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SESK: Stuff Everyone Should Know · sesk.info/hamg-solar-flux

What is the solar flux index?

A measure of solar radio emission at 10.7 cm wavelength.

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  1. Another name for the sunspot number
  2. A measure of solar radiation at 10.7 centimeters ✔
  3. The highest useful frequency between two points

Why: The solar flux index measures the Sun's radio noise at a wavelength of 10.7 centimeters. Hams use it as a quick gauge of how well the higher HF bands should work.

Huh, didn't know that: During low solar activity, 15, 12 and 10 meters are the least reliable bands for long-distance work.

The dad joke
Why is the Sun so popular with hams? It always has great flux.
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References

Answer: A measure of solar radiation at 10.7 centimeters. The solar flux index measures the Sun's radio noise at a wavelength of 10.7 centimeters. Hams use it as a quick gauge of how well the higher HF bands should work.

  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
    A measure of solar radiation with a wavelength of 10.7 centimeters
    Checked 2026-10-10.

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Ham RadioHow does a higher sunspot number affect HF propagation?Ham RadioOn which HF bands do General licensees have some off-limits portions?Ham RadioOn which of these bands is voice (phone) operation prohibited?

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