08-06-2012, 12:56 PM
Introduction of Laser Communications
Laser Communications.ppt (Size: 323 KB / Downloads: 1)
Diode Lasers
Laser diodes emit an elliptical beam with astigmatism
Better units will include corrective lenses for astigmatism and to make the dot appear round
Neither of these problems are inherently bad for DX purposes but correcting them also improves divergence, a big win (more gain).
Modulation
AM
Easy with gas lasers, hard with diodes
PWM (Pulse Width Modulation)
Used by Ramsey in their kit
PFM (Pulsed FM)
Potentially the highest bandwidth (>100kHz)
Mounting Systems
Mounts and stands need only be as accurate as beam divergence
Good laser diodes will be 1-2mR (milliRadian)
A 32 pitch screw at the end of a 2' mount will yield 1mR per revolution. Since quarter turns (even eighth turns) are possible, this is more than accurate enough
Higher thread pitches allow shorter mounts which may be more stable (against wind, vibration, wires)
1mR is 1.5' of divergence every 1000', 3' at 2000 ', etc.
Applications
Transmit voice for miles line-of-sight
Use weak signal modes for “cloud scatter”
Transmit video with cheap pens
Transmit high speed data without WEP
Blind flies for easy extermination.
