Why should an amateur operator normally avoid transmitting on 14.100, 18.110, 21.150, 24.930 and 28.200 MHz?
A system of propagation beacon stations operates on those frequencies✓Bonne réponse
A system of automatic digital stations operates on those frequencies
These frequencies are set aside for emergency operations
These frequencies are set aside for bulletins from the FCC
Explication
Those five frequencies carry the international network of propagation beacons that transmit in a timed sequence from 20 through 10 meters. Transmitting on them degrades a shared propagation resource that many operators rely on, which is what good amateur practice requires you to avoid.