Theory
Anyone who has tried to listen to an AM radio during a thunderstorm will know that lightning creates a radio frequency signal. By 'listening' to this noise with a sensitive directional antenna, it is possible to determine the direction the strike is coming from. By further processing to analyse the strength and nature of the signal, it is also possible to calculate with reasonable certainty the distance to the strike and the type of strike - that is, whether it is positive, negative, cloud-to-cloud or cloud-to-ground.
In practice
That's all very well, but how do you go about doing this in a practical sense? I took the easy option and bought an antenna and signal processor made by Robert Boll of Boltek in the USA. These can be pricey little items, but I managed to get one on super-special from Provantage Corp, also in the USA. They sell an astounding array of stuff, and will ship to Australia at reasonable rates (well, at least they did last time I dealt with them).
The Boltek gadget consists of an antenna and a PCI card for the computer and 15m of black cat5 cable to join the two. You can run up to 60m of cable, but my study (and computer) is directly under where I mounted the antenna, so 15m was enough. The antenna has to be mounted clear of any metal that might cause interference, so I fixed it with cable ties to a length of timber that reaches above the peak of my roof. I'll need to fabricate a PVC house for it, but for now it is protected from the rain with a plastic bag!
Much of the processing occurs in software. Boltek supply a rudimentary programme with their trackers, but it is evident that their expertise is in hardware. Instead, this station uses the excellent third party programme NexStorm, written by Astrogenic in Europe. In addition to the StormVue applet that you can view on this website, NexStorm provides a comprehensive real-time display for local users here on the farm. It also uploads data in real time to the Australian Strikestar network which triangulates lightning strikes with other online Boltek detectors to form an accurate picture of storm activity. Strikestar is the world's only large scale, community-based lightning detection network. This station is designated "URL" after the closest village of Uralla.