THE SID DETECTOR AT VE3EARIn 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 antennas make it through the chain of components of the system.
The signals that I want to receive and process 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. Instead of those, I use an E-Field probe and a H-Field multi-turn loop antenna.
The E-Field probe is based on an AMRAD design, modified by W1VD. It consists of a vertical whip about 30 inches long, connected to a high impedance FET followed by an emitter-follower buffer, that both amplifies
the signal and changes the impedance down to 75 ohms that can be fed via coaxial cable to the shack.
The H-Field loop consists of 25 turns of #22 AWG wire inside of a frame 40 inches square, constructed from
PVC electrical conduit and fittings. The wire is actually an old printer cable, and is shielded by a foil wrap, except for a one inch gap at the top centre, which prevents the shield being a shorted turn that would deter signal pickup.
Both antennas are located outside, the E-Probe being about 20 feet above ground on the roof of my house,
while the loop hangs in a tree at about 10 feet above ground. The feed line for the E-Probe is RG-6 coax, and
the loop’s feed line is a shielded pair audio cable.
Both feed lines enter the house and terminate in the shack, where the probe’s connects to a Bias-T which sends power to the antenna and sends the signals along to the next link in the chain, a 1:1 audio isolation transformer, the output of which is connected to the next link in the chain, as is the loop feed line.
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! I use a sampling rate of 96 kb/s, allowing me to receive signals up to 48 kHz.
The loop antenna’s feed line is connected to input 1 of the 202 using a balanced XLR type connector, and the
E-probe antenna feed from the isolation transformer is connected to input 2, using a TS type connector. Input 1 uses the 202’s internal mic preamp to boost the signal, while input 2 operates at line level. I use the
isolation transformer to prevent ground loops, which could introduce 60 Hz. hum.
Both of the inputs have gain controls to set the proper levels of the signals, as well as allowing either one to be used individually, or both together. At this point, the signals leave in two separate outputs, one via the USB port to the Win10 laptop, and the other via the headphone jack to a second isolation transformer and a Sound Blaster external USB sound card dongle, connected to the Win7 PC.
Now we get to the second part of the SID detection system, the software. In both the laptop and the PC, 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 the correct parameters, I saved that in the 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! It 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, NAA, and NLK.
The plotter windows show 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.
As of the end of July, 2022, I have saved about a dozen screen shots that show sudden changes in the strength of only some, or all of the Watch List signals that may have been caused by events on the sun. I have yet to capture and confirm a genuine SID caused by a CME or Solar Flare, but it’s only a matter of time until one comes my way! That one will be special, which I will share with you all.
73, J.B., VE3EAR