Sferics (short for “atmospherics”) are the radio impulses produced by lightning. You do not need a transmitter: a very simple receiver, a small antenna and an audio input are enough to hear the Earth’s atmosphere at work.
What you hear
Sferics are broadband pulses, with most of their energy below 100 kHz and a peak between 5 and 10 kHz. They travel in the waveguide between the ground and the ionosphere with little attenuation, so they arrive from storms thousands of kilometres away. On VLF they sound like a crackling Geiger counter.
With some luck, and away from noise, rarer sounds appear:
- Tweeks: short “pings”, when the same sferic arrives by two paths, direct and reflected from the ionosphere.
- Whistlers: tones that fall in pitch over a few seconds. A lightning signal travels up along the Earth’s magnetic field lines and dispersion stretches it into a whistle.
- Dawn chorus: a chorus of rising tones shortly before sunrise, caused by the solar wind interacting with the magnetosphere, not by lightning.
A receiver to build
YO6DXE (Ciprian, Romania) has, on his DX Explorer channel, among many other homebrew radio projects, a build of a sferics receiver based on a design by K8TND. It is a single-JFET circuit with a trap against AM broadcast stations, feeding an audio amplifier (for example an LM386) and a telescopic whip. The project files are available under a CC BY-NC-SA licence.
Tips from the video and from similar builds: keep away from power lines to reduce mains hum and its harmonics; keep a hand on the receiver’s ground plane, because otherwise it may self-oscillate; and feed the audio output into the computer’s sound card to record and analyse.
- DX Explorer (YouTube) — YO6DXE's channel, with receiver builds and radio projects
- Pesquisar «sferics» no canal — all the channel's videos about sferics and VLF
- yo6dxe.com — YO6DXE's blog (Ciprian, Romania), with articles and schematics
- K8TND VLF Sferics Receiver (PCBWay) — schematic, bill of materials and Gerber files for the receiver (CC BY-NC-SA licence)
- Homebrew Sferics Receiver (Hackaday) — article about this build
Software
To record and look at what you received: record the audio and analyse the spectrogram afterwards, where sferics show up as vertical stripes and whistlers as falling curves.
- Audacity — record the receiver's audio
- Sonic Visualiser — spectrograms to analyse the recordings
Reading and learning
- Sferics, whistlers and the dawn chorus (Hackaday) — a good introduction to natural sounds on VLF
- Radio atmospheric signal (Wikipedia) — definition, spectrum and propagation of sferics
- Stanford VLF Group — introduction to whistler waves in the magnetosphere
- VLF Natural Radio Experiments — resources and experiments with natural radio on VLF
- WR9STL guide — setup guide for a commercial VLF receiver
- E202VAR Natural Radio Receiver — a more complete homebrew receiver project on GitHub
Lightning in real time
To compare what you hear with the storms that are really out there:
- Blitzortung — community network locating lightning worldwide in near real time