Alexanderson Day will be held on Sunday 29 June this year. This day commemmorates the Swedish radio engineer Ernst Alexanderson and is celebrated by having an open house at the Swedish VLF transmitter, Grimeton. The transmitter operates on 17.2 kHz with the call-sign "SAQ".
Alas, it probably can't be heard in South Africa (Please someone prove me wrong.)
The remarkable thing about this transmitter is that the signal is produced by a rotating machine called an Alexanderson alternator. In essence this is a series of rotating iron discs swith cut-outs arounds the periphery. The field coils are also placed on the periphery and the iron induces an alternating voltage of very high frequency in the field coils. The spaces between the cut-outs are filled with non-magnetic gunmetal to reduce air-resistance.
The signal can be received using a tuned circuit attached to a PC sound card. A program such as "Spectrum Lab" can be used to decode the transmission. There's even an SAQ receiver script for this software.
Alternatively, a TRF receiver could be used.
For those who like to experiment, a stepper motor from an old disk drive can be used as a high frequency alternator, but keeping the frequency steady is another thing altogether.