Thai rescue workers used radio Heyphone, developed by hobbyist in 2001
Heyphone - a communication system for carrying out rescue operations in caves. It was used Thai rescuers rescued 12 Thai guys from the football team and their coach, are trapped in the flooded cave of Tham Luang. All anything, only Heyphone was developed in 2001 and its creator, an Englishman, a radio amateur John Hay, fell in 2016.
The fact of the use of this unusual device for modern realities noticed Facebook user Phil Karn, who "became interested in the technical aspects of salvation from Thailand cave", especially communications. Rescuers used G3TDZ radio (Heyphone), equipment with open source code, specifically created for this purpose. It uses the upper single sideband modulation of 87 kHz to very low frequencies. The antenna consists of two parts, dug into the ground at a distance of 20 meters. At a sufficient current supply they can "cling" other antennas at a distance of 100 meters from them (or down).
John Hay slept in 2016 and never saw so brilliant use of its technology. But his work lives amateur radio.
Moreover, Heyphone no longer recommended for use. He was replaced by John Cooper Micro Heyphone, more modern and updated version of the original device.