Jan Jozef Oksiuta
Brisbane QG62lk
Australian Amateur Radio Station

DC to light, homebrewing, minimalist antennas and projects, QRSS, QRPp and less


The International Telecommunication Union defines ELF (Extremely Low Frequency) as electromagnetic waves with frequencies between 3 and 30Hz, whereas the ULF (Ultra Low Frequency) extends from 300 Hz to kHz according to the ITU nomenclature. The SHF (Super Low Frequency) covers the bandwidth in between the other two.

Various scientific disciplines may have formulated different definitions and frequency boundaries suitable for their partticular uses. Here, for the sake of simplicity, we will talk about signals with frequencies anywhere between DC and 3kHz with some references above and beyond, into the VLF range above 3kHz.

So, what signals can one observe in this range, other than the ever-present 50 Hz mains hum and its harmonics? It may come as a surprise for some but such low frequencies might be the only available communication medium in certain environments. There are of course severe limitations on available bandwidth and therefore the throughput of information. Nevertheless, these signals can penetrate into the ground and under water so they find use in emergency communication in mines, cave exploration or communicating with submarine vessels. Incidentally, they propagate quite well through man-made infrastructure like pipelines or power distribution. It is therefore no surprise that, with the proliferation of more and more complex domestic and industrial appliances, there will be a great variety of strange-sounding (or strange-looking) signals generated by them and travelling far and wide along the power wiring.

Then there are natural signals generated by various environmental phenomena - such as e.g. the interaction of solar wind with the Earth's magnetic field, or the energy of lightning strikes, An excellent overview can be found at http://www.vlf.it/ .

How to receive signals in this range of frequencies? Quite simple - either an induction coil, a length of wire, or two metal probes inserted into the earth - or even tapping to a nearby pipeline. Whatever pickup device is being used, this needs to be followed by some amplification and filtering (especially very aggressive filtering of the 50Hz mains hum), and a computer sound card. At such low frequencies and consequently limited bandwidth it may be necessary to record the signals over extended periods of time in order to observe any meaningful patterns.

The signals can be listened to directly if within the audible range. Lower-frequency signal can be pre-recorded and then played back at higher speed. But by far the most popular is a graphical presentation through a spectrogram image. Frequency  on the Y axis, elapsed time on X, and  signal intensity represented by varying colour or shades of gray.

Here is an example of a two-hour spectrogram recording, taken from a set of earth probes in a suburban backyard:

Looks quite busy over there - of course the 50Hz mains signal is most prominent - but there is quite a lot of activity all the way down to below 1Hz. Most of it probably has some industrial or domestic origin, but I am yet to identify all the various appliances and their spectral signatures, or "fingerprints". Do you have any thoughts about their origins? If so then please write to me and let me know.

Few words about the software: there is a a multitude of commercial, freeware and open-source applications for a number of operating systems and platforms available on the internet that are capable of performing spectral analysis of raw data or audio files. Probably the most popular is Wolf's DL4YHF Spectrum Lab, freely available and described here. The majority of screenshots on this site has been acquired using Spectrum Lab.

Signals Gallery 01 - Sources and locations
Signals Gallery 02 - S
ignals and the mains supply 1 - amplitude-modulated 50Hz mains (coming soon)
Signals Gallery 03 -
Signals and the mains supply 2 - other curious sideband patterns (coming soon)
Signals Gallery 04 - Signals and the mains supply 3 - identified signatures of some mains-powered appliances (coming soon)
Signals Gallery 05 -
TLF - Below 3Hz (coming soon)

Preamplifiers and filters project - home recording with a sound card (coming soon)
8-channel ELF data logger
HF radio link project - for use in remote ELF recording (coming soon)

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Created with CompoZer. Last updated 2014-06-21