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While the 20-meter band and higher frequencies are not significantly attenuated by D layer energy absorption due to their smaller wavelengths, absorption at lower frequencies is usually too severe for DX. Near Vertical Incidence Skywave (NVIS) technique causes lower HF signals to travel through the D layer in the shortest dimension, nearly at a right angle to the layer's spherical shell overhead. As a result, absorption of low frequency signals is minimized and NVIS regional communications is viable during daylight hours, particularly on the 40-meter band.While the 20-meter band and higher frequencies are not significantly attenuated by D layer energy absorption due to their smaller wavelengths, absorption at lower frequencies is usually too severe for DX. You can expect the 20-meter band to work to some extent both day and night. * Frequencies above 20-meter up to MUF propagate better at day.* Frequencies below 20-meter propagate better at night.Concerning the D layer and NVIS: Near Vertical Incidence Skywave (NVIS) technique causes lower HF signals to travel through the D layer in the shortest dimension, nearly at a right angle to the layer's spherical shell overhead. As a result, absorption of low frequency signals is minimized and NVIS regional communications is viable during daylight hours, particularly on the 40-meter band.Solar flares: increased ultraviolet and X-ray radiation from solar flares will affect radio communication in only 8 minutes of the solar event occurring.