VTU Previous Paper BE EC 6th Sem Antenna and Wave Propagation July 07
1 a. Explain the following as related to antenna systems:
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
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.