How not to create a new digital ham radio mode…

You’ve had a C compiler sitting in front of you and some communications textbooks, and so you’ve done some hard work and created a new digital mode that you’d like to see widely deployed. What should you do? Well, let’s begin with what you should not do:

  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.

New DX spot via WSPR…

This wasn’t quite a personal DX record for me, but it was close, and the first time I’ve heard South Africa in quite some time. I spotted ZS1LS twice this morning on 30m, from grid square JF96fd and a distance of 16477 kilometers. I had previously heard ZS6Y (16927 km) and ZS1TX (16492 km), and these three stations are now my most distant received stations via WSPR. Immediately below them are a smattering of VK6 stations (VK6ADF, VK6WS, VK6GOS, VK6RO, VK6BMW, VK6BN, VK6POP) interspersed with DP1POL, who was operating from grid IB59uh in Antartica.

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.

Launching a rocket with hydrogen/oxygen combustion…

A recent issue of Make magazine had an article about launching water rockets via hydrogen/oxygen combustion: basically an electric current is used to break water into its constituent hydrogen and oxygen, which then bubbles up in the rocket, forcing out some additional water. To launch, this hydrogen is ignited, and recombines quickly into water vapor, but also generates a huge burst of pressure and launches the rocket.

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