66. The radiation resistance of an antenna is 72 Ω, and loss resistance is 8 Ω. What is the directivity if the power gain is 16?

1. 1. 17.78

   
   
   

   2. 14.4

   
   
   

   3. 16

   
   
   

   4. None of these

   
   
   

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1. View Hint View Answer Discuss in Forum

   Directivity = G/η , where η = efficiency
   

   =	Rr	=	72	=	72	=	9	= 0.9
   	Rr + Rl		72 + 8		80		10

   
   Now,
   

   D =	16	= 17.78
   	0.9

   Correct Option: A

   Directivity = G/η , where η = efficiency
   

   =	Rr	=	72	=	72	=	9	= 0.9
   	Rr + Rl		72 + 8		80		10

   
   Now,
   

   D =	16	= 17.78
   	0.9

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67. The Gain of an antenna with a circular aperture of diameter 3 m at a frequency 5 GHz is—

1. 1. 22649

   
   
   

   2. 26469

   
   
   

   3. 24649

   
   
   

   4. None of these

   
   
   

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1. View Hint View Answer Discuss in Forum

   Gp =	4π	Aeff
   	λ2

   
   

   =

   4π

   ×

   πD2

   3 × 108

   2

   4

   4 × 109

   
   

   =	4π × 25 × 102	×	π 32	= 24649
   	9		4

   
   

   Correct Option: C

   Gp =	4π	Aeff
   	λ2

   
   

   =

   4π

   ×

   πD2

   3 × 108

   2

   4

   4 × 109

   
   

   =	4π × 25 × 102	×	π 32	= 24649
   	9		4

   
   

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68. A half wave dipole capable of radiating 1 kW and has 2.15 dB over a isotropic antenna. How much power must be delivered to the isotropic omnidirectional antenna, to match the field strength of directional antenna?

1. 1. 1264.7 watt

   
   
   

   2. 1441 watt

   
   
   

   3. 1364.7 watt

   
   
   

   4. 1641 watt

   
   
   

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1. View Hint View Answer Discuss in Forum

   Given P₁ = 1 kW (Power of unidirectional antenna)
   P₂ =? (Power delivered)
   Gain in dB = 2.15
   Now,
   

   2.15 = 10 log10		P2
   		1000

   
   or
   P₂ = 1000 × 10^.215
   or
   P₂ = 1640. 589
   or
   P₂ ≈ 1641 watt
   

   Correct Option: D

   Given P₁ = 1 kW (Power of unidirectional antenna)
   P₂ =? (Power delivered)
   Gain in dB = 2.15
   Now,
   

   2.15 = 10 log10		P2
   		1000

   
   or
   P₂ = 1000 × 10^.215
   or
   P₂ = 1640. 589
   or
   P₂ ≈ 1641 watt
   

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69. An antenna has a power input of 40 π watts and a efficiency of 90%. The radiation intensity has to have a max of 150 W/unit solid angle. Directivity—

1. 1. 13.5

   
   
   

   2. 16.67

   
   
   

   3. 1.333

   
   
   

   4. 1.0748

   
   
   

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1. View Hint View Answer Discuss in Forum

   We know,
   

   D =	G	→ gain of antenna → efficiency of antenna Gain
   	η

   
   

   Gain =	150 × 4π	(Output power)	= 15
   	40π	(input power)

   
   and directivity,
   

   D =	15	=	15	= 16.67
   	η		0.9

   
   

   Correct Option: B

   We know,
   

   D =	G	→ gain of antenna → efficiency of antenna Gain
   	η

   
   

   Gain =	150 × 4π	(Output power)	= 15
   	40π	(input power)

   
   and directivity,
   

   D =	15	=	15	= 16.67
   	η		0.9

   
   

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70. The approximate gain of a parabolic reflector antenna at operating frequency 40 GHz diameter 20 m and illumination efficiency is 55%—

1. 1. 1.139 × 10³

   
   
   

   2. 1.139 × 10⁵

   
   
   

   3. 1.00 × 10³

   
   
   

   4. None of these

   
   
   

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1. View Hint View Answer Discuss in Forum

   Gain GP =	4π				. Ae	4π					. AK
   		λ2						c		2
   								f

   
   

   =

   4π

   ×

   πD2

   . (65)

   3 × 108

   2

   4

   4 × 109

   
   

   =	4π × 42	× π		202		× (.065)
   	0.09			4

   
   = 1.139 × 10⁵

   Correct Option: C

   Gain GP =	4π				. Ae	4π					. AK
   		λ2						c		2
   								f

   
   

   =

   4π

   ×

   πD2

   . (65)

   3 × 108

   2

   4

   4 × 109

   
   

   =	4π × 42	× π		202		× (.065)
   	0.09			4

   
   = 1.139 × 10⁵

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