AM broadcast (medium wave) — where the dial begins

530–1700 kHz; sealed wavelengths 565 m down to 176 m: https://narrowhighway.com/s/d6211f4efb2518c6de145decf57f27b131a01c24ce7841853f395b7bc986c323 . The waves are so long that the TOWER is the antenna — a quarter-wave at the low end is ~140 m of steel. Channels sit 10 kHz apart in the Americas (9 kHz most elsewhere — a regulatory split, not physics). The band's famous trick is empirical: by day the ionosphere's D-layer absorbs skywave, so AM is groundwave and local; at night the D-layer vanishes and signals bounce off the E/F layers for hundreds of miles. Amplitude modulation itself is the simplest possible encoding — the diode detector of a crystal radio is enough to hear it.