But my morning reading found this website on MathWorks, which details the kind of math that I’m implementing in C. I’ll review it again.

Single Sideband Modulation via the Hilbert Transform Demo.

Addendum: Here’s a brief glimpse of the kind of thing that I worked on. I’ve taken a regular mono signal recorded at 48khz, low pass filtered it at around a 3khz frequency, computed the Hilbert transform of it, and then frequency shifted the single sideband version of it up to a carrier frequency a little above 8khz (that would be VLF, I understand, but it is the principle of the thing!). This was computed using a Hilbert transform with 501 taps (250 non zero entries):

If I try to compute the same thing with only 201 taps, you can see that some of the very low frequency signals are visible in the lower sideband opposite their USB constituents.

I need to ponder this some more.

Addendum²: Just as a little test, I tried modulating my voice up by just 350 hertz, simulating reception of a signal on USB, but tuned 350 hz too low. You get the classic Daffy Duck type speech (sans any noise that is typical on HF):

CQ CQ CQ, this is K6HX calling CQ and standing by…

But, of course if you have some compute power handy, it isn’t really that big of a deal. You can use something called the Hilbert transform which does precisely what you want. From a given signal, it generates the quadrature signal that you need. If you plot the input versus the output for a constant amplitude signal, you get a nice circle, precisely in phase. If you apply it to a voice input signal with varying amplitude, you get a series of circular spirals, like the plot below.

It is probably only an hour or so work to generate a complete abstract implementation of the phasing transmitter/receiver. That’s coming, and I’ll document it more here and make the code available for the interested. Till then, wish me luck with the IRS.

Addendum: Tax appointment went as well as could be expected. I didn’t like the straight lines in the digital signal above, which game from its relatively low sampling rate. If you upsample the signal with a high quality, band limited up sampler, you get a much smoother result, as can be seen below (the audio input wasn’t quite the same, so it doesn’t look exactly the same):

Anyway, tonight I was strolling down memory lane, and encountered a link to something I hadn’t seen before: the Bahtinov Mask. It’s a gadget that you can place in front of a telescope to help you focus precisely. It uses a very clever arrangement of masks to produce a diffraction pattern which can be adjusted to a precise focus.

If you dig around, you can find all sorts of testimonials about how terrific they are. I’ll have to make one of these and give it a try. Perhaps if I can find someone who will loan me a laser cutter, I’ll even cut a really precise one.

Addendum: Here’s a link to a nice mask generating program that will output an SVG file for any scope you need.

I understand nostalgia. We all like the cars we wish we could have had when we were teenagers. We like the music that we learned to like as young people. Heck, I still have the first computer that I bought at age 14. In moderation, nostalgia can be a good thing. It reminds us where we came from. It gives us context. It gives us history.

But I think we always have to temper a sense of nostalgia with perspective and vision. Our first cars were our loves, but they were (let’s face it) often gas guzzling, unreliable death traps. Some of our music was probably brilliant, but a lot of it was (let’s face it) crap. And my first computer, while enabling me to explore a world of computation, which eventually led a rich and rewarding career, was by any modern standards less powerful than an alarm clock.

Which brings me to software-defined radio.

I’ve seen lots of people write negatively about software defined radio. Often it’s just pure nostalgia. “Real radios glow.” “Real radios have KNOBS.” “Real radios don’t require your laptop.” “My old radios just sound better.”

I think we as hams should be more visionary than these statements. Software-defined radio is enabled by the remarkable evolution of computing hardware. The speed of computation has in the time that I’ve owned computers increased by a factor of about 200,000. The cycle time of most desktop computers is now low enough that an inexpensive computer can execute dozens of instructions for every cycle of an HF signal. This enables the traditional features of radios like mixing and filtering to be done in software, instead of being cast in hardware. This means that we can have unprecedented control over these processes. And, potentially we can even change many aspects of operation even after the soldering of our radio is complete. Software isn’t a panacea, but it is the source of such great power it can’t and should not be ignored.

Most of the criticisms I see about radio interface seem curiously misplaced. I agree that using your laptop as an interface is often sub-optimal. But if you’ve played with any radios at all, I bet you had some criticism about the conventional radios that you had as well. Modern radios are complex, and this complexity is not often controlled by careful user interface design, whether in software or in conventional hardware front panels. Even if you thought that some radio you had in the past was perfect, I would submit that it would still be beneficial to use software defined radio technology inside.

But even beyond that, I have been playing with an SDR-IQ for the last couple of months. Its ability to monitor all signals on an HF band is enormously compelling. Using digital displays, we can give the user a perspective on the entire band which is undeniably useful. Technology like CW Skimmer can track dozens of Morse signals in real time. Programs like HRD and fldigi can do the same with multiple PSK signals.

I’ve seen a few people abandon the idea of software-defined radio simply because they aren’t adept at software design or have no knowledge of the mathematics and algorithms that underly software-defined radio. They fatalistically claim that they will never or can never understand or learn how software-defined radios work. I am kind of flabbergasted by this attitude. It’s not that it is easy: far from it. Studies show that to become expert in a subject takes the better part of a decade. But as hams, we are supposed to be about self-training and experimentation. Sure, there is no mandate for you to do so, but let’s not be so fatalistic and condemn it merely because it doesn’t coincide with our own abilities or interests.

There is no reason to forget the past, but let’s not worship it either. Let’s look at the best of what was, the best of what we have now, and the best that we can imagine, and experiment, design, build and use the radios that we have yet to invent.

So, of course, I hauled my FT-817 out of the box it has been in since before the holidays, and had to test it out. The first impressions are a bit mixed. I didn’t have any difficulty at all getting it to work with fldigi or Mixw or Spectran, but for some reason WSPR required a bit of tweaking. It would sometimes just act as if it wasn’t getting any receive audio: the RX level would stay at -30. I suspected that it might have something to do with applications which open the sound card exclusively, so I made sure those were all turned off, and eventually ended up running it in Windows Vista compatibility mode (I have a Windows 7 laptop) and then all was well.

And.. since all was well, I decided to do a bit of WSPR transmitting for the first time in months.

I was heard by VK6DI from his new VK2 location, which was nice to see. David was one of my most distant spots, and his old grabber was the funnest thing for me to check out while testing my QRSS/MEPT beacon stuff before. He’s a few thousand km closer now, but still, Australia is nice. ZL2TLD and I managed to exchange packets. But most interesting was a new station I hadn’t seen before: 4Z4TI. That wasn’t a DXCC prefix which I had seen before in my WSPR spots, so I had to look it up. Yep, Israel. Very nice. Toss in EA4BUL and Japanese stations JA2GRC and friendly face JQ2WDO, and that rounds out my list of DX for the night.

1. First, you shouldn’t presume to dictate what frequencies it should be used on without paying some close attention to the bandplans, both voluntary and involuntary, that we as hams have established. A particularly bad choice would be to recommend frequencies such as 14.101, which are uncomfortably close to the 20m beacon that many hams rely on.
2. You should not recommend frequencies that are likely illegal. Modes wider than 1khz, or which use unspecified coding aren’t legal on 30m in the U.S.
3. You should not call your mode something which is either incorrect, or at best misleading. If you say your mode is spread spectrum, it’s going to make a lot of people upset, and you could have avoided that by knowing what words mean.
4. You should not keep the details of your scheme private. As amateurs, we are tasked with improving the radio art through experimentation, and trade secret modulation does nothing to encourage or aid that kind of cooperation. I’d suggest that you make your scheme open source to achieve maximum impact. Private protocols and encodings are basically just encryption.
5. You should not make vague assertions about the mode’s performance. Comparisons of a mode which runs at (say) 16 baud and take 2.25khz of bandwidth have to be carefully made when compared with modes that run at (say) 31.25 baud and only 80hz or so of bandwidth.
6. Lastly, you shouldn’t fabricate communications between yourself and the FCC in support of your new mode. It doesn’t lend much credibility when you later retract all evidence that you did so by deleting posts from your blog.

Cool.

I haven’t been operating in transmit mode much lately, but if I look at stations that have heard me, I see the most distant station is VK6WS, at a distance of 14842 kilometers.

And here was my question: how big of a burst of pressure does it really generate? Is it safe? 2 liter bottles have a burst pressure of around 120psi or so. That’s actually reachable with ordinary mechanical means, and it is pretty easy to understand and monitor: the pressure increases slowly, and can be monitored if necessary by an ordinary pressure gauge.

The chemical reaction isn’t so easy to wrap one’s head around. You actually need to know some physics. Sadly, I’m mostly self taught, so I have to work through these things slowly. A discussion with Michael and Tom over lunch the other day reminded me of the ideal gas law. One mole of hydrogen and one half mole of oxygen combine to create one mole of water vapor. But I must admit: I know very little about the realities of combustion physics.

So, I did what I always did: I googled. And I found this interesting page. It estimates the pressure to peak at around 160psi, well beyond the burst pressure of the common 2L bottle. To keep the rocket from bursting, it is vital then to dilute the reaction by the introduction of ordinary air. The nitrogen won’t combust, and should limit the overpressure.

I’ll have to muddle over the details more sometime, but it’s good reading.

Powering Water rockets with hydrogen combustion

HFAERO.jpg (3397×1781).

I mentioned this to Loren over lunch the other day, since I know that he has done his fair share of water bottle rocket launches, usually using liquid nitrogen as the propellant. He had a few interesting insights, namely that the nozzle design which is common in ordinary rockets isn’t really useful in water rockets, as water is essentially incompressible. This means that you can use Bernoulli’s equation to compute the force generated. Of course, the mass of the rocket is continuously dropping as water is expelled, but that’s not too hard to deal with in the simulation.

What wasn’t clear to me was that after the water is exhausted, there is still residual air pressure in the rocket, and this pressure is significant and must be dealt with differently since the air is compressible. A bit of research led me to this rather nice website:

Rocket Science.

He has an interesting example of a 2 liter bottle which is pressurized to 100 psi and filled 20% with water. It’s fascinating: the “water burnout” (when all water has left the rocket) occurs only 0.042 seconds after launch, when the rocket has an altitude of only 3.2 feet (!). It continues to accelerate though as the air pressure equalizes. In this example, one third of the velocity of the rocket is obtained after water burnout.

It might be fun to make an implementation of this.

Addendum: Here’s another link to a water rocket simulator.
Addendum²: Another simulator.

This article describes how to build a loop antenna for low frequency (LF) and medium wave (MW) reception with remote-controlled tuning. The antenna is extremely sensitive and can be built mostly from parts from old radios and tape recorders. Like all loop antennas, it is highly directional, which allows you to null out unwanted noise sources.

via Remotely-tuned loop antenna for LW and MW.

Addendum: It seems rather likely that the person photographing this was the yutz who tossed the branch up there. Don’t try this at home, folks.

Anyway, another nifty classic text on Google Books:

A Miami based Radio Station – The Disco Palace – has started broadcasting a DRM SW channel of ‘best of Disco music’ for listeners in Europe and North America.

via New DRM channel of Disco Music  :: DRM.

So, today I tuned in and recorded some of it using the SDR-IQ, and then processed it with GNU DREAM, the GPL’ed DRM decoder. The signal was very strong, no drop outs, and well, it’s disco. Here’s a couple minutes of the decode.

The Disco Palace broadcast using Digital Radio Mondiale on 17.755Mhz

Low Voltage Tubes

Whatever the motivation, I’ve been looking around at projects that people have done. A popular project seems to be “twinplex” radio, which uses a single dual triode tube in a regenerative receiver. Staring at circuits and reading Basic Theory and Application of Electron Tubes, I’m beginning to understand the functioning of these circuits. And, it turns out youtube has a lot of nice inspirational videos of people’s projects, such as the following:

Now that the winter is over, our local flea markets at Livermore and De Anza should be starting up again soon. Perhaps I’ll keep an eye out for the components.

Addendum: Here’s another link for inspiration.

Still no trace of him on WA0UWH’s grabber, but KK7CC has no trouble getting him:

Addendum: A few minutes later, I tuned up the band so I could see WA5DJJ, and got this nice screen grab, along with KC7VHS:

International QRP Collection, edited by Rev. George Dobbs and Steve Telenius-Lowe.

It’s not an ideal book from the homebrewer/QRP viewpoint: it includes a bit too much operating/equipment reviews for my taste. Still, it has a number of interesting construction articles, including the “One Transistor Marvel” from Dave Ingram, K4TWJ (SK, sorely missed) which I hadn’t seen elsewhere, but obviously inspired things like G3XBM’s XBM80-2 transciever. It also includes KK7B’s articles on designing and building linear transistor amplifiers that appeared in QST recently. I’d say that’s a waste, but the articles are so good, I’d like to see them in book form (I always seem to lose magazines). It also includes details on some simple minimal rigs that I hadn’t seen before, such as G3MY’s Pippin, and some novel variations on the Pixie.

Overall, I give it an 8/10. Worth having, but not as good as DeMaw’s QRP Design Notebook.

Fifth Position: White to Play and Draw

Without an endgame database, it takes milhouse a 21 ply search to find the right variation that avoids a loss for white.

      +1 : [-5] 0.00s
         : 27-24
      +3 : [3] 0.00s
         : 27-24 11-15 20-16
... researching after failing low (-122)
      +5 : [-124] 0.00s
         : 27-24 11-15 20-16 14-18 21-17
... researching after failing high (-9)
      +7 : [-9] 0.00s
         : 20-16 11x20 27-23 12-16 19x12 20-24 12-8
... researching after failing low (-34)
      +9 : [-37] 0.00s
         : 20-16 11x20 27-23 20-24 22-18 24-27 18x9  27-32 23-18
     +11 : [-35] 0.01s
         : 20-16 11x20 27-23 20-24 22-18 24-27 18x9  27-32 23-18
         : 10-14 18-15
     +13 : [-40] 0.02s
         : 20-16 11x20 27-23 20-24 22-18 24-27 18x9  10-14 19-15
         : 27-32 23-19 32-27 15-10
... researching after failing low (-78)
     +15 : [-116] 0.08s
         : 20-16 11x20 27-23 20-24 22-18 24-27 18x9  10-14 19-15
         : 12-16  9-6  27-32  6-1  32-27 23-18
     +17 : [-119] 0.23s
         : 20-16 11x20 27-23 20-24 22-18 24-27 18x9  10-14  9-6
         : 27-32 19-15 12-16 23-18 14x23  6-1  32-27  1-5
     +19 : [-114] 0.72s
         : 27-23 11-15 20-16 15x24 16-11 10-15 23-19 14-18 19x10
         : 18x25 11-8  24-27 10-6  27-32  6-1  25-30
... researching after failing high (-89)
     +21 : [-11] 2.65s
         : 20-16 11x20 27-23 20-24 22-18 13-17 18x9  10-14  9-6
         : 24-27  6-1  27-31  1-5  31-27  5-9  27x18 19-15 18x11
         : 9x18
     +23 : [-16] 5.54s
         : 20-16 11x20 27-23 20-24 22-18 24-27 18x9  27-32 23-18
         : 10-14 18-15 13-17  9-6  17-22  6-1  22-26 15-10
     +25 : [-17] 13.83s
         : 20-16 11x20 27-23 20-24 22-18 24-27 18x9  27-32 23-18
         : 10-14 18-15 13-17  9-6  32-27  6-1  17-22 15-10

The play listed in this line differs a bit after the capture 18×9, but the Cake endgame database lists the move as drawn after that capture, and it appears that milhouse is able to hold a drawing line. Smith lists 27-31 as a drawing variation, but suggests 10-14 as the drawing move. Using the Cake database, it also appears that 12-16 and 27-32 can draw as well.

Using the endgame database, milhouse asserts that the game is drawn with a 7 ply search, after searching less that 1000 nodes, about a 450 fold increase in speed.

Lindsay: Build Your 1st Vacuum Tube Regenerative Receiver

It’s a reasonable question as to why you might be interested in trying to build such a thing, but frankly, I don’t have a reasoned answer. I just think it would be cool to experience this technology by actually creating a working example of this kind of art. And it’s kind of cool to build a radio that glows.

We’ll see how far this project goes.

A recording of AO-51 – KJ6AKQ.

Either side to play, white to win...

Milhouse has no difficulty finding the winning line on simple positions like this, but there are definitely some more subtle positions, and Spayth’s book provides some insight into many common openings. I’ll probably type in some of these problems, and eventually produce a downloadable version of milhouse for all to enjoy.

Addendum: Here’s the most interesting book: a copy of Spayth’s The American Draughts Player, which was written circa 1860. I’m not sure when this edition was printed, but it’s obviously quite old, and yet in pretty good condition.

Spayth's American Draught Player

For those of you who weren’t lucky enough to find a copy of this book, you can nevertheless find it on Google Books:

Addendum²: My copy is a sixth edition, which means that it probably dates to the mid 1890s.

The Carolina Flashers Photonics Group -- Welcome.

Addendum: They link to a fascinating demonstration of a German WW2 light-based transceiver system. Very cool.