# Antennas Questions and Answers – Critical Frequency

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This set of Antennas Multiple Choice Questions & Answers (MCQs) focuses on “Critical Frequency”.

1. When a wave is incident normally then the acceptable highest frequency at which signal can be returned is the ______
a) critical frequency
b) LUF
c) optimum frequency
d) dominating frequency

Explanation: When a wave is incident normally then the acceptable highest frequency at which signal can be returned is the critical frequency. Beyond critical frequency, wave is penetrated into another region. When the frequency is greater than critical frequency, still some part is returned back by varying the angle of incidence and this frequency is called MUF.

2. What is the critical frequency when the electron density in the F1 layer is 20×1010/cm3?
a) 40.2GHz
b) 4.02 MHz
c) 40.2 MHz
d) 40.2Hz

Explanation: Critical frequency $$f_c=9\sqrt{N\, max}$$
$$f_c =9\sqrt{20×10^{10}}=4.02MHz$$

3. What is the electron density of the layer if critical frequency is 3 MHz?
a) 11×1012/cm3
b) 0.11×1012/cm3
c) 1.1×1012/cm3
d) 0.11×1010/cm3

Explanation: $$f_c=9\sqrt{N\, max}$$
$$N_{max}=\frac{f_c^2}{81}=\frac{(3×10^6)^2}{81}=0.11×10^{12}/cm^3$$

4. For a regular layer, the critical frequency is proportional to the ______ of electron density.
a) Square
b) Inverse
c) Square root
d) Inverse square root

Explanation: For a regular layer, the critical frequency is proportional to the square root of Nmax electron density. $$f_c=9\sqrt{N \,max}$$

5. 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

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 $$f_c=9\sqrt{N \,max} =9\sqrt{0.708 ×10^{12}}=7.57Mhz$$

6. The value of refractive index when the MUF is equal to the critical frequency is ______
a) 1
b) 0
c) 0.5
d) 0.29

Explanation: Refractive index $$n=\sqrt{(1-\frac{81N_{max}}{f^2})}$$
Critical frequency $$f_c=9\sqrt{N \,max}$$
When $$f_c=f_{MUF}, n=\sqrt{(1-\frac{81N_{max}}{f_{MUF}^2})}=1-1=0$$

7. For fMUF ≥ fc, the wave will reflects back irrespective of the angle of incidence.
a) True
b) False

Explanation: For fMUF ≤ fc, the wave will reflects back irrespective of the angle of incidence. For fMUF ≥ fc, the wave will reflects back depending of the angle of incidence (which should be small).

8. Which of the following frequency is greater than the critical frequency?
a) MUF
b) LUF
c) Optimum frequency
d) VLF

Explanation: According to the secant law, fMUF=fc secθi
So MUF is greater than or equal to the critical frequency depending on the secθi

9. When the frequency decreases below _____ frequency, the signal reception becomes too weak and the Noise increases.
a) MUF
b) LUF
c) Optimum frequency
d) Critical frequency