Category: Antennas Objective Questions

Antenna Array Questions and Answers for Aptitude test on “Array of N-Isotropic Sources”.

1. The direction of nulls for broadside array of N –Isotropic sources is given by _____
a) (cos^{-1}⁡([±frac{nλ}{Nd}]))
b) (cos^{-1}⁡([±frac{2nλ}{Nd}]))
c) (cos^{-1}⁡([±frac{2πnλ}{2Nd}]))
d) (cos^{-1}⁡([±frac{nλ}{Nd}]))
Answer: a
Clarification: The nulls of the N- element array is given by (θ_n=cos^{-1}⁡(frac{λ}{2πd} [-β±frac{2πn}{N}]))
Given it’s a broadside array so β=0
(θ_n=cos^{-1}⁡(frac{λ}{2πd} [±frac{2πn}{N}])= cos^{-1}⁡([±frac{nλ}{Nd}]).)

2. The direction of first null of the broadside array of N-Isotropic sources is _____
a) (cos^{-1}([±frac{λ}{Nd}]))
b) (cos^{-1}⁡([±frac{πλ}{Nd}]))
c) (cos^{-1}⁡([±frac{2πλ}{Nd}]))
d) (cos^{-1}⁡([±frac{λ}{2Nd}]))
Answer: a
Clarification: The nulls of the N- element array is given by (θ_n=cos^{-1}⁡(frac{λ}{2πd} [-β±frac{2πn}{N}]))
Given it’s a broadside array so β=0 and n=1 for first null
(θ_n=cos^{-1}⁡(frac{λ}{2πd} [±frac{2πn}{N}])= cos^{-1}⁡([±frac{nλ}{Nd}])=cos^{-1}⁡([±frac{λ}{Nd}]))

3. The direction of nulls for end-fire array of N –Isotropic sources separated by λ/4 is given by ____
a) (θ_n=cos^{-1}⁡([∓1±frac{4n}{N}]))
b) (θ_n=sin^{-1}⁡([∓1±frac{4n}{N}]))
c) (θ_n=cos^{-1}⁡([∓1±frac{2n}{N}]))
d) (θ_n=cos^{-1}⁡([∓1±frac{n}{N}]))
Answer: a
Clarification: The nulls of the N- element array is given by (θ_n=cos^{-1}⁡(frac{λ}{2πd} [-β±frac{2πn}{N}]))
Since its given broad side array (β=±kd=±frac{2πd}{λ}=±frac{π}{2},)
(θ_n=cos^{-1}⁡(frac{2}{π} [∓frac{π}{2}±frac{2πn}{N}]))
(θ_n=cos^{-1}⁡([∓1±frac{4n}{N}]))

4. The necessary condition for the direction of maximum side lobe level of the N-element isotropic array is _______
a) (ᴪ=±frac{2s+1}{N} π )
b) (ᴪ=±frac{2s+2}{N} π)
c) (ᴪ=±frac{2s}{N} π)
d) (ᴪ=±frac{2(s+1)}{N} π)
Answer: a
Clarification: The secondary maxima occur when the numerator of the array factor equals to 1.
⇨ (sin(frac{Nᴪ}{2})=±1)
⇨ (frac{Nᴪ}{2}=±frac{2s+1}{2} π )
⇨ (ᴪ=±frac{2s+1}{N} π .)

5. The necessary condition for the direction of maximum first side lobe level of the 8-element isotropic array is _______
a) (frac{3}{8} π)
b) (frac{3}{4} π)
c) (frac{1}{8} π)
d) (frac{5}{8} π)
Answer: a
Clarification: The secondary maxima occur when the numerator of the array factor equals to 1.
⇨ (sin(frac{Nᴪ}{2})=±1)
⇨ (frac{Nᴪ}{2}=±frac{2s+1}{2} π )
⇨ (ᴪ=±frac{2s+1}{N}π=frac{2+1}{8} π=frac{3}{8} π.)

6. The necessary condition for the direction of maximum second side lobe level of the 4-element isotropic array is _______
a) (frac{5}{4} π)
b) (frac{3}{4} π)
c) (frac{1}{8} π)
d) (frac{5}{8} π)
Answer: a
Clarification: The secondary maxima occur when the numerator of the array factor equals to 1.
⇨ (sin(frac{Nᴪ}{2})=±1)
⇨ (frac{Nᴪ}{2}=±frac{2s+1}{2} π )
⇨ (ᴪ=±frac{2s+1}{N}π=frac{2(2)+1}{4} π=frac{5}{4} π.)

7. The Half-power beam width of the N-element isotropic source array can be known when _____
a) (ᴪ=frac{2.782}{N})
b) (ᴪ=frac{1.391}{N})
c) (ᴪ=frac{1.414}{N})
d) (ᴪ=frac{3}{N})
Answer: a
Clarification: Normalized array factor is given by (AF=frac{sin(Nᴪ/2)}{N frac{ᴪ}{2}}=frac{1}{√2} )
⇨ (frac{Nᴪ}{2}=1.391)
⇨ (ᴪ=frac{2.782}{N} )

8. Which of the following is the necessary condition to avoid grating lobes in N-Isotropic element array?
a) (frac{d}{λ}≤frac{1}{1+|cosθ_m |} )
b) (frac{d}{λ}≥frac{1}{1+|cosθ_m |} )
c) (frac{λ}{d}≤frac{1}{1+|cosθ_m |} )
d) (frac{λ}{d}=frac{1}{1+|cosθ_m |} )
Answer: a
Clarification: Grating lobes are the minor and unnecessary lobes other than the major lobe.
To avoid grating lobes, kd(cosθ-cosθ_m) ≤ 2π
θ_m – Direction of maximum radiation
⇨ (frac{2πd}{λ} (cosθ-cosθ_m)≤2π)
(frac{d}{λ}≤frac{1}{cosθ-cosθ_m} )
(frac{d}{λ}≤frac{1}{1+|cosθ_m |} )

9. Which of the following is the necessary condition to avoid grating lobes in N-Isotropic element broadside array?
a) d < λ
b) d > λ
c) d=λ
d) d < 2λ
Answer: a
Clarification: Grating lobes are the minor and unnecessary lobes other than the major lobe.
To avoid grating lobes, kd(cosθ-cosθ_m)≤2π
(frac{d}{λ}≤frac{1}{1+|cosθ_m |} )
For broadside to avoid grating lobes (θ_m=90)
⇨ (frac{d}{λ}) < 1
⇨ d < λ

10. Which of the following is the necessary condition to avoid grating lobes in N-Isotropic element end-fire array?
a) d < λ/2
b) d < λ
c) d > λ/2
d) d=λ
Answer: a
Clarification: Grating lobes are the minor and unnecessary lobes other than the major lobe.
To avoid grating lobes, kd(cosθ-cosθ_m)≤2π
(frac{d}{λ}≤frac{1}{1+|cosθ_m|} )
For broadside to avoid grating lobes (θ_m=0)
⇨ (frac{d}{λ}) < 1/2
⇨ d < λ/2

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all areas of Antenna Array for Aptitude test, here is complete set of 1000+ Multiple Choice Questions and Answers .

Reflector Antenna online test on “Parabolic Reflector Antenna”.

1. Which of the following wave conversion mechanism is performed in a parabolic reflector antenna?
a) Plane to spherical
b) Spherical to plane
c) Performs both plane to spherical and spherical to plane
d) Elliptic polarization
Answer: b
Clarification: In a parabolic reflector antenna, the wave conversion mechanism used is spherical to plane. It works on the principle of geometric optics. The reflected plane waves travel parallel o the major axis of the reflector.

2. The power gain of the parabolic reflector with circular aperture of diameter 10λ?
a) 100π
b) 10π
c) 600π
d) 360π
Answer: c
Clarification: The power gain of the parabolic reflector with circular aperture of diameter d is given by
(G_p = 6frac{d^2}{λ^2} = 600π.)

3. Find the aperture ratio of the paraboloid with aperture diameter 1m at 1.5GHz frequency?
a) 5
b) 0.2
c) 1.5
d) 7.5
Answer: a
Clarification: Aperture ratio (= frac{d}{λ})
(λ=frac{c}{f}=frac{3×10^8}{1.5×10^9}=0.2m)
Aperture ratio( = frac{1}{0.2}=5.)

4. BWFN of a paraboloid antenna with circular aperture assuming feed is isotropic is ____
a) (frac{140λ}{d} )
b) (frac{115λ}{d} )
c) (frac{58λ}{d} )
d) (frac{40λ}{d} )
Answer: a
Clarification: A paraboloid antenna with a circular aperture has
BWFN (Beam width between two first nulls) = (frac{140λ}{d}, ) (d is diameter of circular aperture)
For a rectangular aperture (BWFN =frac{115λ}{L}, ) (L is length of rectangular aperture).

5. Find the BWFN of a paraboloid with a circular aperture of diameter 10λ?
a) 14 degrees
b) 28 degrees
c) 11.5 degrees
d) 41 degrees
Answer: a
Clarification: A paraboloid antenna with a circular aperture has
BWFN (Beam width between two first nulls) = (frac{140λ}{d}, ) (d is diameter of circular aperture)
⇨ (BWFN =frac{140λ}{10λ}=14 degrees.)

6. What is the ratio of focal length to diameter for practical applications in a parabolic reflector?
a) 0.25 to 0.5
b) < 0.25
c) 0.125 to 0.3
d) 0.5 to 1
Answer: a
Clarification: The ratio of focal length (f) to diameter (d) f/d < (frac{1}{4}) indicates radiation away from the parabolic surface of the reflector. So for practical applications it lies between 0.125 and 0.5.

7. Which of the following is false regarding a paraboloid antenna?
a) Spill over decreases due to back lobe of primary radiator
b) Feed placed at the focus is used to improve the beam pattern
c) Pill box provides wide beam in one plane and narrow beam in other plane
d) At lower frequencies parabolic antennas are not used frequently
Answer: a
Clarification: Spill over occurs due to the non-captured radiation by the reflector. From the primary radiators also some of forward radiation gets added up with the desired parallel beams. This is called back lobe radiation. It increases due to the back lobe of the primary radiator.

8. Which of the following paraboloid reflector is formed by cutting some part of paraboloid to meet requirements?
a) Truncated paraboloid
b) Cassegrain
c) Corner
d) Pill box
Answer: a
Clarification: When a portion of the paraboloid reflector is cut off or truncated it is called as truncated paraboloid. Pill box provides wide beam in one plane and narrow beam in other plane. Cassegrain is a dual reflector antenna and Corner reflector is also a type of reflector antenna.

9. Which of the following is used to produce wide beam in one plane and narrow beam in other plane?
a) Pill box
b) Truncated paraboloid
c) Cassegrain
d) Paraboloid with rectangular aperture
Answer: a
Clarification: Pill box is short parabolic right cylinder enclosed by parallel plates. It produces a wide beam in one plane and narrow beam in other plane. Cassegrain is a dual reflector. Truncated is a type of paraboloid.

10. In a paraboloid antenna, all rays leaving the focal point are collimated along the reflector’s axis after reflection.
a) True
b) False
Answer: a
Clarification: According to the geometry of the paraboloid reflector,
⇨ All rays leaving the focal point are collimated along the reflector’s axis after reflection.
⇨ All overall ray path lengths (from the focal point to the reflector and on to the aperture plane) are the same and equal to 2F.

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