Australian IR tests

VK4EBP reported that he has carried out successful tests over a 10km NLOS (non line of sight) path using infra red.

See Australian Optical DX   https://groups.yahoo.com/neo/groups/Optical_DX/info .

One of my aims for 2015, assuming I am fit enough, is more NLOS tests including IR over greater ranges. To maximise my chances I shall probably use QRSS3 again rather than rely on aural reception. My previous tests, using clear air scatter, was over a 8.5km distance.   Much further should be possible.

Nanowaves in Finland

My own red LED and IR experiments have had to stop as my brain bleed last year has left me (for now) too clumsy. People are still active though. In the Cambridge, UK area there are both IR and visible red beacons GB3CAM active from RAF Wyton. See http://www.earf.co.uk/light_beacon.htm .

This was posted on the UKnanowaves Yahoo Group:

Greetings you light headed fellows :)

 .

 Darker times are coming and the sun is going down earlier and earlier here in Finland.

 I've been designing an IR based repeater/beacon. When not in repeater use, it transmits FSK441 and CW. It has 6 of 850nm RX heads 0, 60, 120, 240, and 300 degree directions. This enables some selectivity by choosing best RX head by SNR from the received signals. It scans through all of the RX:s and if it detects an signal, it will switch to the best SNR (this is done by MCU).

 The TX is basically 10 IR leds transmitting simultaneously in all directions (they are spread over circular 360 degree form with 36 degree increments).

 I have 6 credit card size fresnel lenses that I was thought using in front of the six BPW34. I thought that a little off-focuse would make the system less prone about the direction of the signals. What worries me is the sun. This system will be installed relatively high and it is certain that the sun will be focused to the photodiodes time to time. 

 Using a small IR filter in front of the BPW34 probably isn't the the best idea, because all of the energy from the sun is already focused to a relatively small area. Probably the best idea is to use the same sized IR filter than the primary antenna - the fresnel is. I've heard that exposed colour film would do a good job in this? Any experience?

Why I put two subjects under one message is that I just wanted to let you know what I'm doing, hi.  And all the ideas will be taken in!

73 de Janne OH1SDR

PS. This repeater will have THz input and 70cm output and vice versa.

Optical forward scatter and frequency?

Perhaps some of my readers can help me - what optical frequency is likely to be best for forward scatter off air molecules and dust particles? My tests to date have been over non line-of sight paths up to about 9km at 481THz (red light), but IR should work in daylight with filtered PIN diodes on RX, but I don't know whether infra-red frequencies scatter more easily or worse than red light?  With red light, the RX can easily be de-sensitised by bright sunlight, and this should be (?) less of an issue with filtered IR detectors, I think.

As the atmosphere scatters blue light very well (giving it a blue colour) one might expect IR to scatter less well than shorter wavelengths like visible red or ultra-violet? See http://en.wikipedia.org/wiki/Atmosphere_of_Earth .

Infra-red "over the horizon" tests?

Reading Stuart G8CYW's article on the history of optical communications in the latest RadCom, I am considering repeating my recent NLOS optical test using infra-red (IR) rather than visible light as there may be some advantages with clear air forward scatter propagation by going to lower near optical frequencies. At least IR beams are not visible to the casual viewer, so are less likely to create problems.  However, IR can be more dangerous than visible light because the blink reaction that occurs with visible light does not happen. When using high power IR LEDs it is therefore even more important not to look into the beam and to take care where the beam is aimed, especially at close range. Never ever look into the beam at close range.

A suitable IR PIN diode is the SRH203-FA from Osram, widely available on eBay.  As you can see from the image, the PIN diode detector has IR filtering (black colour) , so may be usable in daylight with less degradation than would be the case with red LEDs.  I already use the SRH213 PIN diode for visible red optical comms.

I am looking for a suitable 1W-3W IR LED and they are available, but I have yet to find one in the 10mm "fat" standard LED package that I use currently on 481THz.

There are 3W IR LEDs available from China that should be suitable. See eBay item 370784927290 for example in the star package. This would be some 10dB more output than I currently use, although I am not sure of the beam divergence which may be greater than with the 10mm package.

Nanowave over-the-horizon experiments are very much like microwaves, except that the test equipment is a GREAT deal simpler!