Question
What does the term “critical angle” mean, as applied to radio wave propagation?
Answer Options
- A) The long path azimuth of a distant station
- B) The short path azimuth of a distant station
- C) The lowest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions
- D) The highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions
Correct Answer: D
Explanation
The term critical angle, in the context of ionospheric propagation, is defined relative to the horizon (takeoff angle). It represents the highest takeoff angle (measured from the horizontal) that a radio wave can use and still be refracted back to Earth for a given frequency and ionospheric condition.
If a signal’s takeoff angle is higher than the critical angle, its frequency is effectively too high, and the signal will penetrate the ionosphere and be lost in space. An operator aiming for long-distance communication seeks to use the lowest possible angle (to maximize hop distance), but the critical angle dictates the absolute upper limit for a successful skip path.
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