# VTU Previous Exam Paper BE EC 6th Sem Antenna and Propagation July 08

1 a. Explain the following terms as related to antenna systems:

i) Directivity ii) Half power beam width iii) Beam efficiency.

b. Obtain an expression for maximum effective aperture for X/2 dipole.

c.   A radio link has 150 watts transmitter connected to an antenna of 2 m2 aperture at 2 GHZ. The receiving antenna has an aperture of 1.5 m2 and is located at 10 km. Find the power delivered to the receiver.)

2 a. Find the total power radiated and directivity for an isotropic point source with

Unidirectional cosine squared power pattern. Also show the pattern.

b.  Derive an expression and draw the field pattern for an array two isotropic point sources with equal amplitude and opposite phase.

c.   Four isotropic point. Sources separated by X12 are placed along a straight line in a broadside fashion. Draw the field pattern. Also find BWFN and HPBW.

3 a. Derive an expression for radiation resistance of a short electric dipole.

b.  A dipole antenna of length 5 cm is operated at a frequency of 100 MHz with terminal current, I0 = 120 mA. At time t – lsec, 0 = 45° and r = 3m, find

i) Er              ii) Ee                  iii) H<j>

c.   With a neat figure, write the far – field equations for a thin linear antenna.

4 a. Derive the far field expressions for small loop antenna^

b.  Derive the expression for radiation resistance:of small loop antenna.

c.   Find the radiation efficiency of a 1m dia loop (c – n m) of 10 mm diameter copper wire at i) 1 MHz ii) 10 MHz.

5 a. Explain the practical design considerations for the monofilar axial mode Helical antenna

b. Determine

i) The length L. aperture an and half angles in E and H planes for a pyramidal e.m horn for which the aperture ae = SX. The horn is fed with a rectangular waveguide with TEio mode.A,      X Take 8 ~ — in the E – plane and 8 = — in the H – plane.

ii) What is directivity? iii) What is the aperture efficiency?

6. Write short notes on the following:

a.   Plasma antennas

b.  Comer reflector antennas

c.   Complementary antennas

d.  Antennas for GPR systems

7 a. Discuss the salient features of Ground wave propagation,

b.  Derive the expression for resultant field strength at a point due to space wave propogation.

8 a. Define the following terms as related to ionospheric propagation:

(i) MUF     ii) Critical frequency    iii) Virtual height    iv) Skip distance.

b.  In an ionospheric wave propagation the angle of incidence made at a particular layer, at a height of200 km is 45°, with critical frequency 6 MHz. Calculate the skip distance.

c.   Calculate the wave tilt in degrees of the surface wave over an earth of 6 millimeters conductivity and relative permittivity of 12 at 2 MHz.