# VTU Previous Exam Papers BE EC 6th Semester

# Antenna and Propagation July/August 2005

**Electronics and Communication Engineering Antenna and Propagation**

**Note: Answer any FIVE full questions.**

1. (a) Explain the following terms as referred to a radiation element so as to highlight the effectiveness.

i) Beam area ii) Beam efficiency iii) Effective aperture iv) Directivity

(b) Calculate the maximum power received at a distance of 0.5km over a free space 1 GHz circuit consisting of a transmitting antenna with 25 dB gain and a receiving antenna gain of 20 dB. Assume the transmitting antenna input is 150 watts.

2. (a) State and explain the power pattern theorem,

(b) Explain field and phase patterns.

(c) Show that the relative £($) pattern-of an array of two identical isotropic point sourcesarranged as shown in the fig, is given by £($) = cos Also show that the maxima, nulls and Half-power points are given by sincj>

where dr = ^ ✓\

Maxima : $ = $in~^{l}

Nulls : $ = sin’^{1} Half power points : $ = sin’^{1}kX2ddfc2fc+iidX$

3. (a) Explain the term linear array and bring out the properties of broad-side and end-fire arrays. Sketch the directional pattern for these arrays.

(b) Using the principle of pattern multiplication show that a linear array with binomial amplitude distribution has a pattern with no minor lobes.

(c) Show that the HPBW of a long uniform broad side array is given by 50.8°/(i/A)

4.(a) Show that the electric field pattern of a thin linear antenna of length L == A/2 is given bycosliv’j 2’COS0\Sint) –^{1}

(b) Show that the radiation resistance of a small loop antenna consisting of (n) terms is given by/ \^{2}R_{T} — 31.200 (n – j—J £1

(c) Calculate the maximum effective aperture of a thin loop antenna O.lA in diameter with a uniform in-phase current distribution. (6 Marks)

5. (a) Explain the important design considerations for the monofilar axial mode helical antenna.

(b) A right handed monofilar helical antenna has 10 turns, 10mm dia and 70mm turn spacing. Calculate the far field pattern at / = l GHz and HPBW, gain. (8 Marks)

6. (a) Write a brief note on the following types of antenna

i) Slot antenna ii) Log periodic antennas

(b) The normalized field pattern £?($) for a rectangular aperture is given by

Starting from the above equation obtain the expressions for the beam width between first …. nulls (BWFN) for circular aperture and for rectangular aperture.

7.(a) Discuss briefly the design considerations of an antenna used for satellite communication.

(b) Explain the phenomenon of ground wave propagtion over an imperfect earth.

8.(a) Explain the term critical frequency with reference to ionospheric propagation and obtain an expression for the same.

(b) Explain a method to determine the critical frequency and the virtual height of an ionospheric layer.

(c) Calculate the maximum usable frequency for ionospheric propagation assuming the maximum electron density is (5 x 10^{6}) electrons/c.c and the signals are reflected by the F-layer at a height of 140 km. (Assume the radius of earth as 3500 km.)