Solar Flux Index (SFI / F10.7) Explained for Ham Operators

Solar Flux Index (SFI / F10.7) Explained for Ham OperatorsThe Penticton 10.7 cm solar radio flux drives F2 propagation on the high HF bands. What numeric ranges mean, how SFI relates to Sunspot Number, and the 11-year solar cycle.The Solar Flux Index measures the sun's radio emission at 10.7 cm wavelength. It's the single best proxy for how ionized the upper atmosphere is, which determines whether the high HF bands are open. Reported three times daily from Penticton.Numeric ranges

SFI < 70: Solar minimum, high bands closed

SFI 70-100: Low, 20m / 40m workable

SFI 100-150: Moderate, 20m / 17m good

SFI 150-200: Good, high bands open most of the day

SFI > 200: Excellent, 10m / 6m F2 openings

Solar cycleThe sun's activity rises and falls on an ~11-year cycle. Solar maximum brings SFI 200+ for months. Solar minimum drops to ~65-70.Sunspot NumberDerived from SFI via Stewart formula: SSN ~ 1.61 x F10.7 - 73.6.Part of the BloomWX learn library — beginner-friendly explainers covering every surface of the BloomWX weather dashboard. Open BloomWX to see live data for any U.S. county.