UK radio amateur John Hey, G3TDZ (SK), was the original designer of special low-frequency radio equipment — the HeyPhone — used in the recent cave rescue in Thailand. Al Williams, WD5GNR, reported in Hackaday that the British Cave Rescue Council (BCRC) was asked for its help and equipped the rescuers with HeyPhones. The HeyPhone is “now considered obsolete, but is still in service with some teams,” Williams wrote. The radio transmits on USB at 87 kHz, which can penetrate deep into the ground, typically via electrodes driven into the ground.
In a 2018 update, the British Cave Research Association (BCRA) Cave Radio & Electronics Group (CREG) HeyPhone Cave Rescue Communication page called the HeyPhone “a pioneering development in cave radio” that “can no longer be recommended for construction.” Several successor products — including the Nicola Mark III, which has been tested by the BCRC — have been developed.