# VTU Previous Question Paper 6th Sem Antenna and Propagation Jan 07

**Note : Answer any FIVE full questions.**

1 a. Discuss the following terms as applied to antennas :

i) Radiation resistance

ii) Isotropic radiation

iii) Effective height

iv) Directivity and gain

v) Beam width.

b. Distinguish between near field and far field as applied to radiating element,

c. A small dipole antenna carrying a uniform rms current of 10A is having a far zone rms field at a distance V meter in a direction making an angle 6 with the conductorgiven by E ~ sin 9 V/m. Find the total radiated power.

2 a. State power pattern theorem and explain. Discuss its applications.

b. Show that the directivity for unidirectional operation is 2(n+l) for an intensity variation of U = U_{m} cos” 0.

c. Calculate the directivity for an intensity variation of U = U_{m} sin 0 sin <j> O<0< ^>2 , 0<(|)<27c. Using:

i) Exact method

ii) Approximate method.

3 a. Discuss the fields of short dipole with respect to spherical co-ordinate system. Derive mathematical relations.

b. Calculate the maximum effective aperture of a short dipole.

c. Show that radiation resistance of ^ – antenna is 73 ohms.

4 a. Explain the different types of apertures and their inter-relationships.

b. Explain Hansen and Woodyard array.

c. For an 8 element array with equal spacing of 0.7X and fed with equal amplitude and phase, find the approximate gain and FNBW.

5 a. Explain Babinet’s principle with illustrations. Discuss features of complementary antenna.

b. Write explanatory note on frequency independent antenna.

c. Calculate horn parameters i)length ‘U ii)flare angle ‘0’ iii)flare angle iv)width ‘a’ if mouth height ^{c}b’ is 10k and the horn is fed by a rectangular waveguide with TEio mode.

6 a. Discuss the features of Antenna for Ground Penetrating Radar (GPR).

b. Explain the structure of Ionosphere.

c. Obtain the field at a distance of 60 km from a transmitter of frequency 2 MHz radiating 200 W from a vertical half wave aerial over earth with s ~ 12 and a = 5 x 10‘^{3} mho/m (assume attenuation factor K = 0.02).

7 a. Derive simplified formula for VHF propagation.

b. Derive an expression for refractive index in case of Ionosphere.

c. For a VHF communication link, a 85 Watt transmitter is operating at 90 MHz. Determine the distance up to which line of sight communication would be possible given that the height of the transmitting and receiving antenna are 40 m and 25 m respectively. Evaluate the field strength at the receiving point.

8 Write short note on any four :

a. Parabolic reflector

b. Duct multiplication

c. Pattern multiplication

d. Helical antenna

e. Loop antenna

f. Ground wave propagation.