# Antennas and Propagation December 2010

1 (a) Define the following with respect to antenna:

i) Radiation intensity                 ii) Gain                  iii) Efficiency             iv) Directivity.

(b)  Derive the expression for effective height of the transmitting antenna.

(c)  Find the directivity and draw the radiation pattern of an antenna whose radiation intensity is given by, u = u, 0<0<

2 (a)Draw and explain the field and pattern of a directional antenna.

(b)Derive the expression for magnetic vector potential of a thin linear antenna.

(c) A thin dipole antenna is long. If its loss resistance is 2.5 Q, find the radiation V10 y resistance and efficiency.

3 (a)Derive the expression for electric field intensity of an array of two point sources of equal amplitude and inphase currents. Reference point is at the centre of the array. Draw the radiation pattern.

(b )Draw the radiation pattern of an array of 4 isotropic sources separated by distance of   and having in phase currents. Derive the equation required.

4 (a)  Derive the expression for electric field intensity of a loop antenna.

(b) Find the radiation efficiency of 1 m diameter loop (C ™ n m) of 10 mm diameter copper wire at, i) 1 MHz ii) 10 MHz.

(c) Explain helical geometry.

5 (a)  Explain the important parameters of the monofilar axial mode helical antenna.

(b) Explain with a neat diagram, Yagiuda array antenna.

(c) Write the diagram of log periodic antenna and explain.

6 (a)  Explain different types of horn antennas.

(b) Give the comparison between parabolic and comer reflectors.

(c) Explain embedded and plasma antennas.

7 (a) Explain line of sight propagation. Derive the expression for maximum line of sight distance.

(b) Derive the expression for electric field intensity of tropospheric waves.

(c) A VHF communication has to be established with 35 W transmitter at 90 MHz.

Find the distance, up to which, line of sight communication may be possible, if the height of the transmitting and receiving antennas are 40 m and 25 m respectively. Also determine the field strength at the receiving end.

8 (a)  Explain the structure of the atmosphere, with a neat diagram.

(b) What is the effect of earth’s magnetic field on wave propagation?

(c) Calculate the critical frequencies for FI, F2 and E layers, for which, the maximum ionic densities are 2.3 x 106, 3.5 x 106 and 1.7 x 106 electrons per cm3 respectively.