# VTU Previous Paper BE EC 6th Sem Antenna and Wave Propagation July 07

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

i)      Directivity.

ii)     Beam width.

iv)    Band width.

b. Find the power density at a distance 3 km from an isotropic source if the power      density at a distance 2 km is 10 mwatts/sq.units.

c. obtain an expression for maximum effective aperture for X / 2 dipole,

2 a. Find the directivity for an intensity variation, U = U sin0sin2<b m where 0 and cp ranges between 0 and tu.

b.  Two vertical short dipoles are separated by a distance X I 3. The power is applied with equal magnitudes having a phase difference n / 3. Obtain the field pattern and find the beam width.

c.   Four isotropic antennas are placed along a straight line separated by a distance X / 2. The power is applied with equal magnitudes. The peak should be in a direction 60° from the axis of the array. Find the phase difference between adjacent elements. Complete the pattern and find BWFN and HPBW.

3 a. Starting from magnetic and electric potentials, obtain the far electric fields for a short dipole. )

b.  A 2-m long vertical wire carries a current of 5 A at 1 MHz. Find the field strength at 30 km in a direction at right angles to the axis of the wire, assuming the wire is situated in free space.

c.   A half wave dipole radiating in free space is driven by a current of 0.5 amps at the terminals. Calculate the electric field strength E at a distance 1 km from the antenna at angles 45° and 90°.

4 a. Obtain the field components for small loop antenna.

b.  Show how a helical antenna can be used as polarization diversity system.

c.   A helical antenna has 10 turns, 100 mm diameter and 70 mm turn spacing. The operating frequency is 1 GHz. What is the directivity and the polarization state.

5 a. A 64 m diameter dish antenna operating at a frequency of 1.43 GHz is fed by a nondirectional antenna. Calculate its

i)          HPBW.

ii)         BWFN.

iii) Gain with respect to X/2 dipole assuming even illumination. (06 Marks)

b.  Determine the cut-off frequencies and bandpass of a log periodic dipole array with a design factor of 0.7. Ten dipoles are used in the structure, the smallest having a dimension equals to 0.3 m.

c.   Write a note on corner reflectors.

6 Write short notes on any three of the following:

a.   Log periodic antennas.

b.  Pattern multiplication principle.

c.   Horn antenna.

d.  Yagi-Uda array.

7 Define the following as related to ionospheric propagation:

i)     Maximum usable frequency.

ii)     Critical frequency.

iii)    Virtual height.

iv)    Discuss the propagation characteristics of radio waves in the frequency range 200 kHz to 200 MHz.

8 a. Derive the expressions for conductivity; and relative permittivity for ionosphere layers.

b. A HF radio link is established for a range of 2000 km. If the reflection region of ionosphere is at a height 200 km and has a critical frequency of 6 MHz, calculate MUF.Estimate the wave tilt in degrees of the surface wave over an earth of 5 millimhos conductivity and relative permittivity of 10 at 1 MHz.