Principles of Communication: Propagation MCQ

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PropagationHardQuestion 20321

Question

A shortwave (HF) signal at 15 MHz is sent from a transmitter. The critical frequency is 8 MHz. Since 15 > 8 MHz, the wave:

A
Is reflected at normal incidence
B
Must be launched at an angle for ionospheric refraction to return it to Earth
Correct
C
Cannot propagate at all
D
Travels as ground wave

Correct answer

Must be launched at an angle for ionospheric refraction to return it to Earth

Explanation

At normal incidence, f = 15 MHz > f_c = 8 MHz → wave passes through to space. But at an oblique angle θ, effective vertical frequency = 15 cos θ. For reflection: 15 cos θ ≤ 8 → cos θ ≤ 0.533 → θ ≥ 57.8°. So the signal must be directed at least 57.8° from vertical (i.e., within 32.2° of horizontal) for ionospheric return.

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