2007 Anna University B.E ECE - EC 431 - ANTENNAS AND PROGATIONS QUESTIONS

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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 X 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 B - (5X 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 )