For the receiver I built the first stage of the optical head described in the RadCom articles (March-May 2011) using an identical red LED (reverse biased) as the detector. I also tried a BPW34 detector, but it was not that different. This was followed by a couple common emitter transistor stages using my ubiquitous 2N3904s feeding a crystal earpiece. With the TX "beacon" running from a 9V battery and aiming out of my garage I walked across the road with the receiver and a 4 inch magnifying glass. Across the road, at about 25m range, this produced a quite respectable signal as long as the magnifying glass was focussed onto the RX LED. This was all a bit "Heath-Robinson" as I had to hold everything in my hand and move things around to get it spot on. There was quite a bit of interference from the street lights nearby.
I have no idea how much "antenna" gain there is in the built-in lens on the TX LED (a few dB?) or with the magnifying glass on the RX but with this set-up correctly aligned I would think 100m range is certainly possible. This is just the beginning of a series of tests, but I am already happy that the optical head is sensitive and that good, well aligned optics will be essential to get decent distances. More TX power is easily available by using a power LED.
Next stages are:
- A better beacon TX capable of operating at higher power on both baseband and subcarrier frequencies.
- Putting the optical head into a screened enclosure, even if a temporary one.
- Starting to think about optics. Using the same LED on both RX and TX will save on optics as just one set is needed at each end of the link.
Hello Roger, The distance you mention is more than I have used in similar tests. I used FM to reach an indoor distance of about 3 to 5 meters, with good quality in mono.
ReplyDeleteI used a red LED side to side with the IR LED for optical aiming of the magnifier glass or my telescope.
Good luck in experimenting. 73, Bert