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.

Optical CW beacon TX now ready

Today I completed my first keyed optical beacon transmitter (with 100mm optics) which I'll be using for tests at greater range than hitherto. The 481THz (red light) beacon sends my callsign in either 10wpm CW, QRSS30 or QRSS60 as well as a continuous carrier or a 30sec on/off sequence. The TX uses a sub-carrier of 1.082kHz or 8.659kHz, the frequency being derived from an HF crystal divided down in a 4060. The message comes from a K1EL K-ID2 programmed PIC. Initial tests this afternoon at dusk allowed me to copy the 1.082kHz 10wpm CW sub-carrier signal by reflection off a wall across the street at decent strength.

My next challenges are to find some local paths of a few kilometres to check the beacon and the receiver. I'd also like to try some non line-of-sight paths, possibly including cloud-bounce.

How sensitive can headphones be?

Having made several short-wave crystal sets in my time I have nearly always used the little "deaf aid" very high impedance crystal earpieces as the acoustic transducer. I thought these were very sensitive being able to copy HF AM signals down to around -55dBm to -60dBm. However I recently read somewhere that with the old WW2 DLR5 low impedance headsets one could hear an applied audio signal as low as 2uV (-101dBm), which is remarkably low.  

So, I began to wonder just what ARE the best headsets to use if one is after the maximum efficiency converting an audio frequency electrical signal into sound pressure in the ear?

This graph (from Wikipedia) shows the sensitivitity of typical humans at different ages. Someone of my age (63) is likely to be less able to hear as low as someone aged 30 years.

As some of these old military surplus headphones can be hard to find nowadays in good condition, I wonder what is the most sensitive headset/earpiece that can be bought new "off the shelf". Any ideas please?