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What the sun means for HF radio. The sun decides how far HF signals travel. Four numbers tell you whether the higher HF bands are likely to be open.
- Solar flux: higher is better. How active the sun is, measured once a day. A busier sun lets the higher bands carry signals farther.
- K index: lower is better. How disturbed Earth’s magnetic field is, from 0 to 9, updated every three hours.
- X-ray flares. Classes A, B, C, M and X, each ten times stronger. M and X flares can black out HF on the sunlit side of Earth for minutes to hours.
- Sunspots. More sunspots usually means higher solar flux. They rise and fall over a cycle of about 11 years, and Cycle 25 peaked around 2024 and 2025.
In the pictures
- Solar flux gauge: under 85, quiet; 85–109, fair; 110–149, good; 150 and up, excellent.
- K index gauge: 0–3, quiet; 4, unsettled; 5–9, storm.
- X-ray gauge, mildest to strongest: A, B, C, M, X.
- Sunspot curve: rising and falling, with Cycle 25’s peak marked.
Key terms
- HF: the long-distance bands, where signals bounce off the sky.
- Ionosphere: the charged layer high above Earth that bends HF signals back down.
- Solar flux: a daily measure of how active the sun is.
- K index: a measure of magnetic storms, from 0 to 9.
Good to know
- 2 m and 70 cm repeaters barely notice the sun.
- Check the K index first, then the solar flux.
- HF suddenly silent at midday? Check for an X-ray flare.
- Hear beacons between 28.200 and 28.300 MHz? 10 meters is open.
In short: High flux, a low K index and no big flares mean the higher HF bands are worth a listen.
Checked October 2026 against NOAA’s Space Weather Prediction Center.
