MYRIACAT

Radio astronomy isn’t as difficult as you might think. Mike Bedford shows how to get going with just your PC audio system and some free software.

With sunspot numbers set to be high for some time yet, there’s every possibility that we’ll have other opportunities to see auroras like the ones many people saw in May. However, adverse weather conditions – by which we mean a cloudy sky – would derail any such sightings. So, while seeing an aurora is often a hit-and-miss process, listening to one is less affected by the weather. We’re not talking about listening to them with just our ears, though. The northern and southern lights generate radio signals that can be heard with suitable equipment, which might be little more than a PC’s internal audio system, an antenna and some suitable software.

We’re looking at signals at the bottom end of the radio spectrum, which is why they can be received without any special equipment. Wi-Fi signals are transmitted at a frequency measured in gigahertz, as are signals from Bluetooth devices. Mobile phones mostly use gigahertz frequencies, and for conventional broadcast radio (not smart speakers) we find FM in the hundreds of megahertz, going down to the 198kHz of BBC Radio 4’s longwave station.

However, while the longwave broadcast band is in the ITU band officially designated Low Frequency (LF), which covers 30kHz-300kHz, the radio spectrum goes much lower. We have Very Low Frequency (VLF, 3kHz30kHz), then Ultra Low Frequency (ULF, 300Hz-3kHz), Super Low Frequency (SLF, 30Hz-300Hz) and Extremely Low Frequency (ELF, 3Hz-30Hz). Much of this comprises frequencies we consider audible, which is the key to receiving them without a radio receiver.

A PC’s microphone input covers frequencies from below 50Hz to the upper limit of human perception, and much further with high-definition soundcards. So, all we need to do to receive these signals is to swap out the microphone – which converts sound to an electrical signal – for an antenna that converts radio waves to an electrical signal. And that antenna might be as simple as just a length of wire.

It’s never going to be an everyday experience unless you live in polar regions, but hearing an aurora might be easier than seeing one because radio waves aren’t blocked by clouds.

CREDIT: www.pexels.com/Stein Egil Liland

A first attempt at receiving signals at the bottom end of the radio spectrum couldn’t be simpler – we’ll start with a few metres of wire as an antenna. To do so, attach wire to a stereo 3.5mm jack plug. These have three contacts, referred to as the tip, ring and sleeve, in order starting from the end that plugs into the socket. Solder the wire to the tip – although, of the three, only the solder tag for the sleeve is the really obvious one. If you’re unsure, insert the plug into the mic socket on your PC and try touching the wire on to the solder tags for the other two in turn while listening on the speaker – see later for details – until you hear a signal.