K3PGP receiver for 481THz

One of the simplest, yet highly sensitive, receivers for optical communications is one designed by John K3PGP. The G8CYW design in Practical Wireless in the March and April 2013 editions is based on this. This works really well in darkness, but is easily overloaded in any light. Many circuits are optimised for speech communications but this one works really well with digital modulation at very low frequencies where the detector sensitivity is highest.

http://k3pgp.org/Construction/Frontend/preamp.gif

Today I've been building a version and I am in the process of optimising it for subcarrier frequencies below 500Hz. I have added an extra transistor gain stage after the basic K3PGP design but want to see if lower noise FETs and transistors make a worthwhile improvement. When satisfied that it is working optimally I shall be trying this at the RX end of my over-the-horizon tests. I am quite excited about trying much lower frequencies in the next test using QRSS3 and continuous carrier.

481THz optical tests over the horizon tonight

This evening I am doing my first "over-the-horizon" optical test this year. The main aim is to check performance with the new receiver and to see how I get on using the iPod Touch 4g running SpectrumView software as the way of seeing the QRSS3 signal. I shall also be taking the laptop running Argo or Spectran.

Latest receiver that seems more sensitive than my older RX

The receiver circuit is the one in Practical Wireless this month by Stuart Wisher G8CYW. This is simple but works well in darkness. I've added an interface to the iPod Touch 4g (a capacitor and a resistor needed to enable the external mic input) and changed the coupling caps and op-amp gain. Using just MPF102 and 2N3904 devices the results look good. The low noise op-amp may be overkill unless one uses super low noise FET and transistor stages too.

After the tests I'll post results in the blog.

Joining the "over 10km" optical club

This morning I got a note from Stuart G8CYW which surprised me:

"Roger,

It has occurred to me that you have joined a "select" group, who have made an optical contact over 10km here in the UK with modulated LEDs. I had a quick count up of the people I know about and I do not think there are 10 yet. (probably more I do not know about though) .....so no small achievement."

So apparently very few of the people experimenting with 481THz red LED optical communications have broken the 10km barrier for 2-way QSOs. This surprised me. At the same time I am even more encouraged to continue this fascinating area of amateur radio as there is so much more I can do. The beauty of this is that test equipment needs are basic, skills needs well within my capabilities, and the results when success comes are extremely satisfying.

Optical DXing and refraction

Although my own experiments at 481THz have so far been limited to around 9km non line-of-sight, the amateur experts around the world have been achieving remarkable ranges both line-of-sight (LOS) and over the horizon (NLOS). In the UK several groups are looking for longer line-of-sight paths up to around 147km.

Some interesting information is coming to light (no pun intended) about the refractive index of the lower atmosphere at optical frequencies. At radio frequencies the refractive index (K) is around 1.33 and I had always assumed that for lightwaves it was just over 1. But tests suggest this is NOT the case at lower levels such as over the near line-of-sight paths now being attempted. A recent paper by Barry Chambers G8ACN on the UKNanowaves Yahoo group suggests a much higher figure may be more appropriate for a lot of the time meaning that what may appear to be a non line-of-sight path actually is one. He cites some observations by G8CYW and others of distant factory chimneys and cooling towers that should not be visible over the distant horizon but frequently are clearly visible.

Many are familiar with optical mirages, but the fact that the optical refractive index is frequently in the 1.1-1.33 range surprises me. It suggests many more distant optical paths may be workable with suitable equipment than was previously thought possible without having to resort to clear air scattering or cloudbounce. With 481THz cloudbounce and clear air scattering the additional path loss (compared with a LOS path) can be in the 30-50dB range, so equipment for NLOS paths is much more demanding than for a quasi line-of-sight path.

Optical RX improvements

G8CYW Optical head (April 2011 RadCom)

I've now bought some NE5534 low noise op-amps so can implement the full G8CYW optical head circuit which many users have successfully adopted. Stuart describes this as "exquisitely sensitive". I have BPW34 PIN diodes but think the sensitivity should be similar to that with the SFH2030 diodes used in Stuart's design.

In the first instance I need to remeasure the noise floor with the optical head in total darkness to see if the changes indeed make it better than the current discrete component circuit.  My first quantitative noise measurements suggest far higher noise floor than I expected.

Assuming this gives improved performance over my 0.4km local "up the road" test range, my next step is to attempt a longer path, so I am looking for a path of around 2-3km to test with my baseband beacon. If this works with decent S/N then I'll retry looking for the GB3CAM optical beacon. At 32km this is a good test of system performance. At the test site at Nine Mile Hill the traffic noise was high (optically and audibly) so I also need to increase the signal level in the earpiece used to align the RX.

There is also soon to be a second optical beacon near Cambridge and this should be a little closer and an easier signal to find. However, I need to do tests before this is installed at Dry Drayton.

Incidentally, I managed to overcome the parallax issue in the optics alignment  so now a distant street light is spot on in the cross-hairs of the spotting scope.