2-42
The D LAYER is the lowest region of the ionosphere and refracts signals of low frequencies back to
Earth.
The E LAYER is present during the daylight hours; refracts signals as high as 20 megahertz back to
Earth; and is used for communications up to 1500 miles.
The F LAYER is divided into the F1 and F2 layers during the day but combine at night to form one
layer. This layer is responsible for high-frequency, long-range transmission.
The CRITICAL FREQUENCY is the maximum frequency that a radio wave can be transmitted
vertically and still be refracted back to Earth.
The CRITICAL ANGLE is the maximum and/or minimum angle that a radio wave can be
transmitted and still be refracted back to Earth.
