5 Dec 2011

481THz progress update

TX beacon optics
This morning I bought a length of 110mm waste pipe from the local Plumb Centre shop. It came in a 3m length and only just fitted in the car to get it home! After lunch I created my first attempt at some optics inserting my Poundland 100mm lens into the end of a cut section of the pipe. At the focal length I mounted my 10 pence high brightness, prefocussed, LED beacon with Bluetac. With this (crude) set-up on an old tripod and with a taped on gun-sight to help with aim, I set the TX baseband beacon running and pointed it from the stairway through a double glazed window aiming down the road. With my handheld baseband head using just the high brightness 10p LED as the detector I walked down the road to see how far I could get. In the beam in daylight I could detect the signal at the receiver at 120m. With a magnifying glass in front of the RX LED signals at 200m were strong (could not test at greater range), but it was difficult to keep the RX aligned. The red LED was quite strong visibly at the 200m test point even though running just 15mA or so through it. The double glazed window no doubt added a small amount of attenuation. What I'm not sure about is how good (or bad) this result is. Clearly with a really high brightness Golden Dragon LED the range even with this system would be considerably further.


Anonymous said...

Have you considered using an AM modulated laser from a light pen etc.? This works very well, using just an audio transformer in the PSU (same technique as the old AM transmitters)
You can get really powerful (2W), cheap semiconductor blue lasers now, with extremely good collimation, or
use a cheap, red lower power light-pen. Hugh G6AIG

Roger G3XBM said...

Hi Hugh,

My understanding is that lasers are problematic because of the very narrow beamwidth and scintillation. Also, AM suffers from interference from street lights etc. So, I will stick (for now) with red LEDs and FM/SSB on a subcarrier. As you will realise, I am "finding by doing" with lightbeam comms.

73s Roger G3XBM