Because of the long wavelength and high powers, the radio waves can be picked up pretty well by small ferrite aerials ('loopsticks'). In most cases, all the rec

Because of the long wavelength and high powers, the radio waves can be picked up pretty well by small ferrite aerials ('loopsticks'). In most cases, all the receiver has to do is detect the presence/absence of a signal; the transmissions start and stop every second, with variations in the starts and stops encoding the time (mostly it's necessary to receive a full 60s of transmissions, to get 60 bits of data that can be decoded to give the time). It's one of the easiest radio signals to pick up, a cardboard box with a few turns of wire around it and single transistor are enough in most of Europe.