SFI stands for Solar Flux Index, and a value of more than 190 means that the Sun is highly active, resulting in excellent conditions for High-Frequency (HF) radio propagation (long-distance communication).
| SFI Range | Solar Activity | HF Propagation on High Bands |
| 65–70 | Solar Minimum (Low) | Very Poor |
| 100–150 | Average to Moderate | Fair to Good |
| > 190 | High/Solar Maximum | Excellent (Peak Conditions) |
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