SFI (Solar Flux Index) – Indicates solar radio emissions at 2800 MHz. Higher values mean stronger ionization and better high-frequency (HF) propagation. Values above 150 are excellent for 20m–10m DX.
A Index – Reflects overall geomagnetic stability. Low values (below 10) mean calm conditions; high values (above 20) indicate disturbed propagation.
K Index – Measures short-term geomagnetic disturbance (3-hour intervals). Low values (0–2) mean stable bands; high values (5+) often result in noisy or closed HF conditions.
Rule of Thumb: High SFI and Low A/K values = great HF propagation. Low SFI or High A/K values = poor HF propagation.
Here’s a really good YouTube explanation of how these solar numbers interact and affect HF radio conditions.
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