Back on June 7, there was a spectacular coronal mass ejection on the sun:

Yes, I did mention this event and gave some links to VLF receivers at the time, but I’ve been thinking about this some more.

As an astronomy/telescope buff, I have built simple telescopes for looking at the sun, but I haven’t done much of that lately, and I have only recently become interested in observing the sun’s effect on the Earth’s radio environment. I’ve also had a fairly longstanding interest in VLF communications, and so the prospect of building a radio system for monitoring radio for SIDs (Sudden Ionospheric Disturbances) seems like a good project. I recall that Mark Spencer, WA8SME whom I’ve had the pleasure of meeting at Pacificon and even chatted with via the AO-51 satellite had published some articles on designing such radios for amateur and educational construction.

The most common way to monitor for SIDs is to try to detect the signal strength of the VLF station in Cutler, ME on 24Khz. Changes in signal strength can indicate the presence of solar flare activity as the ionosphere is bombarded by high energy particles. One common circuit that lots of people use is called The Gyrator VLF circuit, which you can find here:

The Original Gyrator Circuit (PDF)

I had never really looked at this circuit very carefully before, and looking at it tonight, I realized that it was actually fairly interesting and employs a technique which I hadn’t seen before. Instead of making a tuned front end using a (fairly large, because of the low frequency) inductor, they create an equivalent circuit using two op amps and a collection of resistors. This creates an inductor with very high Q, with the side effect that is easier to make, using operational amplifiers that you can still get at Radio Shack. I’ll probably try to simulate some of the basics with LTSpice to gain some intuition as to what’s going on, but it’s a simple enough circuit that just building it would be pretty easy.

The basic circuit has undergone a couple of iterations, and now the Gyrator III schematic is recommended by the AAVSO. You can check out the details here (and surf around, the AAVSO has lots of information on this stuff).