K index — How disturbed the earth's magnetic field is.
Zero to nine. Under 3 is quiet and good. Five and above means a geomagnetic storm is degrading HF, especially on paths toward the poles.
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Related words
- SWR — Standing Wave Ratio.
- Simplex vs duplex — One frequency versus two.
- Sporadic E — Sudden short-lived openings.
- Gray line — The band of twilight moving around the earth.
- CTCSS / PL tone — A sub-audible tone that opens a repeater.
- FT8 — A weak-signal digital mode.
- RTTY — Radio Teletype.
- Ionosphere — The layer that bounces HF signals back to earth.