J5 fo< f otA 1200

B.E./B.Tech. DEGREE EXAMINATION, MAY/JUNE 2007 -

Seventh Semester

Electronics and Communication Engineering

EC 431 - ANTENNAS AND PROPAGATION

Time : Three hours Maximum: 100 marks

Answer ALL questions.

PARTA- (10 x2=20 marks )

1. The radial component of the radiated power density of an antenna is given by

W,o,t =W, d, = 48149 U, watts/m2 where, Ao is the peak value of powerr-

density. Determine the total radiated power.

2. Give the significance of Friss Transmission equation'

3. What are the parameters that determine the overali pattern of an antennaartay?

4. State the expression for the array factor of an N-element array. What is itssignificance 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 baseof 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 anionospheric layer.

10. What is maximum usable frequency?

www.MaanavaN.com

PARTB- (5x16=80marks )

11. (a) (i) Derive the relation between directivity and beam solid angle. (8)

(b)

(ii) Defrne effective aperture and derive a formula for maximum

effective aperture in terms of poynting vector' (8)

Or

(i) What is polarization. Explain the different types of polarization. (8)

(ii) Derive the relation between directivity and effective aperture of an

antenna. (8)

(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 nutl directions and directions of maxima and

iraw the pattern. (10)

(ii) Explain the principle of pattern multiplication with an example. (6)

12. (a)

13 . (a )

14. (a )

Or

(b) Derive the formula to frnd the maxima, null points and half-power points

of an N-element broadside atray and show that the first minor lobe is

13.46 dB down from the major lobe.

For a short d.ipole antenna, starting from the expression for the magnetic

vector potential, derive the expressions for the electric and magnetic field

intensities. ( 16)

Or

For a half-wave dipole antenna, show that total power radiated is

73 .14 I , * , t . ( 16 )

(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 ope:ration' (8)

(16 )

(b)

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(8)

A 1200

10 GHz.

www.MaanavaN.com

1 5 . ( a ) (i) Explain how refraction of waves occurs at the ionosphere. (8)

(ii) Derive the formulae to determine the maximum usable frequencyand the skip distance. (8)

Or

f)escribe space wave propagation. Derive an expression to

determine the range of propagation. (8)

f)erive an expression to determine the voltage induced in the

receiving antenna in terms of the voltage at the transmitting

antenna, assuming space-wave propagation. (8)

(b) (i)

( i i )

A 1200

www.MaanavaN.com