Optical cloudbounce propagation theory

Bernie G4HJW and I are interested in carrying out some "cloudbounce" tests using our 481THz optical kit and we both are none to clear about the physics of scattering of signals from water droplets and dust in clouds or the sky. However, today a new tool to help our understanding was shown to us on the UKNanowaves Yahoo Group in a posting by Barry Chambers.

The free software is available at  http://www.philiplaven.com/mieplot.htm and allows us to work out the intensity of a scattered optical signal as a function of scattering angle, droplet size and wavelength. Scattering is best when the angle of incidence is at grazing incidence and the droplet size is small. If I've understood the results correctly, aiming at the underside of a cloud at 45 degrees would result in a scattered signal some 50dB weaker than if at grazing incidence. So, depending on how far apart the 2 stations are and the angle at which the optical signal hits the underside of a cloud then signals can be quite strong or extremely weak. This is why weak signal modes like QRSS60 may be needed to work a given path by cloudbounce.

A novel QRP power source

FreeCharge 12V is a small, portable generator, similar to that used in portable hand-cranked torches. The output is regulated to give 12-14.2V DC although the product datasheet does not state the available current. I suspect it is in the low milliamps as its main use appears to be to power/charge mobile phones in an emergency.

This could make a novel power source for low power QRP transceivers as no batteries would be required, just a few seconds or minutes of cranking.  Figures given are 360 turns (approx. 3 min wind) gives 9-11 minutes of mobile phone talk time and a 60 second wind gives 2-4 minutes of talk time. Price is £19.99 and it is available from Freeplay or via Amazon UK.

The sales blurb says:

"The Freecharge 12V provides emergency power to a mobile phone, PDA, iPod, GPS receiver, or any other electronic device for which you have (or can get) a cigarette-lighter adapter. Wind its hand crank to generate electricity and put power straight into your device. Never be unable to use your mobile phone, or other important device, just because its battery has run down."

Weak signal modes compared

There has been quite a bit of debate recently about just how good certain weak signal modes are when compared against similar modes. So, ON7YD has done some research and straw polling to compare them and published the results on his wonderfully informative website. He asked people to try to decode some weak QRSS signals by eye - these were at defined S/N ratios and compared the results against OPERA at various speeds and WSPR.

Amongst his conclusions is that "Opera8, QRSS10 (or DFCW10) and WSPR should have a more or less equal performance."  This is very much as I have found from practical experience on 136kHz where QRSS3 does not perform as well as QRSS30 which is somewhat better than WSPR. So, WSPR being much the same as QRSS10 seems spot on. The advantage of WSPR is of course the internet database reporting which allows you to see where you have been received. QRSS reports depend on someone decoding the signals by eye and then manually sending you a report, which rarely happens. So, WSPR remains my favourite weak signal beaconing mode.

ON7YD's graph showing the effectiveness of various weak signal modes

472-479kHz secondary allocation

From the RSGB's website today comes the latest news from WRC-2012 about an amateur secondary allocation to replace 500kHz with something more permanent. It needs ratifying but it does now look very likely indeed to happen.

Agenda Item 1.23 – 500kHz
Progress through Committee 4 (COM4) was a little easier than expected and the frequency band 472-479kHz will be allocated to the amateur service, on a Secondary basis. This is subject to no further objections being received during the two final readings through the plenary meetings, of which the first ‘blue’ reading is expected to be during the plenary this coming Friday, late afternoon.

Free power radios

KE3IJ's always excellent website has a novel receiver circuit that derives its DC power from AC mains hum and noise it picks up on its antenna.  It uses a single 2N3904 in a regenerative circuit. Ideally the circuit would be better with a lower Vbe device i.e. a germanium transistor.

Taking this one stage further, I know from my experiments at VLF that the AC mains hum between a pair of grounded electrodes can be pretty high. There is certainly enough 50(or 60)Hz energy going free in the ground to power something useful. I keep wondering if I could get a few uWs of HF RF power from a low voltage crystal oscillator that would be enough to work some local stations on CW? My nearest station is 0.3km and the next nearest 3km. Now having a QSO using free power from my back garden would be rather fun.

New Elecraft KX3 Photo

The new KX3 HF-6m 0-10W Elecraft All-mode transceiver

N1RX, who has been a field tester for Elecraft's new KX3 QRP rig, posted a photo on the KX3 Yahoo Group today showing the latest version of the transceiver. Deliveries are due to start shortly and it does look a very high specification radio, albeit quite a bit more expensive than the FT817. However it offers SO much more by way of features. It is more like a small K3 transceiver.  A look at its specification and features set is worth it. The guys at Elecraft do know how to design some very elegant QRP products. A copy of the user manual is expected to be available for down load within a few weeks.

Remarkable "all diode" transceiver QSO

DL3PB's "all diode" transceiver

Today I got this most interesting email from Peter DL3PB. Peter must be congratulated on this excellent achievement.

Hi folks,

I'd like to share with you a long-cherished dream, that recently came true, fourty years after I came to read about hams using tunneldiodes to make QSOs when I was aged twelve or so:

Finally I managed a first skywave QSO with my PARASAKI-transceiver, an 'all diode' rig:  Christophe/F8DZY replied to my very first call on 20m band in REF-contest last weekend. I was running 2mW into a temporary vertical dipole on my balcony. Distance between us is  918km - obviously OM Christophe has excellent ears.

Those interested in the cruel details of my circuit, please find attached a schematic and a photo of the pretty ugly setup. The circuit is designed straight-forward with exception of the parametric VXO, derived from Mike/AA1TJ's famous Paraceiver design. (see  http://fhs-consulting.com/aa1tj/paraceiver.html)

The low impedance of the high peak-current tunneldiodes make it very difficult to built a really crystal controlled oscillatorrather than an LC-oscillator, synchronized by the crystal more or less, at least on the higher SW-bands. The Parametric VXO provides a crystal-stable, chirp-free signal on expense of an output power of two milliwatts only instead of ten, but with an amazing spectral purity, no need for a low pass filter or such. Of course it sounds pretty cool making a QSO with a 'bunch of diodes' and a parametrically excited crystal, but believe me or not, I'd preferred to bring that full ten milliwatt into the air - on the other hand that approach allowed  to tune the rig a bit ( ~ 5kHz/per xtal ), which turned out to be much more valuable than a few milliwatts more while being 'rock-bound'.

The receiver in its 'gain-less' version works fine for strong signals - while listening to QRP(p) stations, the moderate gain of the audio amplifier helps a lot. A comfortable frequency shift between receive and transmit is realized by the 5µH inductor at the LO-port of the mixer, with little effect on sensitivity.

Thanks for the bandwidth, OMs, won't bother again you with such mails, unless I make a cross-pond QSO with that rig ( not that likely ) or any skywave QSO with homemade semiconductors ( probably impossible )...

72!

Peter/DL3PB"

481THz optical treebounce and skyscatter

Optical treebounce (0.3km total path) 10wpm CW 0.501kHz 35dB S/N

This evening I did some further tests with optical treebounce and what I'd loosely call skyscatter. The sky was cloudless, but a little hazy. My test kit was as follows:  TX (250mA high brightness red LED, 100mm optics in one room of house), RX in another room KA7OEI based head (doors closed, no light leakage) with 100mm optics.  PC running Spectran positioned to minimise light pollution to RX head.

First test was 10wpm CW off the tree (0.3km path length total) with strong signals received 35dB S/N in 5.4Hz bandwidth. Signal v.clear in the earpiece too. I could just make out the red light glow in the distant tree branches. Aiming critical.

Weak QRSS60 signal received by aiming at clear sky

Second test: TX and RX elevated to aim at roughly same patch of clear sky. QRSS60 signal sent from TX. Signals detectable in Spectran in 0.17Hz bandwidth, weakly, but definitely there. It is less clear on the capture than on the real screen. For this second test I made no great attempt to optimise the RX aim,
just aimed at what I thought was roughly the same patch of sky. Now I can't be sure whether the signal is purely from scattering off mist/dust particles or what, but I think it is unlikely signals are coming off other objects as I am aiming quite high into the sky (about 45 degrees up) clearing nearby stuff.

These tests suggest that with very slow QRSS I may be able to get a non line-of-sight optical signal to G6ALB in the next village 3km away. When the weather improves I'll go out /P with the PC and RX and see if I can detect the "forward" scattered optical baseband signal at much greater range (1-3km). With proper cloudbounce it should be better I think.

This reference looks like it should be interesting (about scattering) http://en.wikipedia.org/wiki/Rayleigh_scattering

A 472-479kHz band?

Latest reports from the WRC2012 conference suggest the possibility of a secondary amateur allocation between 472-479kHz look "promising" although this hinges on a crucial meeting on Tuesday Feb 7th. Some countries remain against an amateur allocation, but the majority do support one, with caveats. A 7kHz wide worldwide allocation would be a good outcome, so keep fingers crossed for a few more days.

QRP and S Points

This very useful little diagram was posted on Wim PC4T's blog today showing how little effect going QRP usually has on operating effectiveness. Going from 100W to 500mW is just 4 S-points, so if your signal is strong you'd still be a decent signal. Of course, when conditions are marginal 100W may be helpful, but QRP is so much more fun.