In early 2022 I set out to build and commission a Sudden Ionospheric Disturbance (SID) detector at my station. There are two main components of a SID detector, 1) the hardware, and 2) the software. I will first describe the hardware, and how signals received by my antenna make it through the chain of components of the system.

The signals that I want to receive and process are from various Naval VLF transmitters, which fall in the range of 19 to 25 kHz., in what is known as the VLF band. The wavelength is quite long at these frequencies, which rules out conventional antennas such as a dipole or a vertical mast.

Instead of those, I use an H-Field multi-turn loop antenna that hangs in a tree outside my radio room window. This consists of 25 turns of #22 AWG wire, in reality an old printer cable, inside of a frame that is 40 inches square, constructed from PVC electrical conduit and fittings. The loop is shielded by the foil wrap of the cable, except for a one inch gap at the top centre, which prevents the shield being a shorted turn that would deter signal pickup.

The antenna feeder is CAT5 cable which is buried between the tree and the entrance to the house. It enters the house and it is wound around an old flyback transformer core as a common mode choke, before terminating in an XLR connector which plugs into the next link of the chain.

That link is a Behringer model UMC202HD USB Audio Interface. Although designed for use in a personal recording studio, I found that it makes an ideal preamp for VLF radio signals, which are just slightly above the hearing range of the human ear! Signals at those frequencies can be either AF or RF in nature!

The loop antenna's balanced feed line is connected to input 1 of the interface, where the 202's internal mic preamp boosts the signal. The signal is then sent to an HP laptop running Windows 10 via USB cable.

Now we get to the second part of the SID detection system, the software. I'm running the program Spectrum Lab by Wolf, DL4YHF. I began by using the Quick Settings option to load the VLF receiver configuration, then once I had everything set to my desired parameters, I saved that in a new file, NavalVLFstationsSID.usr so I could quickly reload if I made a catastrophic error, like hitting the wrong key or clicking the mouse in the wrong place. This does happen, believe me!

When running Spec Lab as a SID detector, only two windows are open. One displays the spectrogram of the signals in the frequency range of 19 to 25 kHz., showing their fluctuations in strength in real time. The second one plots the strength of the Watch List signals, over an extended period of time, and it's here that we look for sudden large changes in signal strength, an indicator of a possible solar event. My watch list is for stations GBZ, NPM, HWU, DHO38, and NAA. The lowest trace (red) is of the broadband noise from 19 to 25 kHz., which represents the system noise floor.

The plotter window shows about two hours worth of signal strength plotting at any given time, but there is a cache memory with a capacity of about 24 hours that you can scroll through to look for any signal changes over that period. If I see something that looks like a SID event, I can take a screen capture of it, which I date and save to a folder created for just that purpose.

I waited several months, capturing dozens of sudden changes in the strength of only some, or at times all, of the Watch List signals, that may have been caused by events on the sun. I knew it would only be a matter of time until a genuine SID caused by a CME or Solar Flare came my way. As you can see in the above screen shot, I had an M-class flare around 19:38 UTC on Friday, December 30th, 2022, which was captured beautifully by the SID detector, in the yellow and blue plots!

73, Bruce, VE3EAR

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