481THz (light) - first test results

Today I carried out my first optical communications tests since 1966. I built a small "baseband" (i.e. not on a sub-carrier based) optical transmitter producing a tone at around 800Hz feeding a standard low cost high brightness red LED at around 10mA current. The LED has a small built-in lens which produces a beam of around 20 degrees.

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.

137kHz transverter picture

137kHz transverter with FT817 and SignaLink USB in the background

Thought people might like to see the current version of the 137kHz transverter used for WSPR transceive. The heatsinks are total overkill as they barely get warm when producing over 20W RF into the loop antenna. I am regularly getting reports from a couple of stations 250km away. M0BMU (69km) and G0WCB (101km) are reporting the WSPR signals almost all the time. It works but needs boxing, when I get time.

A local 478THz (red light) beacon !

Map showing coverage of the 471 THz optical beacon GB3CAM

Guess what - I've just discovered that not only is there a decent level of optical comms activity in my area, but there is also an active 478THz beacon beaming (almost) in my direction from the Wyton site near Huntingdon, location of the GB3CAM beacons on 10GHz and 24GHz. The beacon is a narrow beamwidth (5 degrees wide) signal which is FSK keyed between 1 and 15kHz allowing both baseband and hetrodyne optical receivers to be used. The beacon was designed by Bernie G4HJW. The distance from the beacon to my local /P high spots in the beam would be around 30km, so this will be an ideal test for the optics and receiver, when built.

Google Sites website design

My wife sings in a local choir called the Cambridgeshire Choral Society. They have had a website for some years but it had not been regularly maintained, so I was asked to create this new one. This I have just done using Google Sites. Although I'm no website expert, Google Sites allows quite a credible website to be created without knowledge of HTML coding. I am quite pleased with the result and hope it encourages a few more people to attend the concerts and join the choir.

Their next major concert is in Ely Cathedral in March 2012 when they will be singing Elgar's famous "Dream of Gerontius" which is a wonderful choral work. They are also singing Britten's "St Nicolas" in late January 2012.

In the next few months I hope to tidy up my QRP website and give it a refreshed look.

IC703 Sold

For around 6-7 years I've owned and used an IC703 10W QRP radio as part of my station. Although I have worked all over the planet with it on SSB and CW, it had not been used anything like as much as the FT817, so I decided to "de-clutter"and sell it.  It was bought by a local friend who hasn't got any HF SSB/CW gear apart from a homebrew rig for 20m.  I had thought of trading it in, so offered him the rig at the trade-in value. If he doesn't get hours of fun and enjoyment from it, especially on 10m at the moment, I'd be very surprised. You see very few of these transceivers available on the second hand market, I guess because owners hang on to them.

UKNanowaves Group

G0EHV's lightbeam kit (from the UKNanowaves group photos)

Today I joined the UKNanowaves Group which is dedicated to optical communications.  There is a lot of useful information on this group in the postings and in the files and photos sections. Reading the membership list I noticed several local amateurs interested in optical comms, so when I get my equipment for 481THz working credibly I will have a good chance of some QSOs locally.

Today some of my optical comms electronics parts arrived so I hope to start experimenting with these on the bench shortly. Most gear built for 481THz is simple and homebrew. Apart from designs using transverters to HF or VHF, all kit is in the 0-40kHz frequency range, so easily engineered with simple test equipment. Perhaps, like VLF and LF, this is partly why it appeals to me.

137.5kHz transverter desense - fixed

This afternoon I fixed the problem with RX desense by adding a relay to switch the RF input and to turn off the PA supply when not on TX. It has fixed the RX sensitivity issue and added about 10-15dB to RX sensitivity using the transverter. PA0A was about 30dB S/N on QRSS and DK7FC about 20dB S/N on QRSS. M0PPP is now a better signal with me too on WSPR. The TX is still running at 500uW ERP and GW0EZY is copying me in mid-Wales OK (251km) this evening on 137.5kHz WSPR.

137kHz Transverter RX Desense

When testing my transverter on the bench today I noticed the receiver was being desensitised by the undriven TX strip. I don't switch the TX part off when on RX, I just don't apply 10MHz drive into it. There must be some source of low level noise from these stages when on RX as the desense is around 10-20dB. Despite this I am still able to copy M0PPP at 182km pretty consistently these days on 137.5kHz WSPR. I need to fix this problem before this project is boxed and "finished". When the TX strip is powered down the sensitivity is very good indeed.

The 481THz Band

My tests on 137kHz WSPR are now all but completed and I do not intend to take this work any further now I've "got the measure" of what is possible. I shall put the transverter in a box after a tidy up and use it periodically over the winter.

Now I am about to start something new: 481THz band communications! This is 623nm red light for the uninitiated. This evening I've made a start by ordering some suitable components to allow benchwork to start next week. My initial tests will be low powered beaconing with a portable receiver walking down the road. Assuming this is promising, I'll then refine the kit and organise optics to give some "antenna gain" to allow much greater range.

There are a lot of resources on the internet about light beam communication, especially using high power LEDs and there was an excellent series of inspiring articles about this in the March-May RadCom this year.

10m Chirpy Measurements

Last week G6ALB made a copy of my Chirpy 14 component transceiver for 28MHz CW. Andrew had access to some better calibrated test equipment and carried out some RF power and sensitivity measurements both on his version and on my second original unit - the first was just a rat's nest on the bench.

Both on my version and G6ALB's version the measured RF power out was in excess of 200mW, which is around 2-3dB more than I had crudely measured. The RX sensitivity on both was such that below -100dBm (around 2uV) was audible in our earpieces in a quiet room. Backwave carrier on TX was rather too high at around -10dBm. Second harmonic was also only around -6dB, so a low pass filter is really a necessity apart from casual short tests.

It does seem that the simple design is reproducible and its performance not at all bad for something this simple. The only major shortcoming is the chirp.