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?

Dropbox Problem

When my grabber is running I use Dropbox to automatically upload the captured image to a location on the net where anyone can see the image. Recently Dropbox has been doing something wrong and when I try to launch the program I get an error message (see image). I have tried EVERYTHING to fix this, to no avail so far. Has anyone else had this issue? I looked on the Dropbox website (not too helpful), tried running Ccleaner to see if the registry entries were faulty (no luck), reinstalled the software (about 10 times now in different release versions) and deleted each when the same darn message appeared.

Help!

New beacon chips arrive

Today my new KID2 pre-programmed beacon ICs arrived from www.k1el.com . These are for use in my VLF, 136kHz, 500kHz and 481THz beacons. With 7 pre-programmed messages in 10wpm CW, and various QRSS speeds these are ideal. I have used these before and been very satisfied.  See http://k1el.tripod.com/files/kid2man.pdf for more details of the ICs.

Laptop/Netbook Reliability

Investing in a new laptop or netbook can be a tricky decision: getting the right price, spec, performance and reliability. My Dell laptop failed a year or so ago (keyboard) but I was able to repair it by buying a new keyboard via eBay and following some good fitting instructions found on YouTube.  My wife's school laptop LCD screen just failed suddenly when it was in use on her lap with black "cracks" appearing as if the whole thing had been dropped. This was repaired under warranty thankfully. Then you hear of hard-drives failing.

Today I came across an interesting page giving some reliability statistics for laptops and netbooks. Apparently Asus and Toshiba make the most reliable ones. The paper is by a company selling extended warranties, so they will be keen to show that it is worth buying one. Nonetheless I think the stats are fair.

FT-450D hum: a good response from Yaesu

Last night I wrote to Yaesu UK about the FT-450D hum issue and got a very fast and very helpful, response from Paul Bigwood their Technical Sales Manager. All credit to him.

I will paraphrase Paul's reply below.

He confirms the hum problem has been recognised and that it doesn't affect all radios. Those it does, are only affected in voice modes, when the display brightness is reduced below its normal maximum level. The level of hum is generally low and is usually only noticeable when the received signal strength is high [i.e. with local stations], DX stations don't normally hear it.  He says that steps have been taken to resolve the issue and product shipping into the UK dealers since the beginning of the year have a modification to resolve it. He told me that if I purchased one that does suffer from hum under the circumstances as above, then it can be returned to Yaesu for updating under warranty.

He went on to say,

"Please rest assured that Yaesu does care about its products, their reliability and performance. We certainly look to resolve any problems as soon as they become known to us.  The FT450D is a splendid radio, having done excellently at the T32C DXpedition last year when over 200K QSOs were made with no reports to us of any hum problems."

I thought this was an excellent and totally professional response. Yaesu is a first class manufacturer and it is reassuring to see they have taken on board the hum issue in this otherwise excellent little radio and have now taken steps to properly fix it on new build units.