# Antenna Parameters Questions and Answers – Antenna Bandwidth

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

1. Relation between Quality factor, Bandwidth, and resonant frequency is _________
a) $$Q=\frac{BW}{f_0}$$
b) $$Q=\frac{f_0}{BW}$$
c) Q = BW×f0
d) $$Q=\frac{BW+f_0}{BW-f_0}$$

Explanation: Quality factor is defined as the ratio of resonant or center frequency to the bandwidth.$$Q=\frac{f_0}{BW}$$

2. What is the Bandwidth of the antenna operating at resonant frequency 200MHz with Quality factor 20?
a) 10Hz
b) 5MHz
c) 10MHz
d) 0.1MHz

Explanation: Bandwidth $$Q=\frac{f_0}{Q}=\frac{200MHz}{20}=10MHz$$

3. What is the length of the half-wave dipole with bandwidth 20MHz and Quality factor 30?
a) 5m
b) 0.25m
c) 0.50m
d) 2.5m

Explanation: Operating frequency f0=BW×Q=20MHz×30=600MHz
Now, $$λ=\frac{c}{f}=\frac{3×10^8 m/s}{600MHz}=0.5m$$
∴ Length of half-wave dipole is $$l=\frac{λ}{2}=\frac{0.5}{2}=0.25m$$
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4. Quality factor is defined as ________
a) $$2π×\frac{energy \,radiated\, per\, cycle}{Total\, energy\, stored\, by\, antenna}$$
b) $$4π×\frac{Total \,energy \,stored \,by\, antenna}{energy\, radiated\, per \,cycle}$$
c) $$4π×\frac{energy \,radiated\, per \,cycle}{Total \,energy\, stored \,by \,antenna}$$
d) $$2π×\frac{Total \,energy\, stored\, by \,antenna}{energy\, radiated \,per \,cycle}$$

Explanation: Quality factor Q=$$2π×\frac{Total \,energy\, stored\, by \,antenna}{energy\, radiated \,per \,cycle}$$. Higher Q antennas will have low bandwidth.

5. What is the quality factor of the antenna operating at 650MHz and having a bandwidth of 10MHZ?
a) 65
b) 0.65
c) 15
d) 55

Explanation: Quality factor $$Q=\frac{f_0}{BW}=\frac{650MHz}{10MHz}=65$$

6. In an antenna, the lower frequency limit is determined by pattern, gain or impedance.
a) True
b) False

Explanation: In an antenna, the lower frequency limit is determined by pattern, gain or impedance as the requirements changes. Higher frequency limit is determined by other parameters like the SWR, FBR etc.

7. In the impedance v/s frequency graph of antenna, the antenna impedance at frequencies less than resonant frequency is ____
a) inductive
b) capacitive
c) resistive
d) both inductive and capacitive

Explanation: Antenna is a series R,L,C equivalent circuit.For frequencies less than resonant frequency, the impedance is capacitive.

8. High the Fractional Bandwidth ___________ is the quality factor.
a) low
b) high
c) constant
d) infinity

Explanation: Fractional Bandwidth is the ratio of the frequency range of antenna to the center frequency. If Fractional bandwidth is high, then Bandwidth of the antenna is also more. So the quality factor decreases as it is inversely proportional to the bandwidth.

9. For lower Quality factor antennas, the bandwidth is very high.
a) True
b) False

Explanation: The bandwidth of antenna is inversely proportional to the Quality factor of the antenna.
$$Q=\frac{f_0}{BW}$$

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