‘HeyPhones’ were “delivered to Heathrow in time for their flight, following the welcome assistance of Derbyshire Constabulary and other Police forces”, according to the Council. BTW, for a straight-forward description of UK activities in the rescue, the Council has prepared a page here.
According to the British Cave Research Association (BCRA), “The HeyPhone was designed by John Hey with the support of the BCRC and the Cave Radio & Electronics Group (CREG), which is a special interest group of the BCRA. In the period up to 2001, working with CREG, the BCRC issued over fifty HeyPhones to the UK’s cave rescue teams, allowing communication between the surface controller and one or more underground teams.”
For long-term use, HeyPhone was designed to be maintained, and even built, by electronics enthusiasts.
As electromagnetic waves do not propagate far in the ground, it usually operates conductivity through electrodes spaced 25m to 100m apart (similar to the Fullerphone used in the first world war which Electronics Weekly modelled here), and is also capable of working with an inductive loop.
HeyPhone modulation is 87kHz single (upper) side-band, to improve signal-to-noise ratio – the far-earlier Fullerphone was essentially Morse-code derived dc current filtered for stealth reasons.
Thanks to a correspondent for bringing this subject to Electronics Weekly’s attention, by pointing to this Hackaday page.
BTW, the HeyPhone schematics are here, and it looks like a lovely sensible design.
And look down to ‘Will there be any future developments?’ on this page if anything above has inspired you to offer to lend a hand towards a more modern design.