# WBUT Question Papers CS

## Electromagnetic Waves And Radiating Systems B Tech Sem Forth

Group – A ( Multiple Choice Type Questions)

Choose the correct alternatives for any ten of the following :       10×1 = 10

i)                 The magnetic flux density if and vector potential ^ are related as

a)      f=Vx^   b) ^ = V

c)

£ m V . ^   d) m V .ii)

A potential field is given by V » 3x2 y – yz. The electric Add at P (

2, -1, 4 ) is

a)                                                                                                                                                                 12?- 8? V/m b) 12?-? V/m

c)                                                                                                                                             12″? + 8? + 1c V/m d) – 12? – 8? – 7c V/m.

ill) The electric field lines & equlpotentlal lines

a)               are parallel to each other

b)               are one and the same

c)               cut each other orthogonally

d)              can be inclined to each other at any angle.

‘ . . . ■             x ■                  ‘ ■

iv)              A transmission line of length ^ shorted at far end behaves like

£0 series resonant circuit   b) parallel resonant circuit

c) pure inductor             d) pure capacitor.

v)                Maxwell’s equation V x it * + T\$ represents

a)               Gauss’s law in magnetism

b)              KirchhofFs current law for direct current

c)               Biot-Savart law

d)              Generalized Ampere’s circuital law.

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vi)             In a two region system, a plane waveis arriving at a normal ineMence in air with f ff amplitude E j |* 100 V/m and frequency 100 MHz. The sigh is Teflon ( e r = 2-1,

e 11/e1 = 0 ). The amplitude of the reflected wave is

a) 18-34 V/m                 b) 36-68 V/m

c) – 18-34 V/m               d) – 36-68 V/m.        f

vii)            If the frequency of the incident wave? increases by a factor of 4, the depth to which a wave penetrates a conducting material ‘

a) increases by a factor of 2 ! b) increases by a factor of 4 c) decreases by a factor of 2   d) decreases by a factor of 4.

viii)          The MUF of transmission between two stations 500 km apart at f crit = 9 MHz and minimum value of incident angle i = 45° is

a)                                                                                                                                             0*7 MHz   b) 25-4 MHz

12-7 MHz     ,           d) 18 1®z. V           |

ft) A quarter-wave monopole antenna operating in air at a frequncy of 1 MHz must have an overall length (I) of

a)                                                                                                                                             l»X   b) 150 m

c) 75 m                      d) I«X.

i ’ ■ ■ ■ ‘ .i- ■ ■

x)               The radiation resistance of a dipole, having a length of 4 cm, radiating 50 W of

power at a frequency of 500 MHz is

a)                                                                                                                                             3-5 Q     b) 0-82 £2

c) 1-75 0 i                  d) 7-0 Q .            1

xi)              Ohm’s law is obeyed by

aj conduction current b) convection current c) conduction & convection current d) none of these.

■ ‘ ■    .   …           . ■ *               ■ -V’ ■ .   ‘

xii)           The direction of propagation of EM wave is obtained from

a)                                                                                                                                             E x H b) E . H

1 . : ; • •. . ■ ■- . • ■ 1 ■ “

c)                                                                                                                                             E                      d) H. !

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a

Group – B (Short Answer Type Questions)

–                       .. ;*r                                                      -■ ■ •                                             ‘                                                        .                             ■

Answer any three questions.          3 x 5 = 15

1. Define the term ij VSWR and ii) ‘Reflection co-efficient’ for transmission’line.

Explain the relationship between them.

1. Prove that V x it = if + . The symbols have usual meaning.
2. A 2m long lossless transmission line has an impedance of 300£l . The velocity of propagation is 2-5 x 10 s m/s. The load has an impedance of 300Q with sending end voltage being 60V at 100 MHz. Find :

a)               The phase constant       *                       .

‘                                                                                   i ■ ‘            ■ ■ ■ .                    A7‘ ‘ ‘                                      ■■■ ‘            – ‘ ✓ ‘ – .

d)              The load reflection coefficient &

e)                Standing wave ratio.

1. Why is ionosphere important for radiowave propagation ? Describe the different layers of ionosphere.
2. What do you mesm by magnetic vector potential ? Write down the Maxwell’s equations for timp varying electromagnetic fields, when the medium is lossless, linear, isotropic, homogeneous and source free. 3 + 2

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Group – C (Loag Answer Type Questions)

Answer any three questions.         3 x 15 = 45

1. a) Find the expression of Radiation resistance of a short electric dipole with

uniform current distribution.                            7

b)               Define complex Poynting vector.                          3

<4 Explain the concept of skin depth & find out an expression for that.                                                5

1. a) Derive an expression for the input impedance of a lossless transmission line,

in terms of relevant parameters, when the line is terminated in load impedance

7                                                        6

b)               Show that for a lossless transmission line the impedance of a line repeats over every j distance.                       5

c)               A transmission line with air as dielectric has Z0 = 50Q and a phase constant of 3 rad/m at 10 MHz. Find the inductance & capacitance of the line.                                         4

1. a) What is electromagnetic interference ?                    2

b)               Why does the short wave radio signal propagate with very low attenuation at night ? Describe the sky-wave propagation of EM waves.                                           3 + 6

c)               What is fading ? Briefly describe the diversity techniques to reduce the effect of fading.                       1+3

1. a) Obtain the Poynting theorem for the conservation of energy in an electromagnetic

field and discuss the physical significance of each term in resulting equation. 6

b)               In free space E (z, t) = 50 cos (tot-fiz) V/m. Find the average power crossing a circular area of radius 5m in the plane x = constant. 5

c)                Derive the equation of continuity for time varying fields. 4

1. a) State & explain Faraday’s law.                            4

bj Derive the induced emf when a stationary loop is in the time varying B fields. 4

c)                Determine the magnetic field intensity at a point P due to a current carrying filamentary conductor AB carrying current I along Z axis with its upper and lower ends subtending angles a, and a2 respectively.                                            7

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