# Microwave Engineering Questions and Answers – Antenna Basics-2

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This set of Microwave Engineering Questions and Answers for Entrance exams focuses on “Antenna Basics-2”.

1. As the beam area of an antenna decreases, the directivity of the antenna:
a) Increases
b) Decreases
c) Remains unchanged
d) Depends on the type of the antenna

Explanation: Beam area of an antenna and the directivity of the antenna are inversely proportional. As the beam area is reduced, the directivity increases, meaning smaller the radiating area of the transmitting antenna, more directed is the emitted energy.

2. If an antenna radiates over half a sphere, directivity of the antenna is:
a) Two
b) Four
c) Three
d) One

Explanation: Since the antenna radiates over half the sphere, beam area of the antenna is 2π, directivity of the antenna is given by 4π/ beam area. Substituting for beam area, the directivity of the antenna is two.

3. The half power beam width of an antenna in both θ and φ are 400 each. Then the gain of the antenna is:
a) 23
b) 25
c) 14
d) 27

Explanation: Approximate gain of an antenna is given by the expression 40000/ (HPBW) 2. Substituting the given values in the above expression, the gain of the antenna is 25. In dB scale the gain of the antenna is 14 dB.

4. The number N of radio transmitters or point sources of radiation distributed uniformly over the sky which an antenna can resolve is given by:
a) 4π/ ΩA
b) 2π/ ΩA
c) π/ ΩA
d) None of the mentioned

Explanation: Resolution may be defined as equal to half the beam width between first nulls. In the above expression the resolution N is given as 4π/ ΩA.. Here ΩA is the beam area.
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5. Ideally, the number of point sources an antenna can resolve is numerically equal to:
a) Gain of the antenna
b) Directivity
c) Beam efficiency
d) Beam area

Explanation: The number of point source an antenna can resolve is given by 4π/ ΩA Directivity of an antenna is mathematically given by the relation 4π/ ΩA . Numerically resolution and directivity are equal.

6. Effective aperture is a parameter of the antenna that gives the physical aperture of the antenna.
a) True
b) False

Explanation: Effective aperture defines the amount of the total aperture of the antenna that is utilized for radiation of energy. Higher the effective aperture of an antenna, more is the aperture efficiency.

7. Effective aperture in terms of beam area and operating wavelength is given by the relation:
a) λ2/ ΩA
b) ΩA / λ2
c) λ2× ΩA
d) No such relationship exists

Explanation: Effective aperture is given as λ2/ ΩA. Here ΩA is the beam area. If the beam area is specified in terms of the operating wavelength λ, then effective are of the antenna can be made operating wavelength independent.

8.________ of an antenna is defined as the ratio of the induced voltage to the incident electric field.
a) Effective height
b) Gain
c) Directivity
d) Loss

Explanation: At the receiving end, effective height of an antenna is defined as the ratio of the induced voltage to the incident electric field. Otherwise, if the effective length of the receiving antenna is known and if the induced voltage is measured, then the field strength can be determined.

9. The directivity of an antenna in terms of the aperture efficiency and operating wavelength is given by:
a) 4πAe/λ2
b) 2πAe/λ2
c) πAe/λ2
d) None of the mentioned

Explanation: The directivity of an antenna in terms of aperture efficiency is given by 4πAe/λ2. Here Ae is the aperture efficiency. λ is the operating frequency. With an increase in the effective aperture area of an antenna, directivity of the antenna can be increased making the radiated beam narrower.

10. A radio link has 15 W transmitter connected to an antenna of 2.5 m2 effective aperture at 5 GHz. The receiving antenna has an effective aperture of 0.5 m2 and is located at a 15 Km line of sight distance from transmitting antenna. Assuming lossless, matched antennas, the power delivered to the receiver is:
a) 20 µW
b) 15 µm
c) 23 µm
d) 25 µm

Explanation: The power delivered to the receiving antenna is Pt (A1 A2/ r2λ2). Substituting the given values in the above equation, the power at the receiver is 23 µm.

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