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G8AGN's FREE-SPACE LASER COMMUNICATIONS PAGE |
Here are some pictures of my gear and details of tests
updated 9 January 2011
NEW NEW - UK Optical DX Record broken using LEDs, 8 January 2011
| My first laser Tx was based on a keyring laser, modulated at 488 Hz using G0MRF's Mk 1 modulator circuit . Notice the liberal use of duct tape! The sighting scope has x 4 magnification. |
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| My first Rx was also held together
with duct tape! It is based on G0MRF's Mk 1 circuit
It uses an OPT210 sensor, followed by a 500 Hz active band-pass filter, buffer and LM386 AF amplifier. Light is collected using an inexpensive 4 inch magnifying glass. |
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The gear was first tested on 14 July 2004, over a 600m path, with the help of Peter, G3PHO. At the Tx end, the camera tripod was only just adequate for beam positioning even over this short distance. Note how bright the laser is when seen at only 600m. Although this test was carried out at dusk, the sky seems much brighter than it really was due to the long exposure time on the digital camera. |
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The next step was to get rid of the duck tape on the Tx and Rx. Here is the finished Rx. The Rx body is a Marley TED-1 fitting, which is meant for fitting an extractor fan through the wall of a building. TED-1 has an internal diameter of 4 inches, but Marley also make a 6 inch one. |
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The next test was carried out over a 6.7km path on 26 August 2004 between Edgemount and Roper Hill, both on the west side of Sheffield. The Tx and Rx were now mounted on big microwave tripods and the Tx had a G4FRE CW ident keyer. |
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The need for better laser beam steering was now apparent and so a lever system was built up. Two 18 inch levers are used - one for vertical and one for horizontal movement. The vertical lever was moved by an 0BA threaded rod (25 threads per inch but M6 thread is identical). The horizontal lever was moved using a micrometer head (again an 0BA thread). The hinge for the vertical lever was a standard B&Q item, but the horizontal hinge was made from a bulldog clip C spring. |
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The next test was carried out over a 20.3 km path on 15 September 2004 between Edgemount, near Sheffield, and the base of the Emley Moor TV mast, near Huddersfield. Here is a view of the laser as seen from the Rx end. The picture is rather poor due to camera shake, but the laser is clearly visible on the horizon to the left of the picture. |
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On 8 October 2004, G3PHO and I tried a 45.9km path between Roper Hill, near Sheffield, and Gringley, near Gainsborough. The pictures show the Tx on its lever positioner, at Roper Hill and the Rx site at Gringley in daylight. |
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Received signals were S1-2 but the CW ident of my callsign was clearly readable. Can you spot the laser as seen from the Rx site at night? (Find the red dot to the right of centre and about 1/2 of the way up the picture). As seen by eye, the laser was unmistakeable. |
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This test revealed that the lever positioning system produced some unintended periodic horizontal movement of the laser beam as the vertical lever was adjusted. Hopefully, this has now been cured using the guide structure shown. |
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A Mk2 laser Tx has recently been constructed. It is capable of transmitting in 4 different modes: 1. Laser on all the time 2. 488 Hz tone - c.w. or keyed 3. Voice using G7JTT's PWM circuit (Note: the audio input capacitor should be1uF rather than the 1nF shown) 4. Slow speed FSK using K0SM's LaserScatter software
Circuits and other details are given here |
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The new transmitter was first tested on 10 November 2004, with the help of Gordon G0EWN, over the 600m Redmires path. The picture shows the laser as seen from the Rx end. Note the reflection from the surface of the reservoir. The speech quality, using G7JTT's PWM circuit was very readable |
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The picture shows an optical test source for optimising receiver sensitivity. Basically, it's a 555 driving a red LED whose brightness can be varied to simulate a weak signal source. The LED is placed behind a small lens to give some degree of beam control.
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Contact Barry, G8AGN mailto:B.Chambers@shef.ac.uk
