VLF/LF Radio and Whistlers
Updated 13.10.05

73kHz, 136kHz, 160-190kHz and 500kHz band operation

A few years ago a new UK amateur band was allocated on a limited basis at 73kHz. Only a few, very dedicated amateurs made use of the allocation but results exceeded expectations with contacts being made at remarkable ranges. One way reception was achieved across the Atlantic which is remarkable considering that it took over a year to achieve useful 2-way communications ranges when the band was first released. The biggest challenge was the very small antennas possible and their exceedingly low ERP. Also, interference from large commercial stations and the high background noise levels was "challenging". Sadly, this UK only allocation has now been withdrawn. Perhaps someone can persuade the "powers that be" to re-allocate it at a later date? Certainly those who used the band achieved some real scientifically useful results.

A narrow allocation has been made more generally at 136kHz. Better equipped stations have been making contacts right across Europe and more recently to the USA, Canada and beyond using both very narrow bandwidth modulation methods as well as CW. SSB is impractical in the available bandwidth except over very local distances.

In the USA operation is allowed between 160-190kHz under FCC Part 15 conditions (less than 1W DC input to antennas not longer than 50 feet). Results have been remarkable. In New Zealand this spectrum is also available to radio amateurs.

There are now rumours that an allocation in the UK and possibly Europe will be granted around vacated spectrum just above 500kHz. This could be a most interesting allocation! Already this is taking shape in the USA. See the RSGB's 500kHz proposal.

Whistlers and Tweeks (natural emissions mostly below 10kHz)

If you've never heard a "whistler" or a "tweek" then you are in for a surprise. These are natural radio emissions usually triggered by lightning strikes and which are audible at very low radio frequencies. Equipment to receive these emissions is simple - just a sensitive audio amplifier, ideally with some rejection of mains hum and frequencies above about 10kHz, and a loop antenna.

Natural radio sounds can be quite unreal with whistlers sounding like a long descending pure or raspy whistle and tweeks sounding like the "ching" one gets when someone walks on a gravel path. Other emissions, called "chorus" sound like birds in a dawn chorus. The definitive work on the subject is "Whistlers and Related Ionospheric Phenomena" by R.A. Helliwell published by the Stanford University Press in the mid 1960s. The atlas section at the rear of the book contains some remarkable spectrogram pictures of VLF emissions. You can hear some of these at the links below.

Some years ago I recall hearing a whistler recording made in the atmosphere of Jupiter by a space probe - proof that lightning occurs in the Jovian atmosphere too.

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