At 04.27 14/03/2004 +0000, you wrote: >On Sat, 13 Mar 2004 15:44:10 GMT, Dave Dilatush >wrote: > >>Andrea wrote... >>> For the receiver I'm thinking about using a small telescope, pointing >>> it to the transmitter (hopefully 1+ Km away.. ehm). Will the lens be >>> transparent for the incoming 875nM or 950nM light? > >Surely at that sort of range you would be better going with an RF solution >no? At those sort of distances atmospheric scattering, dust, fog & heat >lens effects will be tremendously problematic. Yes, this document in particular was enlighting: http://www.freespaceoptics.com/AirFiber-Physics-FSO.pdf They state that 500m is the typical max range, trading bit error rate or bandwidth to achieve more distance gives very little in terms of what one loses (and they use lasers!). Also, it someway answers to my question if 875nM is better than 950nM by showing that water vapor is absorbed at 950nM much more than at 875nM. So I'll use my IR system just for short range applications. However, as you suggested now my interest moved into RF. I would like to understand what wavelengths are more suited for long range communication, and why (well, basicly I understand that different substances absorb certain frequences more than others, so in trying to find the "average best wavelength for communications" it would be useful to have tables of EM absorption of different materials, rocks, terrain, water, etc..). Also, I still like the idea of the pulse-time modulation. One way to apply it to RF would be to build a high Q resonant system (a capacitor and a inductor, being the damper (resistor) the antenna?), and then feed it a high current (maybe stored in a capacitor) to send a short burst at that frequency. I reckon that pure pulses would violate all EMI laws on the planet, and thus are to be avoided.. but a tuned system shouldn't be illegal (provided that the use of that frequency isn't illegal as well.. and that, ok, one obtains a licence to transmit at such high power levels, even for short bursts O;-) ). Back on the frequency selection issue.. any idea? -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics