What should be done to continue a long-distance contact when the MUF for that path decreases due to darkness?

Объяснение

As the path goes into darkness the F-region electrons recombine, the electron density falls, and the maximum usable frequency drops below the band in use, so moving to a lower band puts the signal back under the MUF. Switching higher would leave the signal above the MUF and it would pass out through the ionosphere, and no antenna takeoff angle or beamwidth can restore refraction the ionosphere is no longer able to provide.