[implements]

> My pet idea is to make some use of longwave! […] At 60 kHz the wavelength is 5 kilometres long …

Dim memory from my Ham Radio days that you’d need an antenna length of 1/4 the wavelength, which wouldn’t be very convenient for portable devices, unfortunately.

[lxgr]

Only for most efficient transmission. Reception just scales with antenna size, so as long as your transmitter is powerful enough in terms of effectively radiated power, you can make the receiver arbitrarily small.

Many wristwatches are capable of receiving these LF time beacons, despite usually having antennas more compact than several kilometers.

[sidewndr46]

The 1/4 wavelength is derived from the half-wave dipole. It's not necessary to have an antenna that is 1/4 wavelength. It'll probably perform better if it is closer to that size, but it isn't required.