# Antennas Questions and Answers – Half Power Beam Width

This set of Antennas Quiz focuses on “Half Power Beam Width”.

1. Beam area and directivity are related as _____
a) $$D=\frac{2\pi}{B}$$
b) $$D=\frac{4\pi}{B}$$
c) $$D=\frac{B}{2\pi}$$
d) $$D=\frac{B}{4\pi}$$

Explanation: Beam width/ beam area is the measure of directivity of an antenna.
$$D=\frac{4\pi}{B}$$

2. What is the Beam area in radians when the HPBW in perpendicular planes are given by 60°, 30°?
a) $$\frac{\pi^2}{8}$$
b) $$\frac{\pi}{2}$$
c) $$\frac{\pi^2}{18}$$
d) 2

Explanation: Beam area B=θE θH=60°×30°=$$\frac{\pi}{3}×\frac{\pi}{6}=\frac{\pi^2}{18}$$

3. Beam width is the measure of directivity of antenna in terms of angles either radians or degrees.
a) True
b) False

Explanation: Beam width is defined as the measure of angular width at two points on the major lobe where the power decreases to half of the maximum radiated power. So beam width is the measure of directivity of antenna.
$$D=\frac{4\pi}{B}$$

4. Find the directivity when the half power beam widths are 45° and 60° in perpendicular planes.
a) 12.58
b) 17.98
c) 22.91
d) 15.28

Explanation: Directivity in terms of HPBW (in degrees) is given by $$D=\frac{41257}{θ_E θ_H} =\frac{41257}{45° × 60°}=15.28$$

5. What happens to the beam-width of antenna if the frequency of operation is increased?
a) Beam-width will decrease
b) Beam-width will increase
c) Beam-width is independent of frequency
d) Beam width may increase or decrease

Explanation: If frequency is high, power radiated is low (From Radar range equation). That means directivity is high as $$D=\frac{4\pi U}{P_{rad}}.$$ Beam width and directivity are inversely proportional to each other. So beam width will be low.

6. Half-power beam width is also called as 6-db beam width.
a) True
b) False

Explanation: Half-power beam width is also called as 3-db beam width. It the angular width calculated between the points when the power drops to half of the maximum power radiated.

Sanfoundry Global Education & Learning Series – Antennas.