WBUT question of EM theory of 4th sem

2006
Group-B
2(a). What do you mean by self inductance?
(B). A 3 cm long solenoid carries a current of 400mA. IF the solenoid is to produce a magnetic flux density of 5 mWb/m², how many turns of wire are needed?
3 (a) State Ampere's circuit law.
(b) A hollow conducting cylinder has inner radius a and outer radius b and carries current I along the positive Z-direction .Find H everywhere.
4(a) States Faraday's law. From the fundamental principle establish the relation

∆×B= -∂B/∂t

5. What do u you mean by effective height of an antenna? Calculate it for linear and sinusoidal distribution of current.

6. what do you mean by skin effect? Calculate the skin depth for radio waves of wavelength 3 m (in free space) in copper, the electrical conductivity of which is 6×10⁷ s/m.

Group C

7(a) Write the maxwell’s equation for time varying electromagnetic fields, when the medium is homogeneous, source free, loss less, isotropic and linear. Explain the properties of the parameters of the medium.

(b) Explain the inconsistency present in the Ampere’s law. How is the low modified by Maxwell?

8(a) Derive the expression of input impedance, Z_in of a lossless transmission line in terms of relevant parameters, when the line is terminated in load impedance, Z_L.

(b) Give a neat sketch of variation of Z_in as a function of electrical length of the line, when the line is terminated in a (i) short circuit and (ii) open circuit. Discuss the significance of the plots.

9(a) Explain the mechanism for tropospheric scatter propagation.

(b) What is radio horizon?

(c) Derive the expression for critical frequency for sky wave propagation.

10(a) Explain the formation of standing wave pattern on transmission line.

(b) Deduce the relation between the reflection co-efficient and VSWR.

(c) A 70Ω lossless line has S=1.6 an θ_k=300⁰. If the line is 0.6λ long, obtain

(i) K, Z_{L, ,} Z_in (ii) the distance of the first minimum voltage fro

11 Find the relationship between the effective aperature and directivity of an antenna. Show that the effective aperture for isotropic antenna is 0.079 λ². Derive an expression for the Friis formula in communication system.

m the load.