Good point about this service, I almost forgot that it still exists (although reception is somewhat spotty indoors).
The basic advice is sane ("do not rely on side effects"), yet "third-party systems provided free of charge and without obligation" sounds exactly like that, except now it's putting all your eggs in a basket that's controlled by one entity (be it DCF77, or the GPS temporal component). I would think that the AC-as-a-clock is usually chosen for convenience rather than high reliability - "the juice is needed anyway, why bother with an extra receiver?" - and if you actually need to tell the time, checking multiple independent sources would be encouraged :)
DCF77 surely cannot be that bad indoors on the Continent?
When living at what was then often quoted as the extreme fringe of the coverage area - Trondheim, Norway, on 63,5 degrees of latitude - I had reliable coverage as long as the clocks were kept in window sills (hence, effectively outdoors)
However, just moving down to my current home (on 62 degrees, or -roughly speaking- 11% closer to the transmitter site), I now have reliable coverage everywhere (granted, in a wooden house - but the DCF77 alarm clock in the basement synchs every hour, too)
Citizen, the Japanese watches maker, offered a passive (without batteries) "enhancer" for their AT series, IIRC was a ferrite block where to put the watch, I think I've seen a photo.
Or one can use a PC to transmit the DCF77 signal locally (it could be illegal) to a clock using a pair of headphones, I've seen at least a couple of examples but never tried them.
More importantly, you would need to know how the antenna is oriented within the enclosure of your clock. I'm pretty sure that that wasn't mentioned in the manual of the one I bought.
Beats me - I'm within reasonable driving distance to the antenna (one border away), yet clocks fail to acquire signal unless outdoors/directly at a window. Just a bad coverage area, I guess.
This is completely outside of my area of expertise, but doesn't the radio signal strength decrease exponentially? So that last 11% could be a big deal?
Line-of-sight attenuation of an EM wave in the empty space is quadratical, whilst - for example - attenuation of an electrical signal in a wire is exponential.
