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Posted By: b.thirumaya prabhu Member Level: Gold Posted Date: 22 May 2008
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2007 Anna University B.E Electronics and Communication EC 431 - ANTENNAS AND PROPAGATION 07 Question paper
B.E./B.Tech. DEGREE EXAMINATION, MAY/JUNE 2007.
Seventh Semester
Electronics and Communication Engineering
EC 431 —— ANTENNAS AND PROPAGATION
Time : Three hours Maximum 2 100 marks
• Answer ALL questions.
• PART A -— (10 >< 2 = 20 marks) .
1. The radial component of the radiated power density of an antenna is given by
Wrad = Wr ar = (A0 Sin? /r2) ar watts/m2 where, AO is the peak value of powerdensity. Determine the total radiated power.
2. Give the significance of Friss Transmission equation.
3. VVhat are the parameters that determine the overall pattern of an antenna array?
4. State the expression for the array factor of an N-element array. Wvhat is its significance in pattern multiplication?
5. Find the gain of a half-wave dipole in the direction of maximum- power.
6. An oscillating dipole has an effective height of 100 m. If the current at the base
of the antenna is 450 A (rms) at 40,000 Hz, what is the total power radiated?
7. What are the common feeding methods for micro strip antennas?
8. Draw and explain the need for components in a Yagi-Uda antenna.
9. State the relation between critical frequency and electron density of an
ionospheric layer.
10. What is maximum usable frequency?
PART Bi - (5 >< 16 = 80 marks)
11. (a) (i) Derive the relation between directivity and beam solid angle. (8)
(ii) Define effective aperture and derive a formula for maximum
effective aperture in terms of poynting vector. (8)
Or
(b) (i) What is polarization. Explain the different types of polarization. (8)
(ii) Derive the relation between directivity and effective aperture of an
antenna. (8)
12. (a) (i) For an array of 2 isotropic point sources, fed with currents of same
magnitude but, in phase quadrature, determine the radiation
pattern. Evaluate the null directions and directions of maxima and
draw the pattern. (10)
(ii) Explain the principle of pattern multiplication with an example. (6)
Or
(b) Derive the formula to find the maxima, null points and half—power points
of an N-element broadside array and show that the first minor lobe is
13.46 dB down from the major lobe. — (16)
13. (a) For a short dipole antenna, starting from the expression for the magnetic
vector potential, derive the expressions for the electric and magnetic field
intensities. p . (16)
Or
(b) For a half—wave dipole antenna, show that total power radiated is
73.14
14. (a) (i) Explain the construction, radiation pattern and features of the
rhombic antenna. (8)
(ii) Justify how the log—periodic antenna can be used as a wide-band
antenna, by explaining its method of operation. (8)
Or
(b) (i) Explain the construction and features of micro strip antennas. (8)
” (ii) Design a rectangular micro strip antenna with dielectric constant of
substrate as 2.2, h : 0.1588 cm so as to resonate at a frequency of
10 GHz. (8)
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