[hrpyultg] 21 cm toys

Create toys and trinkets featuring the length 21.10611405413 cm, the wavelength of the astronomical hydrogen line.  One of the most important lengths in the universe happens to be human-scale.  Usually things are much smaller or much larger.

Most interesting would be an antenna whose resonant frequency is 1420 MHz.

Incidentally, how can the value 21 cm be derived from quantum first principles?  It must have originally been done so to make the astronomical prediction, because the spontaneous transition is too rare to be observed in a laboratory, and hydrogen masers had not been invented yet.