Antennas & Propagation - Jun 2014

Electronics & Communication (Semester 6)

TOTAL MARKS: 100
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks. 1 (a) Define the following terms with respect to antenna:
(i) Gam (ii) Isotropic radiator (iii) Beam area
(iv) Radiation resistance(8 marks) 1 (b) Prove that maximum effective aperture for a λ/2 antenna is 0.13λ².(6 marks) 1 (c) The effective apertures of transmitting and receiving antennas in a communication system are 8&lamba;² and 12λ² respectively, with a separation of 1.5 km between them. The E.M. wave is travelling with a frequency of 6 MHz and the total I/P power is 2.5 kW. Find the power received by the receiving antenna.(6 marks) 2 (a) Derive an expression of power radiated from an isotropic point source with "sine squared power pattern". Also find directivity "D" and draw power pattern.(6 marks) 2 (b) Find the power radiated and directivity for the unidirectional point source, having the following point source power patterns:
i) U=U_m cos<ssup>2 θ sin³ ϕ, 0?θ?Π, 0?ϕ?Π
ii) U=U_m sin² θ sin³ϕ, 0?θ?Π, 0?ϕ ? Π</ssup>(6 marks) 2 (c) Eight point source are spaced λ/6 apart. They have a phase difference of Π/3 between adjacent element. Obtain the field pattern. Also find BWFN and HPBW(8 marks) 3 (a) Show that the radiation resistance of a half wave $$ \left ( \dfrac {\lambda}{2} \right ) $$ dipole antenna is 73Ω.(6 marks) 3 (b) Write an explanatory note on folded dipole antenna, giving neat figures.(6 marks) 3 (c) A magnetic field strength of 20 μA/m is required to be produced at a point 2.5 km from the antenna in the broadside plane, in space. How much power is transmitted by,
i) A hertz dipole with $$ l=\dfrac {\lambda}{15} $$ ii) a half wave dipole and
iii) a monopole antenna.(8 marks) 4 (a) Discuss the features of a loop antenna. Derive an expression for the far field components of a loop antenna.(10 marks) 4 (b) Explain Babinct's principle with illustrations. Discuss features of complementary antennas, with neat figures.(10 marks) 5 (a) With a neat figure, explain the working of Yagi-Uda antenna. Write the design formulae for different components, used in Yagi-Uda antenna. Also mention the applications of Yagi-Uda antennas.(8 marks) 5 (b) Determine:
i) The length L aperture 'a_n' and half angles in E and H planes for a pyramidal Horn antenna, for which a_E=10λ. The horn is fed with a rectangular wave guide in TE₁₀ mode. Let $$ \delta = \dfrac {\lambda}{12}\ in \ the \ E- plane \ and \ \delta=\dfrac {\lambda} {6} $$ in the H-plane.
ii) Calculate directivity 'D'.(8 marks) 6 (a) Write note on:
i) Plasma antenna.
ii) Embedded antenna(8 marks) 6 (b) With a neat sketch, explain the principle of lens antenna.(6 marks) 6 (c) A parabola reflector of 2m diameter is used at 10 GHz. Calculate the beam width between first nulls (BWFN) HPBW and gain in dB.(6 marks) 7 (a) Discuss various forms of radio-wave propagation.(8 marks) 7 (b) Derive the expression for resultant electric field strength (E_R) at a point, due to space wave propagation.(6 marks) 7 (c) Derive the expression for 'Line of sight' distance (LOS) between transmitting and receiving antennas.(6 marks) 8 (a) A high frequency radio link is to be established between two points on the earth 350 km apart. The reflection region of the atmosphere is at a height of 250 km and has a critical frequency of 8 MHz. Calculate the maximum usable frequency (MUF), for the given path in case of flat earth.(6 marks) 8 (b) Define skip distance. Derive an expression for skip distance (D), for a flat earth.(6 marks) 8 (c) Define critical frequency. Find the critical frequency for a particular ionospheric layer with Nm=9×10⁶ /cm³. Also find maximum usable frequency (MUF), if the angle of incidence ∠i-60°.(8 marks)