MICROWAVE ROUND TABLE
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.
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.
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.
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.
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
G8LSD, (see in the picture on the left, setting up his equipment)
has for a main system a solid-state laser and beam collimator combination
with 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. 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.
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 is described above.
Just to ensure that microwaves
were not forgotten, Derek G3GRO talked about his new 23cm
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.
The exchange of ideas
generated at Crawley has led to Allan G8LSD setting up a web site
devoted to laser communications. Visit www.lasercomms.org.uk
to see some of the ideas and equipment used.
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.
Chris Whitmarsh G0FDZ