481THz (light communication) progress

This afternoon I did a bit more on the optical front. I repeated my outdoor range tests using a low-cost high brightness LED as both a baseband optical transmitter and receive detector. With around 10mA TX current into the LED I was able to copy a 1kHz tone at 20m with another high brightness LED as the detector, without any additional optics other than the LED's built-in lenses at each end.

A 100mm lens properly focussed has a gain of >24dB (nearer 30dB if correctly adjusted). Assume 24dB "antenna" gain at each end of the link and we will have 48dB system gain over the simple LEDs on their own. Based on these calculations my optical transceiver should have a range of at least 5km.  If the gain of the lens is 30dB then the range could be as great as 20km. This is without using high power LEDs as the TX or using larger Fresnel lenses which would have even higher gain.

I now have the 100mm lenses I intend to use as well as 2 gun sights bought off eBay. The next step is to buy some drain pipe to house the optics. I have still to decide whether to build fully self-contained FM transceivers (simpler) or to build the optical transceive heads with separate transverters to use with the FT817.

RF skill levels

In my professional life I interviewed many graduates aspiring to become RF engineers. Very few, in recent years, had what I would call "the knowledge". By this I mean a "gut instinct" for RF that does not come from an academic course, important though this aspect is.  Rather, this "jizz" comes by living and breathing RF through building RF things yourself, however simple. A great many 2:1 graduates in communications electronics knew almost nothing about RF, had never touched a soldering iron ever and were rejected. In all honesty I believe I could sense who would make a good RF engineer within 2 minutes of the interviews starting.

Today I received a request from a Spanish amateur who professed (more or less), "I`ve a problem. I know nothing about electronics and would be unable to build the SAQ converter on your website." He then offered to pay me to build and ship the converter to him. Surely, a radio amateur in ANY country should have learnt basic RF skills as part of his training in preparation for his licence? At the most basic level the understanding may not be deep, but how can a radio amateur really not know how to put together a basic circuit?

In the UK we have a growing, and very serious, issue with poorly educated science and engineering graduates who are simply not coming out of universities with the skills needed to start work in industry. One answer was the sandwich course in which young A-level students were accepted on a company training scheme that married "on the job" skills training with educational training, usually to HND or degree level. People spotted young, with real RF "jizz" (easily judged in interviews) usually went on to become the best engineers we had.

Sunspot peak to be over 100?

The latest NOAA sunspot predictions show cycle 24 now peaking in May 2013 at a smoothed sunspot high of exactly 100. This is UP on earlier forecasts and the actual trend since this forecast was made is still upwards, I believe at a rate faster than NOAA was predicting.

So, despite the gloomy predictions of even 6 months ago, we seem to be enjoying a decent solar cycle after all with every prospect it will be better than even current forecasts suggest. Whatever, conditions on HF will continue to be good for 5 years to come, so go and enjoy them.

Today my QRP WSPR was copied in Australia a couple of times on 10m yet again. It appears that every time I fire up it gets >16000km. I just wish a few more East Coast USA stations would  come onto 6m WSPR as this is where the excitement will be for the next  few weeks if F2 MUF reaches 50.293MHz.

Busy amateur VLF weekend

G3ZJO's reception of DK7FC on 8.97kHz

Yesterday's test by DK7FC from his fixed location was a resounding success with reception all over Europe by many stations including G3ZJO, G3WCD and G3KEV, and what is believed to be the first transatlantic reception of an amateur VLF signal by a station in the Eastern USA. Results are being checked as I write.

Stefan's ERP was later re-calculated as around 800uW. It is incredible that such a tiny VLF signal can be detected so far away. If confirmed, this certainly heralds the dawn of a new and exciting phase of VLF amateur work. With another 3-6dB ERP, worldwide amateur VLF DX is now a real possibility. Of course such VLF DX reception requires incredible frequency stability, very narrow RX filters and long signal integration times; signals are far too weak to hear.

Also, Marcus DF6NM ran a 8.97kHz test today using a kite antenna. G3ZJO received a good signal in IO92ng 20dB above the noise in 424uHz bandwidth. Marcus was also well received by many other stations across Europe.

In summary, this weekend has been a BUSY one on the Dreamer's Band.

More POWER - why??

Today I see that amateurs in New Zealand are to be allowed 1kW power in future; their limit was 500W earlier I believe. I've read something about amateurs in Eire asking to be allowed 1kW in contests and I think something similar is being requested of OFCOM here.  My simple question is why?

As an example, last evening I had a totally solid QSO on CW with KT5E in Denver, Colorado on 28.060MHz 2-way QRP with 5W each end.  My antenna was a small wire halo. Like many people, I have worked well over 100 countries with QRP on CW, PSK31 and SSB and always with simple wire antennas like dipoles, never a beam.

When more power is used it just creates more splatter, and more unnecessary QRM. OK there are times when 100W rather than 5W would make QSOs much easier with fewer repeats. But do people really need to run 1kW? OK you may blast a signal through the pile-up, but do you go to bed at night with any more satisfaction than someone working DX with a few watts or even milliwatts? I very much doubt it. Power is about egos, pure and simple.

28MHz WSPR with 50mW

50mW WSPR Reports 28MHz

It really is too easy now on 28MHz! In just a few minutes, these were the reports when using just 50mW to my small halo antenna. Some reports suggest 1mW would have been enough. I need to build a bigger attenuator.

CB interference to the 28MHz band

Interference from CBers on the 28-29.7MHz band is getting worse. I guess they've been there a long time now but with good conditions their presence is more evident. Doesn't seem to cause problems with CW and WSPR operation though. The availability of rigs covering 26-30MHz capable of being easily switched from CB coverage to 10m coverage must be one of the problems and to CB operators the wide open spaces must be attractive. Use it or lose it.

More 481THz lightbeam progress

670nm receive head and converter to 80m

Today I did some further light beam experiments, this time using a 25kHz modulation signal on the light beam and receiving the signal on my FT817 with this head/converter unit above. It consists of a BPW34 photo-detector feeding a cascode FET/transistor stage into an emitter follower and SBL1 mixer to convert the signal to around 3.584MHz. With a current into the TX LED of just 10uA (a very dim glow from the red LED) the signal was 20dB S/N in 0.67Hz bandwidth on Spectran at a distance of 25cms without optics. If my calculations are correct, this means a range of around 100m could be obtained even with this miniscule power if 100mm lenses were added at each end to give some 27dB gain at each end. Using the same TX LED at 10mA (1000 times the current) then the range is already in the many km region, and this is without using power LEDs. This is encouraging progress. Tomorrow I want to repeat the test with the same LED as the TX as the detector.

80m FETer outing

This evening I came on 80m to listen for G6ALB who was taking part in the valve QRP day. Using my  18mW output FETer transceiver (18 parts total) I was able to hear almost all of the stations active around 3.56MHz using QRP.  G6ALB was worked (599 each way, but we are only 3km apart). I had forgotten how effective this little transceiver was and it was a pleasure to use it once again.

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.