Question
What is meant by the term “critical frequency” at a given incidence angle?
Answer Options
- A) The highest frequency which is refracted back to Earth
- B) The lowest frequency which is refracted back to Earth
- C) The frequency at which the signal-to-noise ratio approaches unity
- D) The frequency at which the signal-to-noise ratio is 6 dB
Correct Answer: A
Explanation
The term critical frequency, in the context of radio propagation, refers to the highest frequency at which a radio wave, transmitted straight up (vertical incidence), will be refracted back to Earth by an ionospheric layer. If a signal’s frequency exceeds this critical frequency, it will pass through the ionosphere into space.
For signals transmitted at shallow angles (oblique incidence), the concept is generalized to the Maximum Usable Frequency (MUF). The MUF is always higher than the critical frequency (by a factor known as the Secant Law) because a shallow ray angle spends more time in the ionosphere and is easier to refract.
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