alt Hacker News> atomic clock time signal that (was/is?) broadcast nationwide
Probably DCF77 or WWVB.
> I think the actual antenna and hardware were relatively large since it's a long wave signal
Casio has some normal sized wristwatches that synchronizes to DCF77, it would definitely fit into a stove, microwave, or basically anything.
I believe it was a longwave broadcast so probably WWVB which would apparently imply a 60mm antenna, but it was a standard old school "GE digital clock radio" form factor so size wasn't at a premium.