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SESK: Stuff Everyone Should Know · sesk.info/hamg-k-index

What does the K-index measure?

The short-term stability of Earth's geomagnetic field.

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  1. The short-term stability of Earth's geomagnetic field ✔
  2. The relative position of sunspots
  3. The solar radio flux at Boulder

Why: A low K-index means a calm geomagnetic field, which is good for HF. A high number means a disturbed field, often meaning poorer HF conditions.

Huh, didn't know that: The A-index measures the long-term stability of Earth's geomagnetic field.

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You know you're a ham when the propagation forecast means more to you than the weather forecast.
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References

Answer: The short-term stability of Earth's geomagnetic field. A low K-index means a calm geomagnetic field, which is good for HF. A high number means a disturbed field, often meaning poorer HF conditions.

  1. NCVEC 2023-2027 General Class Pool and Syllabus (6th Errata, Feb 4 2026) ncvec.org/downloads/General%20Class%20Pool%20and%20Syllabus%202023-2027%20Public%20Release%20with%206th%20Errata%20Feb%204%202026.pdf
    The short-term stability of Earth’s geomagnetic field
    Checked 2026-10-11.

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