nice project Peter … not much sun lately 
I received last week some supercaps, small ones 0.47 uF and bigger ones to do some measurements, like on the gammon site
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The LIBRARY basement part 1
I’m afraid they don’t deliver enough Tx current and that the voltage drop to much, but I haven’t started testing yet
Oh and you should really talk to Charles CH2I … he is THE lowpower low voltage specialist 