Optical receiver noise floor measurements

A PC with some simple free software makes a very useful piece of test gear.

For example, today I wanted to check the noise floor of my optical receiver head between 0-20kHz in total darkness and in daylight and compare this with the noisefloor of the PC with the head turned off. SM6LKM's excellent little software receiver designed to listen to SAQ's transmissions on 17.2kHz was ideal. This tunes from 0-22kHz. Here are the results (see plots) which show noise goes up by about 5dB in the daylight, although noise at 0-2kHz seems to go down in sunlight. By connecting the optical receiver head to the PC I was able to use this little package to see the noise floor changes.  G4JNT thinks the FET is possibly being biassed to a less optimum position, or it is saturating, in daylight. By changing the bias I may be able to get a lower noisefloor and better sensitivity. That I shall try tomorrow.

481THz update: 1.6km test

Today I did my first test beyond the end of my street and in daylight.  I set up my 1.082kHz subcarrier CW beacon and 100mm optics pointing out through my double glazed bedroom window and aimed it at a local feature called the Devil's Dyke which is 1.6km (1mile) exactly from home. This is the furtherest line-of-sight (LOS) path I have from home.

Then I went up to the Devil's Dyke and started looking with my handheld 100mm optics receiver. Much to my joy and surprise I heard the beacon before I spotted it by eye. The beacon could be copied over a stretch about 50m along the path. S/N I'd guess at around 20dB (by ear) in speech bandwidth in daylight. Next time I'll take the laptop and measure S/N with Spectran.  1.6km is my best distance so far. I'm using a BPW34 detector with some reverse bias with the PIN diode's anode connected directly to the FET gate in a KA7OEI optical head. This feeds into a feedback biased common emitter stage into a crystal earpiece. Recovered audio was a bit low in the wind.

Some progress in the right direction.

More Sixboxes and Fredboxes

Joe Milbourn has a nice photo of his version of my Sixbox QRP 6m AM transceiver. It looks he's made some changes including the addition of a beefier audio stage on receive, perhaps to drive a loudspeaker.  It also looks like he has added a mic pre-amp. Click the link for Joe's original size image. Incidentally, there are some great photos on Joe's picture site.

Then I noticed a version of my 2m AM Fredbox on a Romanian website. See http://www.garajuluimike.ro/electrice/emitator-receptor-144mhz.htm . This version also uses an LM386 audio amp to drive a loudspeaker.

Latest Solar Data

Jan Alvastad's excellent page on solar data shows that for the last couple of months the trend in solar activity is downwards, not upwards. Now it is not uncommon for solar cycles to show more than one peak, with the second being greater than the first, but I hope that was not "it" and that we are now sliding down to the next minimum already! The image below is on Jan's site and I hope he does not mind me linking it here.

More on the German "Chirpy"

Today I got this email from Martin about his build of Chirpy:

Hi Roger,
Somehow the text of my email got lost when I send it from my phone. I like your blog very much, especially your homebrew projects. Yesterday I took the time to get the chirpy from the breadboard into a nic enclosure. I have plenty of this tins since my xyl likes to eat some paste that comes in them. My rebuild of chirpy puts out 210 mW and has a lot of chirp. Perhaps we can have a chirpy qso when the band is open.

Thank you very much for the design and your nice blog.
73 de Martin, DL8MAR

Slight progress towards a new 500kHz band

The latest reports from WRC2012 about a possible new allocation just below 500kHz sound slightly more hopeful than a few days ago with China and Russia softening their positions, but unfortunately the Arab group seems dead set against an allocation at all.  We'll have to wait until Working Group 4C (next level up) discusses it.  In the meantime let's keep our fingers crossed.

VLF Talk - Cambridge Club Feb 10th

On Feb 10th I have been invited to talk to the Cambridge and District Amateur Radio Club on VLF through the ground although I intend to widen the scope to include all aspects of amateur radio communications experiments below 9kHz. In the last year or so this aspect of our hobby has come on leaps and bounds and what was once thought of as impossible being achieved. Surprisingly, this is a very accessible part of the spectrum in which to experiment. If you want to come along and are not a CDARC member I am sure you'd be made welcome by club members.

Another 10m Chirpy

Martin Spreemann (callsign not known) in Berlin has sent me a picture of his version of Chirpy, the minimal component transceiver for 28MHz CW. I wonder what results people who have made this (rather chirpy) transceiver have obtained? Please let me know if you have built and used one. Remember the design uses a fundamental crystal for 28MHz, not a 3rd overtone. The design should work equally well on 24MHz (slightly less chirp too) and 21MHz, although I suspect broadcast breakthough will start to become more of an issue.

Optical "tree bounce" tests

Spectran screenshot of 481THz "treebounce" test 30.1.12

Last night I made my first recordings of my optical beacon in action. In one room of the house the 100mm optics beacon was set running QRSS3 and aiming at a tree 150m away across a field. This was a deciduous tree with just bare branches. In another room of the house, optically well isolated from the TX, I set up the 100mm optics KE7OEI head feeding an earpiece and my PC running Spectran. The signal scattering off the tree (total path length 0.3km) was 30dB over noise in a 2.7Hz bandwidth. This suggests (I think) that other non line-of-sight paths with a total distance of up to 1km might be possible by reflection off trees, buildings etc. I have still to try a true line-of-sight path at any distance with my current system but it will certainly be good for many kilometres.

Yesterday my 1W ultra-bright LEDs arrived from Hong Kong. When fitted to the beacon these will be VERY bright and will extend the range possible on 481THz yet further. All good fun!

A Czech Chirpy from OK1CDJ

Ondra OK1CDJ kindly sent me some photos of his version of my 10m chirpy transceiver. Currently he is getting around 50mW out, a little lower than on samples made here. I have suggested some things to check and possibly change. I really must find a way of making it chirp less, HI.

See http://translate.google.com/translate?client=tmpg&hl=en&u=http%3A%2F%2Fblog.ok1cdj.com%2F&langpair=cs|en for an English translation of Ondra's blog.