THAI CAVE RESCUE

On 23 June 2018, 12 boys went exploring in Thailand's Chiang Rai province with their football coach - and ended up trapped deep inside a cave underneath a mountain. Locals found their bikes, the bags, and some football shoes outside. They raised the alarm. The boys and coach eventually found themselves marooned on a small rocky shelf about 4km from the cave entrance. Over a week later, divers eventually found them and a huge and dangerous rescue exercise eventually saw them all safely repatriated with their parents.

Source: BBC News

Not many people realised that essential communications underground were acheivable thanks to the invention of a radio amateur.

Phil Karn KA9Q describes the 'Heyphone':

Naturally I got interested in the technical aspects of the cave rescue in Thailand, particularly communications. They used the Heyphone, a voice radio designed by a UK radio ham, John Hey, G3TDZ, as open-source hardware specifically for cave rescues.

It uses upper (single) sideband voice on 87 kilohertz in the VLF (very low frequency) band. (That's what it says -- 87 kHz is actually LF).

The antenna consists of two stakes driven into the ground about 20m apart. Enough of the current between them fringes outward to couple to another antenna up to a few hundred meters away (or down).

John Hey passed away in 2016 so he didn't get to see his work used here. But ham radio should get some of the credit.

Source: KA9Q Facebook