Antennas Multiple Choice Questions on “Directivity”.
1. The ratio of maximum power density in the desired direction to the average power radiated from the antenna is called as _______
a) directivity
b) directive gain
c) power gain
d) partial directivity
Answer: a
Clarification: The ratio of maximum power density in the desired direction to the average power radiated from the antenna is called Directivity. The ratio of power radiated in a particular direction to the actual power input to antenna is called Power gain. Directive gain is the ratio of power radiated in desired direction to the average power radiated from the antenna. Maximum Directive gain is called as Directivity. Partial directivity is the part of radiation intensity in a particular polarization to radiation intensity in all directions.
2. What is the Beam area for Directivity to be 1 in Steradian?
a) 4π
b) 1/2π
c) 2π
d) 1/4π
Answer: a
Clarification: The Directivity in terms of the Beam area ΩA is given by D=(frac{4pi}{Omega_A})
⇨ (frac{4pi}{D})=4π steradians.
3. If directivity of antenna increases, then the coverage area _____
a) decreases
b) increases
c) increases and then decreases
d) remains unchanged
Answer: a
Clarification: As the directivity increases, the beam area decreases. So, the coverage area of beam decreases and takes more time to scan a target for target detection.
4. If half power beam width in one plane and other plane orthogonal to it are equal to π then the directivity is ____
a) π
b) 4π
c) 4/π
d) 2π
Answer: c
Clarification: Beam area ΩA≈ θ1r θ2r where θ1r, θ2r are half-power beam widths in radians.
Directivity (D=frac{4pi}{Omega_A}=frac{4pi}{theta_{1r}theta_{2r}}=frac{4pi}{pi.pi} = 4/pi)
5. Directive gain with maximum radiation intensity is called as Directivity.
a) True
b) False
Answer: a
Clarification: Directivity is defined as ratio of maximum power density in desired direction to the average power radiated in all directions. This is simply, maximum Directive gain. Maximum Directive gain is obtained if maximum radiation intensity is present in desired direction.
6. How the directivity and effective aperture related to each other?
a) Inversely proportional
b) Directly proportional
c) Independent
d) Proportionality depends on input power
Answer: b
Clarification: The directivity D is directly proportional to the effective aperture of antenna. (A_e=Dfrac{lambda^2}{4pi})
7. What is the directivity of half-wave dipole?
a) 1.64
b) 1.5
c) 1.43
d) 1.44
Answer: a
Clarification: Directivity (D=frac{U_{max}}{U_{av}} = frac{4pi U_{max}}{P_{rad}} )
Prad=36.54I02 Since the radiation resistance for Half-wave dipole is 36.54Ω.
Maximum radiation intensity is given by (U_{max}=P_{max}.r^2=frac{mid E_thetamid_{max}^2}{2eta}.r^2)
Where (E_theta=frac{jeta I_0 e^{-jkr}cos(frac{pi costheta}{2})}{2pi rsintheta})
⇨ (D=frac{4pi}{1}frac{eta I_0^2}{8pi^2} frac{1}{36.54I_0^2}=1.64.)
8. What is the directivity of antenna having effective aperture 1 m2?
a) (frac{4pi}{lambda^2})
b) (frac{lambda^2}{4pi})
c) 1
d) 4π
Answer: a
Clarification: Directivity D=(frac{4pi}{lambda^2}A_e)
Given effective aperture Ae=1m2
⇨ D=(frac{4pi}{lambda^2}A_e = frac{4pi}{lambda^2} × 1 =frac{4pi}{lambda^2})