## 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 0.13λ^{2}.(6 marks)
**1 (c)** The effective apertures of transmitting and receiving antennas in a communication system are 8&lamba;^{2} and 12λ^{2} 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 sin^{3} ϕ, 0?θ?Π, 0?ϕ?Π

ii) U=U_{m} sin^{2} θ sin^{3}ϕ, 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 TE_{10} 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=9×10^{6} /cm^{3}. Also find maximum usable frequency (MUF), if the angle of incidence ∠i-60°.(8 marks)