# Antennas Questions and Answers – First Null Beam Width

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This set of Antennas Multiple Choice Questions & Answers (MCQs) focuses on “First Null Beam Width”.

1. Angular width between the first nulls or first side lobes is called as _______
a) half power beam width
b) full null beam width
c) beam area
d) directivity

Explanation: Angular width between the first nulls or first side lobes is called full null beam width. Half power beam width is the angular width measured between the 3dB power points of the major lobe. Beam area is the product of HPBW in perpendicular directions. Directivity is the maximum directive gain.

2. If the HPBW is 30° then FNBW is approximately _____
a) 60°
b) 30°
c) 15°
d) 20°

Explanation: FNBW ≈ 2HPBW=2×30°=60°

3. Total beam area is sum of major and minor lobe areas.
a) True
b) False

Explanation: Total beam area is sum of major lobe area ΩM and minor lobe area Ωm.
ΩA = ΩMm

4. If beam efficiency is 0.87 then the stray factor is ____
a) 1.87
b) 0.13
c) 1.30
d) 0.87

Explanation: Stray factor ∈s is the ratio of minor beam area to total beam area.
Given beam efficiency ∈B=0.87
B+∈s = 1
s = 1-0.87 = 0.13

5. If the antennas revolution time is 10 sec and 3 dB beam width duration is 150ms then the antenna beam width is _______
a) 5.4°
b) 54°
c) 15°
d) 1.5°

Explanation: Antenna beam width = $$\frac{Beam \,duration}{rotating \,period}×360° = \frac{150m}{10} ×360°=5.4°$$

6. Relation between beam efficiency and stray factor is given by ____
a) ∈B+∈s=1
b) ∈B-∈s=1
c) $$\frac{\in_B}{\in_s}=1$$
d) ∈B+∈s=2

Explanation: Stray factor ∈s is the ratio of minor beam area to total beam area. Beam efficiency ∈B is the ratio of major beam area to total beam area. Total beam area is sum of major lobe area ΩM and minor lobe area Ωm.
$$\in_B=\frac{\Omega_M}{\Omega_A}, \in_s = \frac{\Omega_m}{\Omega_A}$$
B+∈s=1

Sanfoundry Global Education & Learning Series – Antennas. 