The basic principles of radiolocation are simple. Rock absorbs radio waves at commonly-used frequencies (which is why your GPS and cell phone don’t work underground). What has been proven to be useful is a magnetic signal produced by a circular coil underground. With the plane of the coil horizontal*, the imaginary lines of the magnetic field curve from one side to the other like iron filings between the poles of a bar magnet.
At the point on the surface directly vertical to the transmitter the lines of magnetic force will be vertical. This point is conventionally called “ground zero”. Seen from above, these lines will appear to radiate outwards from ground zero:
The receiver loop is held vertically, and the signal heard in the earphones will vary depending on how the loop is orientated in relation to the lines of the magnetic field. The more lines that go through the loop, the stronger the signal. When the loop is exactly aligned with the magnetic field lines, the signal strength will be at a minimum, and this is called the “null”.
When a null is found with the receiver, the plane of the loop antenna is pointing directly towards ground zero. When the loop is exactly at ground zero, the magnetic field lines are exactly vertical and therefore the signal will be nulled whatever direction the receiver loop is pointing in, as long as the loop is exactly vertical.
Unfortunately our old ferrite core cave radiolocation beacon (at the bottom of the picture) did not survive its trip to Kentucky. Despite Matt Vinzant's valiant attempts to repair it in the mud of Roppel Cave, the dislocation of the core during the trip unwound some turns from the coil and detuned it to about 10-15% of its full output power. It was never really built to withstand dry caving abuse.
Rewinding and retuning the coil is time-consuming and tricky, and luckily I had a replacement version in the works with a smaller, slightly lighter but more powerful coil and improved electronics which incorporate a low battery warning, low battery shutoff and the ability to program a delayed start, a pulsed signal or timed shutoff (top of the picture). The weight reduction has allowed me to remove 2.5 inches (10cm) from the length of the housing. The mounting system has also been substantially strengthened, so it might survive the next trip.
Andy Pitkin and Brett Hemphill performing a cave radiolocation at Lineater