On one night I found out that solar radiation and comsic rays actually produce radio emissions in the audible radio range, IE radio waves from ~20Hz to ~20kHz. That means that by building a reciever, the radio signal tapped directly can be heard with no form of demodulation, and this piqued my interest.

I had a fair bit of interest into what caused the pings and clicks I heard from the reciever and wanted to see if there was a coorelation from the time of day to VLF activity. I found that in the earing mornings, there is a wash of sound, referred to by people who research VLF radio as “dawn chorus”.

Right at dawn one morning I woke up, set up the antenna, and sat down. I listened and began to hear a symphony of chirps and whistles as the sun’s light and charged particles slammed into the ionosphere above me, and soon after the horizon began to glow with sunlight.

In the embedded video, I have set up my antenna in a field relatively far away from residential or commercial spaces, but you have to travel very far from power lines to escape the 60Hz VLF hum.

With a headphone amplifier and a long, shielded audio cable, I was able to hear my first solar activity.

The box itself has not only the reciver (the red PCB) but also an arduino datalogging system with a peak detector for measuring average actvity. I don’t have the data anymore but I did notice a lot of spikes in activity around when lightning/thunderstorms where within a very large radius of my area, which I found to be pretty neat. The datalogging portion consists of an SD card adapter, real time clock, arduino micro, and some active analog signal processing ciruitry on the board.

When building this thing, apart from the insane amount of AC 60Hz line noise in my building, I was able to hear all my devices such as my phone and computer’s internal whirring and data transfers through the VLF antenna. As a result of it’s strong sensitivity to any low frequency signal, I ran all my testing by planting it out in a far away meadow in a forest far from human activity, running on a 12V SLA battery for a week straight. The knobs shown on the antenna side where never actually used despite them being intended as low/high cut EQ settings.

I learned heaps about the ionosphere and about low frequency RF which made me really appreciate all the things that I can’t see or hear, but are always there.

VLF Radio