The following lines were added (+) and removed (-):
=== angle of incidence ====== D layer absorption ===In the low altitude D layer, the density of atmospheric particles is much greater than the density in the higher E and F layers.The D layer absorbs HF frequencies below the 20-meter band. When the D layer dissipates after dark the lower frequencies are free to propagate by skywave or skip from the E and combined F layers. Because of this, ham radio operators use the higher HF bands of 20-meters and above during the daytime, and use the lower bands at night.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.=== angle of incidence ===