51. The radiation resistance of a circular loop of a turn is 0.01 Ω. The radiation resistance of five turns of such a loop will be—

1. 1. 0.002 Ω

   
   
   

   2. 0.01 Ω

   
   
   

   3. 0.05 Ω

   
   
   

   4. 0.25 Ω

   
   
   

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1. View Hint View Answer Discuss in Forum

   Radiation resistance of loops of N turns is given by
   

   Rr= 31200		N	A
   			λ2

   
   A → Area of the loop

   Correct Option: C

   Radiation resistance of loops of N turns is given by
   

   Rr= 31200		N	A
   			λ2

   
   A → Area of the loop

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52. A loop is rotating about the y-axis in a magnetic field
    
    

    →	→
    E = B0 cos (ωt + φ)	axT

    .The voltage in the loop is—

1. 1. Zero

   
   
   

   2. Due to rotation only

   
   
   

   3. Due to transformer action only

   
   
   

   4. Due to both rotation and transformer action

   
   
   

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1. View Hint View Answer Discuss in Forum

   Since magnetic field is sinusoidal so the voltage in the loop will be by transformer action and rotation both.

   Correct Option: D

   Since magnetic field is sinusoidal so the voltage in the loop will be by transformer action and rotation both.

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53. For field of an antenna varies with distance r as—

1. 1. 1

      r

   
   
   

   2. 1

      r2

   
   
   

   3. 1

      r3

   
   
   

   4. 1

      √r

   
   
   

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1. View Hint View Answer Discuss in Forum

   It is clear from the expression provided in synopsis.

   Correct Option: A

   It is clear from the expression provided in synopsis.

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54. When N is the maximum electron density in per cubic metre. The critical frequency (fc) is given by—

1. 1. 9 N

   
   
   

   2. 9N^3/2

   
   
   

   3. 9√N

   
   
   

   4. 9

      √N

   
   
   

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1. View Hint View Answer Discuss in Forum

   f_c = √81 N_max ⇒ 9√ N_max

   Correct Option: C

   f_c = √81 N_max ⇒ 9√ N_max

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55. A line of length nλ/4 (n is integer) in short circuited at the far end. The input impedance is—

1. 1. Zero

   
   
   

   2. Infinite

   
   
   

   3. Z₀

   
   
   

   4. None of these

   
   
   

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1. View Hint View Answer Discuss in Forum

   For the line of shorted and multiple of λ/4. Input impedance is given by
   Z = j Z_d tan βd
   so for  

   d =	nλ
   	4

   
   

   β =	2∏	we get '∞' impedance.
   	λ

   Correct Option: B

   For the line of shorted and multiple of λ/4. Input impedance is given by
   Z = j Z_d tan βd
   so for  

   d =	nλ
   	4

   
   

   β =	2∏	we get '∞' impedance.
   	λ

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