Question

Find the critical frequency when the refractive index of the layer is 0.54 and MUF is 9MHz?

(a) 0.84MHz

(b) 7.57MHz

(c) 0.75MHz

(d) 8.4MHz

I had been asked this question during an online exam.

My question comes from Critical Frequency in division Virtual Height, Critical Frequency and Muf of Antennas

Answer 1

Correct answer is (b) 7.57MHz

Explanation: Refractive index \(n=\sqrt{(1-\frac{81N_{max}}{f^2})}\)

⇨ \(N_{max}=\frac{(1-n^2)f^2}{81}=\frac{(1-0.54^2)(9×10^6)^2}{81}=0.708 ×10^{12}/cm^3\)

⇨ critical frequency \(f_c=9\sqrt{N \,max} =9\sqrt{0.708 ×10^{12}}=7.57Mhz\)