{"id": "card_n_a735e61f7ca1", "kind": "note", "title": "Shortwave (HF) — around the world on the ionosphere's mirror", "body": "3–30 MHz; the 19 m band's wavelength seals at 15 MHz: https://narrowhighway.com/s/d6211f4efb2518c6de145decf57f27b131a01c24ce7841853f395b7bc986c323 . Here the ionosphere's\nF-layer refracts waves back to the ground, and a few watts can cross an ocean by skipping\nbetween sky and sea. The geometry seals; the MIRROR is empirical — the maximum usable\nfrequency rides the solar cycle, day/night, and season, which is why shortwave listening is\nhalf physics and half weather report. This is the band of world services, ham operators, and\ntime stations; the same skip explains why it fades and returns as the Earth turns.", "source": {"label": "Concordance assay — 2026-07-09", "url": "https://narrowhighway.com/s/d6211f4efb2518c6de145decf57f27b131a01c24ce7841853f395b7bc986c323", "ref": "spectrum_walk", "authority_tier": "engine_derived"}, "shelf": "science", "box": "radio_spectrum", "bands": ["shortwave", "hf", "skip", "ionosphere", "f-layer", "ham radio", "19m band", "muf"], "connections": [], "author": "engine", "created_at": "2026-07-10T01:06:08.948754+00:00", "updated_at": "2026-07-10T01:06:08.948754+00:00", "visibility": "public", "lifecycle_stage": "public", "volatility": "permanent", "surface": "secular", "metrics": {"paperclips_count": 0, "helpful_count": 0, "not_helpful_count": 0, "cite_count": 0, "walks_through_count": 0, "flagged_count": 0}, "source_hash": "2084ed5bf89c050ee517f069bc38f477ed76c33642b4e753b26742117fa3d7c7"}