Johnm.... quote.... 'there's nobody to talk to except D&D on the way through Wales'
Is this because D&D have better radio coverage than London Info, with more radio repeater stations on Welsh hill-tops?
Or are you expecting a reply from an Air-France 777 on route from Paris to Toronto (at FL380.)?
I'm just interested in the Radio Engineering systems involved.
Edit... As I am so interested in the 121.5 coverage, I Googled it and came up with ...
https://www.caa.co.uk/docs/33/srg_gad_D&D_poster.pdf
So No special equipment.... just a Radio and Aerial somewhere Down South.
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