SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/hamg-twenty-seven-day

What causes HF conditions to vary in a 26 to 28 day cycle?

The Sun's rotation on its axis.

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  1. Cyclic variation in Earth's radiation belts
  2. The position of the Moon in its orbit
  3. Rotation of the Sun's surface layers around its axis ✔

Why: The Sun spins about once every 27 days or so. Active regions rotate into and out of view of Earth, so propagation conditions can repeat on that schedule.

Huh, didn't know that: Charged particles from solar coronal holes generally disturb HF communication.

The dad joke
What did the old astronomers do when they got tired of waiting for sundown? They decided to call it a day.
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References

Answer: Rotation of the Sun's surface layers around its axis. The Sun spins about once every 27 days or so. Active regions rotate into and out of view of Earth, so propagation conditions can repeat on that schedule.

  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
    Rotation of the Sun’s surface layers around its axis
    Checked 2026-10-11.

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