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Question 20254
JEE Physics · Medium

Principles of Communication: Propagation MCQ

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PropagationMediumQuestion 20254

Question

For sky-wave propagation, the critical frequency f_c is the maximum frequency that can be reflected by the ionosphere at normal incidence. It is related to electron density N_max by:
  1. A
    f_c = 9√N_max
    Correct
  2. B
    f_c = 9N_max
  3. C
    f_c = N_max / 9
  4. D
    f_c = 81N_max

Correct answer

f_c = 9√N_max

Explanation

fc = 9√Nmax Hz where Nmax is peak electron density in electrons/m³. If Nmax = 10¹² /m³, fc = 9×10⁶ = 9 MHz. Above fc at normal incidence, the wave passes through into space.

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