See also the HeyPhone and Nicola systems built by underground boffins in the UK and France: https://www.shropshirecmc.org.uk/radio.html https://www.shropshirec

See also the HeyPhone and Nicola systems built by underground boffins in the UK and France: https://www.shropshirecmc.org.uk/radio.html https://www.shropshirecmc.org.uk/radio.html http://www.scavalon.be/avalonuk/technical/radio1.htm http://www.scavalon.be/avalonuk/technical/radio1.htm https://www.electronicsweekly.com/blogs/engineer-in-wonderland/british-electronics-thai-cave-rescue-2018-07/ https://www.electronicsweekly.com/blogs/engineer-in-wonderla... https://www.caverescue.org.uk/nicolaradio/ https://www.caverescue.org.uk/nicolaradio/ http://site2.caves.org.uk/radio/comms_in_caves.html http://site2.caves.org.uk/radio/comms_in_caves.html