[Laser] successful 46km daytime 850nm NIR contact

[Stuart]

Successful 46km 850nm contact


Stuart, G8CYW, Brian G8KPD and Peter, G8POG, and Gordon G8PNN set out at 1200 BST today to attempt to increase the distance worked on 850nm NIR in the middle of the day. The intention was to be operating at 1307 BST which is the sun transit time today (highest in the sky). We were thwarted by a number of factors including rain, grey clouds and mist at both ends of the path when we arrived at our respective locations, plus Gordon's car had hit a large pothole and damaged a wheel and tyre on the way. Brian and Peter went to his help and eventually the RAC were called and Gordon had to return home, so we were one 850nm station fewer. Brian and Peter returned to their site later, more rain and wind were in evidence, but after nearly two hours and a weather radar report from Eddie G0EHV said  that things were getting better, we set up.
The stations were both using LED transceivers and single optics.
This time we had decided there were to be no visual aids such as beacons or strobes, instead both remaining stations had aligned their optics to a higher accuracy than ever before hoping simply to beam accurately at each other. Stuart began radiating a 20kHz subcarrier on 850nm and almost immediately Brian picked up a tone on LSB, which was confirmed by Stuart keying it on and off.  Both rigs were then adjusted for aim and we were in QSO around two minutes later, the quickest set up time we had ever managed. Signal reports were exchanged at 59 both ways on SSB,  FM was also tried, signals were strong but fluttering so much it made copy difficult. The wind which was forecast to be at 10mph, but it was at least 30mph on the tops, causing the gear to vibrate and much turbulence in the atmosphere. This was another case where SSB gave a better result.
We then went into conversation mode on SSB which as stated, won the day over the atmospheric conditions. We actually made contact within a two hour period around local noon, but I did not get round to recording a sample until a few minutes after this. We maintained the contact on various tests for around 90 minutes. After the rain the sky cleared up and there was even sunshine at times.
So having dispensed with the visual aids and made contact despite the inclement conditions, we moved on to the next trial, which was to use an iris plate attenuator to reduce the signal by a known amount. Put simply, a 6dB attenuator (which reduced the area of the Fresnel lens by a factor of four to roughly a rectangle 10cm by 14cm) reduced signals on SSB to 57 one way and 58 the other. This roughly simulates the signal level at twice the distance without taking account of the larger extinction coefficient at that distance. 
The iris plate can also acheive a 12dB reduction in signal strength,(which meant we were operating through a silly little hole roughly 5cm by 7cm in a sheet of cardboard in front of the lens) was duly tried next. Although Stuart clearly copied Brian with it in place, the wind ripped the cardboard attenuator off the front of the rig and hurled it across the road. Stuart recovered this before it got run over by a passing car. We were not able to fully explore this due to some re-aligning  being necessary due to the rig being buffetted by the strengthening wind, but it indicated that a further signal margin was available. We gave up with the attenuator at this point as it was felt that the result at 6db alone was useful.
We also noted that the received noise level due to daylight was around S3-4 using 850nm long-pass filters in the rigs. Removing my filter increased the noise to S6-7 so the use of a long-pass filter is a benefit, particularly as it had no effect on the wanted signal. The iris plate had exactly the predicted effect on the noise level as was expected, the 6dB aperture reducing the noise(and signal) by 2 S-points.
So our result is that subcarrier communication "in the middle of the day" is possible using 850nm NIR, at 46km distance.