LF: QSY

["Dick Rollema" <d.w.rollema@freeler.nl>]

To All from PA0SE

Reading Practical Wireless of June 2002 I came across the
following item (conversion by OCR):


Laser Comms

David Bowman GOMRF and Allan Wyatt G8LSD have achieved a laser communication record with a c.w. QSO over 49.3km

David Bowman GOMRF and Allan Wyatt G8LSD have been conducting tests over
the last four months together with Derek Atter G3GRO and Lech Laszkiewicz G3KAU in a bid to develop laser cornmunication distances. The distances gradually developed from a few
tens of metres to 20km, then 40km and then finally on Saturday 5 April 2003 to the record 49.3km. The path for the QSO was from Fairlight east of Hastings to Capel le Ferne east of Folkestone.
David, accompanied by his wife, arrived on site at 183OUTC and saw a good sunset but no view, as it was quite misty. The power station at Dungeness some 30km away disappeared completely by 1900UTC and by 2125UTC they had a sighting of the powerful white lights (used for aiming) as a dirn yellow disc.
The laser was aimed at the disc and at 213OUTC the laser was switched on and Allan (?  PA0SE) GOMRF received the signal immediately and within a few minutes both systems were aligned. Reports of 559 and 579 as well as both callsigns and additional characters were exchanged by 2145. The distance was verified by using the G4JNT software.

Semiconductor lasers operating at 670nM were used at both ends of the contact.  Three to a beam expander at the G8LSD end, and 1OmW barefoot at the GOMRF end.

Both receivers were designed independently and operated at audio baseband without a carrier. The 488Hz tones were generated in the transmitters for modulating the lasers by the dividing down of a crystal oscillator. Keying of the tone providing the c.w. and both receivers featured audio filters to reduce bandwidth.
Each station had separate receiving and transmitting equipment. David GOMRF used a purpose built telescope using a 1OOmm lens to gather light on receive, and Allan used a Russian 4.5in reflector telescope and swapped the eyepiece for the detector head. Both transmitters used tripod bases with vernier adjustments mounted above. Finally, the transmitter was mounted on the vernier mechanism.
The aiming of the laser was by rifle sight secured to the transmitter housing. Strong and very cold winds worked against long term holding of the beam position. But even when the laser dimmed from the brightest light in the sky to only just visible with binoculars, the signal remained audible.
The PW Editorial team congratulate David and Allan on their achievement!