141. A 1 km long microwave link uses two antennas each having 30 dB gain. If power transmitted by one antenna is 1 watt at 3 GHz. The power received by the other antenna is approximately—

1. 1. 6.34 × 10^–2 watt

   
   
   

   2. 6.34 × 10^–3 watt

   
   
   

   3. 6.34 × 10^–1 watt

   
   
   

   4. 6.34 × 10^–5 watt

   
   
   

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1. View Hint View Answer Discuss in Forum

   PR = PT GT GR		λ		2
   		4πd

   
   where,
   P_R = received power = ?
   P_T = transmitted power = 1 watt
   G_T = G_R = 30 dB = 1000
   G_T = gain of transmitting antenna
   G_R = gain of receiving antenna
   

   λ =	c	=	3 × 108	= 0.1
   	f		3 × 109

   
   d = 1 km = 1000 m
   Now,
   

   PR = 1 × 1000 × 1000 ×		0.1		2
   		4π × 1000

   
   = 6.34 × 10^–5 watts.
   
   

   Correct Option: D

   PR = PT GT GR		λ		2
   		4πd

   
   where,
   P_R = received power = ?
   P_T = transmitted power = 1 watt
   G_T = G_R = 30 dB = 1000
   G_T = gain of transmitting antenna
   G_R = gain of receiving antenna
   

   λ =	c	=	3 × 108	= 0.1
   	f		3 × 109

   
   d = 1 km = 1000 m
   Now,
   

   PR = 1 × 1000 × 1000 ×		0.1		2
   		4π × 1000

   
   = 6.34 × 10^–5 watts.
   

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142. In a uniform linear array, four isotropic radiating elements are spaced λ/4 apart.The progressive phase shift between the elements required for forming the main beam at 60º of the end fire array is—

1. 1. – π radians

   
   
   

   2. −	π	radians
      	2

   
   
   

   3. −	π	radians
      	4

   
   
   

   4. −	π	radians
      	8

   
   
   

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1. View Hint View Answer Discuss in Forum

   We know,
   

   ψ =	2π	d (cos φ – 1)
   	λ

   
   where, Q = 60º, d = λ 4
   so,
   

   ψ =	2π	.	λ	(cos 60º – 1)
   	λ		4

   
   = – π/ 4

   Correct Option: C

   We know,
   

   ψ =	2π	d (cos φ – 1)
   	λ

   
   where, Q = 60º, d = λ 4
   so,
   

   ψ =	2π	.	λ	(cos 60º – 1)
   	λ		4

   
   = – π/ 4

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143. In a broadside array of 20 isotropic radiators, equally spaced at distance of λ/2, the beamwidth of the first nulls is—

1. 1. 11.46º

   
   
   

   2. 5.73º

   
   
   

   3. 10.46º

   
   
   

   4. None of these

   
   
   

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1. View Hint View Answer Discuss in Forum

   For broad side array beamwidth of first null is given by
   

   2θ = beamwidth = 2 sin–1		λ
   		ηd

   
   where, η = no. of radiators
   d = spacing between the radiators
   so,
   

   2θ = 2 sin–1	λ			= 2sin-1		1
   		20 .	λ			10
   			2

   
   = 2 × 5.73 = 11.46°

   Correct Option: A

   For broad side array beamwidth of first null is given by
   

   2θ = beamwidth = 2 sin–1		λ
   		ηd

   
   where, η = no. of radiators
   d = spacing between the radiators
   so,
   

   2θ = 2 sin–1	λ			= 2sin-1		1
   		20 .	λ			10
   			2

   
   = 2 × 5.73 = 11.46°

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144. 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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145. 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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