Shortwave (HF) — around the world on the ionosphere's mirror

3–30 MHz; the 19 m band's wavelength seals at 15 MHz: https://narrowhighway.com/s/d6211f4efb2518c6de145decf57f27b131a01c24ce7841853f395b7bc986c323 . Here the ionosphere's F-layer refracts waves back to the ground, and a few watts can cross an ocean by skipping between sky and sea. The geometry seals; the MIRROR is empirical — the maximum usable frequency rides the solar cycle, day/night, and season, which is why shortwave listening is half physics and half weather report. This is the band of world services, ham operators, and time stations; the same skip explains why it fades and returns as the Earth turns.