Crawley Microwave Round Table

by Chris Whitmarsh G0FDZ

Laser equipment and operators. From left to right: Derek G3GRO, Allan G8LSD, Nick G4ACW, David G0MRF, Chris G0FDZ, John G7JTT

At the Crawley event in 2002 there was some discussion that showed that a number of those present were interested in lightwave communication. With that in mind, it was decided that the theme for this years round table would be lightwave (laser) communications, and what a success it turned out to be.

Around twenty five microwavers attended the event at the Crawley Radio clubs QTH, attendees included several people who weren't strictly interested in microwaves but came along because they too were interested in laser communications.

Activity outside the Crawley clubhouse. G0FDZ's ststion in the foreground with G8LSD by his red VW minibus (mobile shack)

During the morning period, the weather being pleasant, a number of lasers systems were set up outside the hut and much testing and short range QSO's resulted. It was extremely pleasing to have the chance to chat and exchange ideas with other 'lightwavers', where previously a number of us had been working in isolation and had never met up before.

Allan G8LSD with Dave G6KIE

Although most of the systems shown used lasers (both solid state and helium/neon), there were some interesting transmitters using stacks of LED's. Discussion also centred on the practicalities of laser alignment over long paths, and also the pro's and con's of various modulation methods with a number of ideas presented.

David G0MRF with his record breaking laser station.

So far, the majority of laser contacts have taken place during the hours of darkness, but the use of optical filters allowing possible contacts during daylight were mentioned, but these tend to be very expensive with narrow bandwidths giving compatibility problems between different frequencies of red- light solid state and gas lasers. I think that nighttime operation will continue to be the norm for the foreseeable future.

After lunch, the first talk was by Chris G0FDZ on 'Laser Comms - The simplest approach'. Chris showed his laser system built back in the 80's using a Maplin helium/neon laser. The transmitter uses a beam chopper to create a modulated beam, and a solenoid optical shutter, which could be keyed to produce a simple CW transmitter. He also incorporated a simple AM modulator for speech that would give low-level modulation at around 10%. On receive, Chris had a choice of two detectors, the first is a simple photo-PIN diode system, and the second uses a photo-multiplier. A four-inch diameter lens provides the light gathering optics.

Allan G8LSD followed on with a talk on more advanced systems, one of which he had used earlier in the year to cross the English channel, and also to make the UK record of 49kms. He presented a simple cheap transmitter option using a laser level obtainable from B&Q and other retailers. His main system uses a solid-state laser and beam collimator combination, and pulse width modulation for speech. An 488Hz tone is available for CW keying. On receive, Allan uses an OPT211 detector and a Russian astronomical telescope for the optics. See here for pictures The use of a cheap strobe, directed at the distant station to pinpoint his exact location, allows the time taken for the alignment of the lasers to be greatly shortened, and for receiver checking both locally and at the distant end. Some discussion was made with regard to propagation in various weather conditions, bearing in mind experiences gained when crossing the channel, and the possible usage of DSP techniques such as those already used to great effect on 137kHz.

Just to ensure that microwaves were not forgotten, Derek G3GRO talked about his new 23cms masthead mounted system that he used successfully during the recent VHF NFD. The system produced an impressive 80 watts easily.

This event has surely to be the largest gathering of UK laser enthusiasts so far, with six lightwave operators with working systems, and a number of others who departed the event with various laser and optical components in hand sufficiently enthused to have a try themselves.

Many thanks to Derek G3GRO and the Crawley Radio club for organising this event. Sufficient interest in lightwaves has been generated, that the idea of organising a 'Lightwave Round Table' early in the New Year now seems to be likely.

Held on the 1 SEPT 2003

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