bits end. But the second one looks like it has an echo. That is with my computer playing over built-in laptop speakers, and receiving 15 inches away in the mic at the top of the screen. I guess he could place his stuff tighter than that, with the mic right on the speakers, and transmission would have to be one-way.Aliasing seems to really weaken the signal even sampling at 8 times the frequency, since even if I have 20 cycles per bit, they are all aliased in about the same way. If anybody has any advice on acoustic modulation, I would be curious to hear it. For now I am going to take some time off and think about it.
 
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farmer
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February 19th, 2012, 8:05 pm

Anyway, here is the source code if anyone wants to show how to build an acoustic modem by example. (Needs DXSDK 9.0).
 
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rmax
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February 20th, 2012, 11:54 am

QuoteOriginally posted by: farmerQuoteOriginally posted by: outrunI'm certain your pc has a plug somewhere that'll allow you to connect external speakers or a headphone. Some older pc's have audocards with lots and lots of plugs...I can see it now...farmer: I'm not getting anything. Did you plug the other end of the cord into your radio?ham radio expert: Yes.farmer: Is your radio set to 29 megahertz?ham radio expert: No, what? It is set to 29.00 these are frequencies, what is megahertz?ham radio expert: Hold on a minute. My PC was not turned on.LOL.Anyway. I asked some people about the microwave link. Microwaves follow the earths curvature a little, hence you can perform your LOS calculations on 4/3 earths radius. If if live in foggy conditions, you need to work with 2/3 earths radius. When placing your dish you also need to be aware of the Fresnal clearence zone. This is where the diffraction of the side lobes reflects on a surface (earth or building etc) and generate an out of phase reflection at the recieving end.
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