Question

Suppose a ray is incident normally in the ionosphere region with electron density 36×10^10/cm^3,  then the critical frequency is _____

(a) 5GHz

(b) 54MHz

(c) 5.4MHz

(d) 324GHz

This question was posed to me by my school principal while I was bunking the class.

Origin of the question is Maximum Usable Frequency in section Virtual Height, Critical Frequency and Muf of Antennas

Answer 1

Correct answer is (c) 5.4MHz

To explain I would say: \(f_c=9\sqrt{N \,max}\) where Nmax= electron density and angle of incidence is ἰ = 0 (given)

                    ∴ \(f_c =9\sqrt{36×10^{10}} = 5.4MHz\)