SFI below ~70 (solar minimum conditions): These low values indicate very weak solar activity. The MUF might only be in the low HF range (10 MHz or below) . You can expect poor high-band conditions – 20 m (14 MHz) may be marginal, and 15 m (21 MHz) or 10 m (28 MHz) will likely be closed for long-distance DX. Long-distance HF communication will be mostly confined to lower bands like 40 m and 80 m (which rely less on F2 ionization) . In summary, SFI < 70 is considered “poor” for upper HF and is typical of solar minima .
SFI ~70–90: Still on the low side, but slightly improved. 20 m daytime openings become more reliable as SFI climbs through the 70s and 80s. This range might be described as “poor to fair” HF conditions. You may get some short or sporadic openings on 15 m around peak sun hours, but 10 m will mostly remain quiet. Many HF operators consider SFI in the 80s to be low-average conditions .
SFI ~90–120: This is a moderate level of solar flux, often occurring in the rising or falling phases of the solar cycle. Here we reach “average” HF conditions, turning toward good. Frequencies up to ~21–24 MHz (15 m to 12 m bands) can open consistently in daytime . DXers will notice 20 m is solid all day, 17 m/15 m open most days, and even 10 m (28 MHz) may open on strong days when SFI is above ~100 . An SFI in the 100+ ballpark generally indicates the Sun is active enough to make high-band HF interesting again.
SFI ~120–150: These values signify significant solar activity. HF conditions in this range are “good” up through 10 m on many days . Expect regular openings on 10 m, especially at lower latitudes or during peak sun hours, and even 6 m (50 MHz) sporadic-E or F2 layer openings can occur when SFI nears the 150 mark in combination with seasonal factors . In this range, all the HF bands from 80 m through 10 m can offer long-distance propagation at various times of day or night. Operators with modest antennas will start to enjoy worldwide DX on high bands when SFI consistently sits in the 130–150 range .
SFI > 150: When the solar flux index rises above about 150 and stays there, we reach ideal HF conditions for the higher bands . This is typically seen near the peak of the 11-year solar cycle. An SFI of 150–200 means “excellent” propagation on all bands up through 10 m, with 6 m openings also likely . Worldwide communications on 10 m become routine; even daylight 40 m and 30 m propagation improves thanks to increased ionization at all altitudes. Experienced HF DXers often say that SFI sustained over 150 is the magic ingredient for consistent global DX on 10 m and 12 m. Moreover, values above 200 (when the Sun is very active) indicate maximum peak conditions – the F2 layer is highly charged, and one can expect the absolute best HF propagation, with daily 10 m openings and daytime MUFs potentially exceeding 50 MHz . During such times (usually the crest of the solar cycle), even small portable radios or compromise antennas can make surprising long-range contacts on HF bands . Keep in mind, values in the 200–300 range are usually seen only for relatively short periods during the solar maximum, but they truly light up the ionosphere.