Antenna Parameters Questions and Answers – Basics

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

1. The relation between vector magnetic potential and current density is given by ______
a) ∇.A=J
b) ∇×A=H
c) ∇2 A=-μJ
d) ∇2 A=∇×H
View Answer

Answer: c
Explanation: Magnetic Flux density B is expressed as B=∇×A
Taking curl on both sides, we get ∇×B=∇×(∇×A ) = ( ∇.A)∇-(∇.∇)A
From Maxwell’s equation, ∇.B=0 =>∇.A=0
⇨ ∇×B= -∇2 A and B= μH , ∇×H=J
⇨ ∇×μH= -∇2A
⇨ ∇2 A=-μJ.
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2. The induction and radiation fields are equal at a distance of _______
a) λ/4
b) λ/6
c) λ/8
d) λ/2
View Answer

Answer: b
Explanation: For an Hertzian dipole, equating the magnitudes of maximum induction and radiation fields we get,
\(\mid \overline{E_θ}\mid_{max⁡Radiation} =\mid \overline{E_θ}\mid_{max⁡Induction} \)
\(\frac{I_m dl}{4\pi\epsilon} (\frac{ω}{v^2r})=\frac{I_mdl}{4\pi\epsilon} (\frac{1}{r^2v})\)
\(r=\frac{v}{ω}=\frac{\lambda f}{2\pi f}=\frac{\lambda}{6}.\)

3. The ratio of radiation intensity in a given direction from antenna to the radiation intensity over all directions is called as ________
a) Directivity
b) Radiation power density
c) Gain of antenna
d) Array Factor
View Answer

Answer: a
Explanation: Directivity of antenna is defined as the ratio of radiation intensity in a given direction from antenna to the radiation intensity over all directions. \(D = \frac{U_{max}}{U_0}.\)
Radiation Intensity is power radiated from an antenna for unit solid angle. \(U_0= w_r.r^2\frac{watts}{steradians}.\)
Gain of antenna is ratio of radiation intensity in given direction to the radiation intensity of isotropic radiation. Array factor is a function of geometry of array and the excitation phase.
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4. What is the overall efficiency of a lossless antenna with reflection coefficient 0.15?
a) 0.997
b) 0.779
c) 0.669
d) 0.977
View Answer

Answer: d
Explanation: For a lossless antenna, the radiation efficiency ecd=1.
Overall efficiency of antenna is given by eo =ecd (1-\(\mid\gamma^2\mid)\)=1×(1-(0.152))=0.977.

5. The equivalent area when multiplied by the instant power density which leads to free radiation of power at antenna is called as _______
a) Loss area
b) Scattering area
c) Captured area
d) Effective area
View Answer

Answer: b
Explanation: Scattering area is the equivalent area when multiplied by the instant power density which leads to free radiation of power. Loss area leads to power dissipation and captured area leads to total power collection by the antenna. The relation among them is given by,
Captured area= effective area + loss area + scattering area.
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6. Equivalent circuit representation of an antenna is ______
a) Series R, L, C
b) Parallel R, L, C
c) Series R, L parallel to C
d) Parallel R, C series to L
View Answer

Answer: a
Explanation: Antenna is represented by a series R, L, C equivalent circuit. Antenna is used for impedance matching and acts like a transducer.

7. Radiation resistance of a Hertzian dipole of length λ/8 is ________
a) 12.33Ω
b) 8.54Ω
c) 10.56Ω
d) 13.22Ω
View Answer

Answer: a
Explanation: Radiation resistance of a Hertzian dipole of length l is
R=80π2\((\frac{l}{\lambda})^2=80\pi^2 (\frac{\lambda/8}{\lambda})^2=12.33\Omega\)
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8. Relation between directivity and effective area of transmitting and receiving antenna is ________
a) Dt At=Dr Ar
b) Dt Ar=Dr At
c) At Dt=∈Dr Ar
d) Dt At=∈Dr Ar
View Answer

Answer: b
Explanation: The power collected by the receiving antenna is
\(P_r = \frac{P_tD_tA_r}{4\pi R^2} => D_tA_r = \frac{P_r}{P_t}4\pi R^2\) = Dr At
∴ Dt Ar=Dr At

9. The axis of back lobe makes an angle of 180° with respect to the beam of an antenna.
a) True
b) False
View Answer

Answer: a
Explanation: The axis of back lobe is opposite to the main lobe. So it makes 180° with beam of antenna. It is also a side lobe which is at 180 to main lobe.
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10.Radiation resistance of a half-wave dipole is ______
a) 36.56Ω
b) 18.28Ω
c) 73.12Ω
d) 40.24Ω
View Answer

Answer: c
Explanation: Since radiation resistance of quarter-wave monopole (l=λ/4) is 36.56Ω, then for a half-wave dipole (l=λ/2) it is given by 36.56×2 = 73.12Ω. Hertzian dipole is an ideal dipole of infinitesimal dipole.

11. The radiation efficiency for antenna having radiation resistance 36.15Ω and loss resistance 0.85Ω is given by ________
a) 0.977
b) 0.799
c) 0.997
d) 0.779
View Answer

Answer: a
Explanation: The radiation efficiency \(e_{cd}=\frac{R_r}{R_l+R_r}=\frac{36.15}{36.15+0.85}=0.977.\)
Radiation efficiency is also known as conductor-dielectric efficiency. It is the ratio of power delivered to the radiation resistance to the power delivered to it when conductor-dielectric losses are present.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn | Youtube | Instagram | Facebook | Twitter