# VTU Previous Exam Paper BE EC 6th Sem Antenna and Propagation July 09

PART-A

1. Define: i) Radiation intensity ii) Power density derive their relation.

State and prove reciprocity theorem for antennas.

Determine the directivity of the system if the radiation intensity i) U = Umcos30 ii) U = Umsin0 sin2<|).

(b)Derive an expression for maximum effective Aperture, Aem– Also show that Ae

of dipoleb.is 0.13k .

2. Derive a relation that relates total received power and total transmitted power in terms of directivities.

(b)C  If ‘P’ is power radiated and ‘G* is gain of the antenna, then show thatE = (3QpG)

is —.The power is applied with equal magnitudes and a phase difference -dr. Obtain the

3.(a)Derive an expression for array factor of an array of N-isotropic sources.

(b)A Linear antenna consists of 4 – isotropic sources. The distance between adjacent elements k 2 field pattern and find HPBW.

(c)Explain the principle of pattern multiplication.

4. (a)Derive the expressions for the field components of a short dipole starting with expressions of electric potential and vector magnetic potential. Also (b)determine the far field components.

Derive an expression for radiation resistance of a short-dipole.

PART-B

5. Derive the expressions for the field strengths E<j, and He in case of small loop. (b)The radius of a circular loop antenna is 0.02k. How many turns of the antenna will give a radiation resistance of 350?

6.(a)Explain the necessity of flaring of walls of waveguide in case of Horn antennas. Describe a Helical Antenna. Explain its two modes of operation with relevant expressions.

(b)Explain the operation of log-periodic antennas.

7. (a)List the merits and demerits of lens antenna.

(b)Derive an expression for space wave field intensity and show that it varies sinusoidally.

8. (a) Explain Duct propagation. Find the maximum range of a tropospheric transmission for which transmitting antenna height is 100 ft and receiving antenna height is 50ft.

(b)Explain the mechanism of Ionospheric propagation. Also derive an expression for the refractive index of an Ionospheric layer. Discuss the effect of Earth’s magnetic field on Ionospheric propagation.

(c)A high frequency radio link has to be established between two points on the earth 200 km away. The reflection region of the ionosphere is at a height of 200 km and has a critical frequency of 6 MHz. Calculate the MUF for the given path in case of flat earth.