Roman,

Many thanks for pointing these out. Some aspects of the French experiment were 
also described in CREGJ 89 and 92.

FERRA was also mentioned in CREGJ 89.

John Hey (SK) would be delighted for his cave radio to be described as 
“Professional”. It undoubtedly was of that standard, but was not manufactured 
commercially as we could not find a company to do that. Many people built 
Heyphones, over a hundred sets were built and approximately 80 were built by 
John himself.

73 John F5VLF

> On 23 Jul 2018, at 19:41, Roman <[email protected]> wrote:
> 
> Hi John, LF!
> 
> The radioamateur experiment in France several years ago with 3.3 khz PSK31 
> communication in caves.
> 
> http://f6kcz.free.fr/Actualites/2011/Carrieres%20Fleury/Carrieres%20Fleury.htm
> 
> Profesional cave phone:
> 
> http://bcra.org.uk/creg/heyphone/
> 
> Russian communication system FERRA from Omsk radio-factory:
> 
> http://www.radioscanner.ru/forum/topic46535-3.html
> 
> 
> 73!
> Roman