IonoFlux field guide
Solar flux (SFI / F10.7)
What the solar flux index means for amateur radio and HF propagation.
In simple terms
Think of SFI as the Sun's background 'volume'. It is measured in solar flux units (sfu) and is mainly useful as a broad signal of how well the ionosphere may support longer-distance HF contacts.
How to read it
Higher SFI can raise the usable ceiling of the ionosphere, especially on 14 MHz and above. It is not a guarantee: local time, geomagnetic activity and the path itself still matter. Compare SFI with the live map and a path prediction rather than using it alone.
A practical example
If SFI is 160, 20 m and sometimes 17 m may be lively. If it falls near 70, 40 m and lower bands may be the more dependable starting point.