[Laser] Voice communication using 940nm IR LED in full daylight

[I]

 Optical communicaton using near-infrared wavelengths


The light evenings now in the north-east UK at 55 degrees north place a severe restriction on operating conventional optical 


communications using red LEDs. Looking back at previous UK records (the time and date on audio files of contacts), it has 


been found that communication has taken place only when the sun dips 12 degrees below the horizon, a condition known as 


"nautical twilight". This now will now not occur here before 10pm until the end of August, or early October at a more 


reasonable 8pm. This places restrictions on activity over the summer months.




Stuart, G8CYW has been experimenting using near-infrared LEDs producing either 850nm or 940nm radiation, and filtered 


photo-diodes to communicate in the IR part of the spectrum. The experiments have been carried out not only in the hours of 


daylight, but in bright sunlight also. A 1km one-way signal path was successfully trialled some weeks ago. The challenge 


therefore, is to communicate using these wavelengths during daylight hours, ie. between sunrise and sunset. It is hoped that 


other groups will join us on this band in the spirit of healthy competition to push the boundaries and see what can be 


achieved in terms of maximum distance worked. An internet search has not revealed any reports of amateur voice 


communication using IR and only a single attempt at daylight operation over a short distance,(that is not to say that there 


has been little or no activity, just that I have been unable to find any), and so any contacts made could be regarded as 


both UK and world records if no other information comes to light (pun intended!). The unique challenge here is that there is 


no light to be seen in the distance from the other station on which to visually line up the equipment! So it is back to 


compasses, binoculars and careful adjustment.




On Tuesday evening, 26th April at 7.30pm BST, in full sunlight, Stuart, G8CYW located at his home QTH IO94CX70, and Brian, 


G8KPD/P at IO95CA63 made their first two-way FM and SSB contacts on 940nm over a modest distance of 6.3 km. Signals were 


5/9+ both ways on both modes. For this contact both stations went back to the earlier equipment described in the RadCom 


articles using plumbing pipe and 100mm glass lenses. Brian used separate transmit and receive heads connected to his optical 


transverter and FT817. The transmit head red LED was replaced by an Osram Golden Dragon 940nm 2.2W device and the receive 


photo diode was replaced by an IR filtered version of the same device, a SFH2030F. Stuart used an IR version of his LED 


transceiver connected to his optical transverter and FT817, both transmitting to Brian and receiving Brian's signal on the 


same 940nm LED.


The contact was witnessed at Brian's end by the farmer and his wife who had given him permission to use their farmyard, a 


nice level concrete surface, Brian had only to contend with an inquistive horse who stood exactly in the path of the beam 


until encouraged away by the farmer with a little extra feed! Stuart's wife Christine witnessed the other end, taking photos 


and even helpfully kicking the tripod taking a photo over the optics whilst in use! On this photo, the sunlit conditions can 


clearly be seen.


A number of local amateurs are actively building gear for this which will hopefully enable a number of contacts at 


increasing distances to be made, the signs are that this is possible as at one point Stuart covered 90% of the lens with his 


hand to estimate the potential the system has in reserve, and the signal from the other end was still clear of noise.




Now a request for help, I have searched for information on atmospheric absorption of IR and come up with imprecise and even 


conflicting data. Does anyone know whether 940nm would be better than 850nm for long distance pathe through the air? Power 


LEDs at these two wavelengths are readily available. What I need is a really accurate plot of absorption in the near-IR 


region.




Stuart, G8CYW