VK4EBP's NLOS Lightwve Experiments - Part 3

F1AVY's Cloudbounce System

 "Returning to Steve's questions and our general discourse:

..is it (the RX) fairly small and portable...and lensless?

I do have a lens in the RX I now prefer to use - although it happens to be a convex lens from a tabletop magnifier - about 7x12cm giving me a narrow reception beam and a fair bit of gain compared with naked photodiode. There seems to be a general agreement that increasing the size of receiving aperture will capture more light and is the best way to greater gain.

Here I need to qualify gain requirements for my particular applications. I am currently exploring relatively short-distance NLOS paths in urban environments with abundant QRM from city and suburban lights, and this already limits my gain requirements. I developed my own qualitative test of satisfactory sensitivity - by pointing my receiver at individual stars in the sky. If I can hear them twinkle (yes they do twinkle!!!) then I am generally satisfied with the project.

On cloudy nights, pointing the RX into the clouds brings an interesting combination of mains harmonics and general hash from neon lights, clicking pulses from aeroplane lights, sweeping sounds from airport and marine lighthouses. In an environment like this, going to extreme lengths as one would do for very long-range cloudbounce or LOS comms in dark and secluded areas is simply not required and uneconomical. I initially did invest time and money in large receiving boxes with the best components there are, only to find that, unless I moved out into the bush, I could not take a real advantage of the improved performance, and did equally well with smaller and more wieldy portable designs.

"Is the idea, when talking about NLOS, to light up as much sky as possible...and that is why no focusing lenses are employed? Would a focused emitter, as produced by the typical Fresnel lens light box, produce too narrow a beam than desired? It would seem likely that with precise aiming combined with the gain to be had by utilizing fresnels (for example), would produce longer paths??"

There are several separate issues there. One is the implied requirement for a large Fresnel lens in order to obtain a narrow beam. First, how narrow is narrow enough? If we can be satisfied with say +- 5 degrees then there are more wieldy solutions available - most of the high power LEDs now have clip-on small spot lenses awailable separately - usually from third-party manufacturers. These were unheard of until recently. Standard-sized LEDs are available with very narrow radiation pattern, e.g. the SFH4550 at barely +-3 degrees. And one would expect an array of 200+ of them to behave quite like our stacked Yagis, many times over! Hence narrow beam is quite possible even without an external lens.



Osram SFH4550 IR LED

Of course the extremely narrow beams that one would prefer for LOS DX comms would be next to impossible to align in NLOS situation. (Invisible infrared is not a problem in LOS as it can be easily supplemented with e.g. a plain strobe light for alignment purposes.) I find my +-3 degrees array (with similar +-3 deg aperture at receiving end) hard enough to align over the horizon, and it is probably the limit of how narrow one can go to remain practicable in NLOS.

"to light up as much sky as possible..."

It is possible that cloudbounce and obstruction-bounce will develop into separate NLOS techniques, along with atmospheric scatter.  Of course one can think of all of these in some topographies.

I had some success in inter-suburban cloudbounce with high beam elevations at both ends - 30 to 60 degrees. Also with 45 degrees or less elevation with TX and RX about 1km apart side by side and pointing at the same cloud in the distance. Power used around 10-20W, beamwidth +-5 to 10 degrees, red and infrared with similar results (including 950nm). The received signal was just strong enough for telegraphy or qrss, but never near enough for any type of voice modulation. Results were affected by the height of cloud cover necessitating frequent elevation adjustment, and an additional liaison channel on VHF. Signals disappeared entirely as soon as the clouds started producing rain.

I never got any usable results with clear night sky and the TX and (imaginary) RX beams crossing each other at similar elevations. Only at very close distances - the best signals being received with blue and infrared. Presumably Rayleigh scatter with blue. (This raises the possibility of usable atmospheric scatter with UV?)

Only recently I commenced preliminary tests with close-to-ground, low-elevation NLOS and immediately the infrared proved itself to be a clear winner with quite reliable and repetitive results. (Needless to say the signal strengths are marginal compared with direct LOS over similar distance.) Intuitively, one would suspect general scatter and bouncing of nearby objects being the main sources of some of the energy reaching over the horizon, with some contribution perhaps of atmospheric/dust/aerosol scatter.

This adds an additional dimension to your question.. to light up as many objects around the TX(or around RX, or around the obstruction) as possible, or send the focused beam as far as possible? I guess it will very much depend on topography. I can imagine a situation where I would prefer to use a broad floodlight rather than a focused beam - e.g. to illuminate as many as possible of the surrounding tall buildings if transmitting from a ground floor apartment. Or from the foot of a mountain. An opposite rule would apply if it is the receiver that is just behind an obstruction - here a focused TX beam and a broad RX aperture might prove advantageous. And where the main obstruction is midway between TX and TX than the usual narrow-beam configuration would apply at both ends. I think it might be worth trying an infrared TX with interchangeable lenses, and same for the RX...
The portable TRX image shows the general view. Most of the electronics is contained in a plastic box and includes an SLA battery, the RX audio bandpass filter, power amp and speaker, and most of the PWM AM TX circuitry except the final LED driver. The DIN socket provides connection to external RX front end TX output drivers and LEDs, usually held together on a tripod. The header socket is for the external microphone connection, and there are switches for power and speaker and a headphone jack. The crudely made corflute box beside it is the RX front end with lens, photodiode and preamp.

The RX detail 01 shows the business end of the photodiode, and the separate lens box (slides in and out for focus adjustment). Detail 02 shows the preamp with interchangeable photodiode modules - SFH213 for visible to 850nm, and the SFH213-FA for 850nm and below.

Multi-wavelength TX 01 (front and back image) includes several groups of fat LEDs (red, blue and 850nm) with matching lenses providing approx. +-5 degrees apertures. Also the PWM driver on 2x CAT4101 in parallel, external high current power connection, intensity control, and a PICAXE-controlled strobe or audio callsign/frequency sweep beacon.The whole assembly connects to the TRX control box (above) and can provide power to it, draw power from it, or have both DC sources in parallel.Enclosure includes a photo tripod mount on the bottom plate, and velcro strips on top to hold the RX front end.

Pocket-sized 10W IR TX is a small holiday project and still work in progress. The 7x LEDs are 2W each SFH4783 with an intrinsically narrow beam of +-10 degrees - quite appreciable for a naked LED of this size and possibly obviating the need for external optics in some applications. My usual CAT4101 drivers are included and capable of providing 2A at 50% duty cycle. Another Picaxe beacon is on the other side of the heatsink, and the voice modulator is still in progress.

The low-power multi-wavelength TX is another one day project, built to compare relative performances of 850 and 950nm IR. Three almost identical beams of red, 850 and 950 are transmitted in sequence with approx 100mA PWM running each diode. Unique CW ID has been coded for each wavelength.

6x SFH213 photodiode with individual preamps - quite a respectable low noise, high sensitivity front-end that works very well without external optics (SFH213 have a +-10 deg intrinsic apertures). A matching array of small fresnel lenses has been built and tested for narrow-beam reception, but a permanent box is yet to be added.. Some of the other gadgets seen in the foreground."

As mentioned earlier, Jan's experiments with NLOS signals utilizing IR is of particular interest to myself as I would like to continue experiments with VE7CA, on the other side of Georgia Strait...hopefully with Markus not having to move to a LOS position, but simply aiming across the Strait in a direct path, from his yard. Using Argo and QRSS3 mode, it will be interesting to see if any signals can be recovered from the cloud bottoms on some overnight runs.

Don't forget Clint's (KA7OEI) superb Optical Communications / Moduulated Light pages!