Thursday, 19 May 2016

Lightwave Madness

The 288 km path  courtesy: REAST

One of the local lightwave builders, Mark (VA7MM), brought my attention to some outstanding lightwave work conducted several years ago, by a group of very dedicated amateurs in Tasmania.

A pair of articles describes their successful attempts to send signals, via cloudbounce, over the astounding distance of 288km (180mi), crossing Bass Strait between the north Tasmanian coast and southern Australia.

What did it take to transmit lightwave signals over such a distance? Basically a system similar to the ones recently employed in our own local lightwave experiments but on a grander scale ... much grander!

The receiver is based on one of the KA7OEI designs, with modifications to increase its sensitivity. The receiver, and several other designs, can be found on Clint's website here, probably the best source of information on amateur lightwave available anywhere.

The lightwave receiver  courtesy: REAST
Although the basic receiver used a typical-sized fresnel lens, what really set it apart from most was the use of a large (10mm x 10mm) Avalanche Photo Diode (APD) for the detector, to maximize the field of view produced by the fresnel and gather every bit of light possible ... at a cost of $1200!

The 10mm x 10mm rx APD courtesy: Hamamatsu

The transmitter was also big, consisting of an array of 60 red Luxeon III LED's, similar to the Red Rebel Luxeons used in our own local tests. Each LED had its own 12cm square fresnel lens, heatsink and method of focusing. Certainly this was a mammoth project, by amateur lightwave standards.

The 60 LED TX array courtesy: REAST
One of the biggest problems when using such a high-gain system, is the difficulty in pointing. They found that aiming in altitude was simply a matter of pointing a few degrees above the horizon but azimuth pointing was much more critical, requiring accuracy to within a half-degree.

Earlier long-haul tests out to 209 km used the digital JT65 mode for signal decodes but the 288 km test used a fairly esoteric weak signal mode called WSC built on the Spectrum Lab software. This mode is capable of digging almost 20 db deeper into the noise than JT65, down to almost -50db.

An in depth description of the two long-haul events, including equipment schematics, can be found in "288 km Cloudbounce from Tasmania to the Australian Mainland" and in "209 km with Narrow Beamwidth Transmitter".

The 288 km crossing project evolved over several years and is all very well documented, from the first early steps, at the Radio and Electronics Association of Southern Tasmania's (REAST) website here.

This adventuresome project was largely the work of VK7MO, VK7JG, VK3HZ and VK7TW. Their work is most inspiring and much can be learned from seeing what they discovered when transmitting into the cloudy nighttime skies.

Such an endeavour as this makes the local, much shorter Georgia Strait crossing, seem like a cake-walk, but I can't imagine using anything that big and bright here without causing trouble ... it would probably appear much too 'laser-like' to talk one's way out of a jam. Pointing anything resembling a laser light into the air these days is simply begging for trouble.

I can however, envision a scaled-down version, perhaps consisting of an array of four Luxeons ... at least on my end of the path, but even pointing one of those from the city could be problematic. Perhaps any NLOS lightwave attempts across Georgia Strait will need to be well away from Vancouver and its two-million sets of eyes.

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