Which of the following are the least reliable bands for long-distance communications during periods of low solar activity?
80 meters and 160 meters
60 meters and 40 meters
30 meters and 20 meters
15 meters, 12 meters, and 10 meters✓Bonne réponse
Explication
The 15, 12 and 10 meter bands sit at 21 to 29 MHz and need a high F2 critical frequency to be refracted back to Earth. At solar minimum the reduced EUV flux leaves the F2 region too thin, the MUF frequently falls below 21 MHz, and signals on those bands pass through into space instead of returning.