Antennas & Propagation - Dec 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) With the help of Maxwell's equation, explain how radiation and reception of EM takes place?(6 marks) 1 (b) Explain the following terms as related to antenna system:
i) Directivity ii) HPBW iii) Effective length iv) Beam efficiency.(8 marks) 1 (c) Show that the directivity for unidirectional operation is 2(n+1) for an intensity variation of U-U_mcosⁿθ.(6 marks) 2 (a) Write a neat diagram, obtain an expression for maximum effective aperture of a λ/2 dipole.(7 marks) 2 (b) Derive relationship between effective aperture and directivity of an antenna.(8 marks) 2 (c) Find the maximum power received at a distance of 0.75 km over free space 110 MHz circuit consisting of a transmitting antenna of 30dB gain and a receiving antenna of 25dB gain. If the power i/p to the transmitting antenna is 120 watts.(5 marks) 3 (a) Starting from fundamentals derive the equation for radiation resistance of Hertzian dipole.(8 marks) 3 (b) A dipole antenna of length 5cm is operated at a frequency of 100MHz with terminal current, I₀-120mA. At time t=1 sec, 0=45° and r=3m, Find: (i) E_r; (ii) E_θ and (iii) H_ϕ(8 marks) 3 (c) Calculate the radiation resistance of a length=λ/5. (Assume triangular current distribution).(4 marks) 4 (a) Derive the far field expressions for small loop antenna.(8 marks) 4 (b) Derive an expression and draw the field pattern for an array of two isotropic point source with equal amplitude and opposites phase. Take d=λ/2.(8 marks) 4 (c) Find half power beam width directivity of a linear broadside array of four isotropic point sources of equal strength widht d=λ/2?(4 marks) 5 (a) Write explanatory note on: i) Folded-dipole antenna; ii) Yagi-Uda antenna.(10 marks) 5 (b) Find the length, L-H plane aperture and flare angles 0_E and θ_H of a pyramidal horn for which E-plane aperture is 10λ. Horn is fed by a rectangular wavelength with TE₁₀ mode. Assume ?=0.2λ in E-plane and 0.375λ in H-plane. Also find E-plane, H-plane beam widths and directivity.(6 marks) 5 (c) A dish antenna operating at a frequency of 1.43 GHz has a diameter of 64mts and is fed by directional antenna. Calculate HPBW, BWFN and gain with respect to λ/2 dipole with even illumination.(4 marks) 6 (a) Write short notes on:
i) Parabola reflectors
ii) periodic antenna(12 marks) 6 (b) Determine the cut off frequencies and bandpass of a log periodic dipole array with a design factor of 0.7. Ten dipoles are used in the structure, the smallest having a dimension L/2 equal to 0.3m.(8 marks) 7 (a) Define a wave tilt of a surface wave propagation. Also prove that wave tilt $$ \alpha = \tan^{-1} \dfrac {E_n}{E_v} = \tan ^{-1} \left [ \dfrac {1}{\sqrt{\epsilon_r}} \cdot \dfrac {1}{[1+x^2]^{1/4}} \right ] $$(10 marks) 7 (b) Derive the expression for resultant field strength at a point due to space wave propagation.(5 marks) 7 (c) For a VHF communication link, a 35 watt transmitter is operating at 90MHz. Determine the distance up to which LOS would be possible given that height of the transmitting and receiving antenna are 40 m and 25m respectively. Evaluate the held strength at the receiving point.(5 marks) 8 (a) Define the following: i) MUF; ii) Critical frequency; iii) Virtual height; iv) Skip distance.(8 marks) 8 (b) Calculate the value of the operating frequency of the ionosphere layer specified by refractive index of 0.85 and an electron density 5×10⁵ electron/m³. Calculate the critical frequency and MUF of the system with θ₁=30°.(6 marks) 8 (c) Calculate the critical frequencies for f₁, f₂ and f₃ and E layers, for which the maximum ionic densities are 2.3×10⁶, 3.5×10⁶ and 1.7×10⁶ elections/cm³ respectively.(6 marks)