The other day I heard a thunderclap and as is my Pavlovian response, immediately ran to my shack to disconnect the antennas from their respective transmitter and transceiver. I added a diary entry to my phone to note that I’d disabled my WSPR, or Weak Signal Propagation Reporter beacon, which generally runs 24/7. A few hours later I remembered that my beacon was off and I returned to put it all back in working order.

That’s not always been the case. From time-to-time I’ve managed to turn off the beacon and forgotten to re-enable it, once for months. Same with my main radio, which at one point was disconnected from its antenna, taking a good half hour of stressful troubleshooting to discover one memorable Saturday morning, when I tried to set-up for my weekly net. Now it’s the first step, before I turn on the power supply.

I’ve mentioned in passing that lightning protection and grounding requirements are not globally uniform and I’m purposefully refraining from advising you how to do this in your shack. When in doubt, talk to your local community and a licensed electrician, mainly because there’s plenty of poor advice around that could result in your applicable household electrical regulations being breached.

Back to my shack.

Wouldn’t it be cool if I could automate this, something like a remote controlled relay, preferably wireless so stray RF doesn’t make it into the wrong circuit. Switching aside, it occurred to me that I’d also need some way of detecting lightning to trigger this system, you know, disconnect the antennas, turn off the beacon, then reverse the process when all is clear.

Speaking of automation, a few years ago I purchased a beautiful electromechanical coax switch. I’m still trying to discover how to drive it and I’m not game to power it with the risk of letting the magic smoke out, so if you have specific details for a K&L 6LMP-28-F-N-I-TTL six position coax switch, a data sheet would be wonderful, get in touch, cq@vk6flab.com is my email address.

Searching for lightning detectors reveals a boatload of circuits and tools. There’s websites galore, an active community surrounding “blitzortung.org” with an absolutely mesmerising homepage, and plenty of other projects to play with. One thing I noticed was a distinct lack of SDR, or Software Defined Radio implementations. The number isn’t zero, but I didn’t find any turn-key solutions.

One recurring theme, other than arguing which frequency to tune to for the best result, was the notion that a simple AM radio receiver could be used. The idea often promulgated is to tune between stations and to listen for clicks and pops. It occurred to me that using an SDR as an AM receiver is pretty much the first thing you do after plugging it in.

Combining these ideas, it seems obvious, of course it’s unlikely to be, but you know, why spoil the fun, to do this within the 2.6 MHz bandwidth of an RTL-SDR dongle which might actually detect some wideband RF, the signature of a lightning strike.

That started me down the path of splitting a signal in GNU Radio, routing it to multiple AM decoders, looking for peaks, all that fun, when I realised that I know of a tool that does pretty much all of that, “rtl-airband”. With a simple text file to describe the frequencies you want to monitor, decode, apply squelch, mix, position and route, most of the job is done.

When it’s running, you might have just created a lightning detector to start your journey on this road, using stuff you already have. I intend to create an example configuration file for exactly this purpose, perhaps even a script to generate it, and I’ll publish it on GitHub.

Word of warning, based on the number of tools alone, the rabbit hole is deep.

I’m Onno VK6FLAB