What does the term “critical angle” mean, as applied to radio wave propagation?
The long path azimuth of a distant station
The short path azimuth of a distant station
The lowest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions
The highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions✓Correct answer
Explanation
For a given frequency and ionospheric state there is a steepest launch angle whose wave is still returned to Earth; steeper than that and the wave meets the layer too near vertical to be bent enough and passes through. That upper limit on takeoff angle is the critical angle.