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2005 Anna University Chennai B.E ANTENNAS AND WAVE PROPAGATION Question paper



Course: B.E   University/board: Anna University Chennai





Code: A-19 Subject: ANTENNAS AND WAVE PROPAGATION
Time: 3 Hours Max. Marks: 100
NOTE: There are 11 Questions in all.
Question 1 is compulsory and carries 16 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.
Answer any THREE Questions each from Part I and Part II. Each of these questions carries 14 marks.
Any required data not explicitly given, may be suitably assumed and stated
Q.1 Choose the correct or best alternative in the following: (2x8)

a. The magnitude of the velocity of electromagnetic waves reflected from an infinite plane separating two ideal dielectric semi-infinite media
(A) gets multiplied by the reflection coefficient.
(B) gets divided by the reflection coefficient.
(C) remains unchanged.
(D) becomes zero.

b. The half-wave dipole antenna of the Yagi-Uda antenna, commonly used for receiving TV signals, is folded primarily
(A) To reduce the beamwidth of the antenna.
(B) To increase the maximum directivity of the antenna.
(C) To improve the matching between the antenna and its feeder.
(D) To facilitate its mounting on the boom.

c. As the main beam of a uniform, equally-spaced linear array is steered from the end-fire direction to the broadside direction by changing the progressive phase shift of the array, it
(A) Becomes broader and broader.
(B) Becomes narrower and narrower.
(C) Remains unchanged.
(D) Becomes broader, attains a minimum and then increases.

d. Which of the following antennas gives circular polarization?
(A) Helical antenna in its Axial mode.
(B) A single dipole.
(C) Pyramidal Horn antenna.
(D) Yagi-uda antenna.

e. The radiation pattern of a paraboloidal reflector antenna is
(A) the same as that of the feed antenna.
(B) the square of the feed antenna pattern.
(C) the square root of the feed antenna pattern.
(D) does not depend on the feed antenna pattern.
1
Index Next>>f. The medium wave broadcast signals also get propagated via the ionosphere during
(A) both day and night. (B) day only.
(C) night only. (D) day in winter and during night in summer.

g. The critical frequency, fc, of an ionospheric layer divided by the square-root of its maximum ionic density, is
(A) proportional to Nmax. (B) proportional to .
(C) a constant. (D) proportional to .

h. Tropospheric forward scatter communication is usually used in

(A) LF range of frequencies. (B) HF range of frequencies.
(C) VHF range of frequencies. (D) UHF range of frequencies.
PART I
Answer any THREE Questions. Each question carries 14 marks.
Q.2 a. State and explain the Maxwell's field equations in their integral forms. (5)
b. A uniform plane wave is incident normally on an infinite plane surface separating two ideal dielectric media. Derive expressions for the reflection & transmission coefficients. (9)

Q.3 The radiation field of a wire-antenna is given by:

where symbols have their usual meanings.
Calculate the total power radiated by the antenna in the direction of maximum directivity. (14)

Q.4 a. Define, explain and hence bring out the difference between the characteristics, maximum power gain and maximum directive gain (directivity) of an antenna. (7)
b. An antenna has a loss resistance of 10 ohms, maximum power gain of
13.0103 dB and maximum directivity of 13.4242 dB. Calculate its radiation
resistance. (7)
2
<>Q.5 a. Describe and explain the important features of a 'small' loop antenna. What are its practical applications? (7)

b.
A 20-turn, square loop of side 0.625 m, is used for direction finding at a frequency of 300 kHz. When oriented to produce the maximum output, the loop produces an emf of 100 V. Calculate the electric field strength picked up by the loop. (7)

Q.6 a. What must the progressive phase shift be so that major lobe of a very long uniform, linear array of N equally-spaced isotropic radiators, is located along an angle, , off the line-of-array. (7)

b. Calculate the sidelobe level and the width of the major lobe between the first nulls of the array described above in Q.6 (a) above. (7)
PART I
Answer any THREE Questions. Each question carries 14 marks.
Q.7 Describe and discuss the principle of operation of parabolic reflector antennas.
How are such antennas fed? What factors govern the design of their feeds? What is aperture-blockage, what are its effects and how it may be avoided? Give some of the applications of these antennas. (14)

Q.8 What is meant by the term, Impedance Matching? Why is it necessary to match an antenna to its feeder? What are the various methods employed to effect matching? Explain how a single stub matching system is designed. (14)

Q.9 Write an essay on the Sky-wave propagation bringing out clearly the role played by various layers of the ionosphere. (14)
What are the antenna and the polarisation commonly employed for short-wave reception?

Q.10 Derive the formula for the power received by the antenna at the other end of an LOS hop, assuming a perfectly conducting flat earth. (14)
What are the effects of the curvature of the earth on the LOS propagation? Sketch the variation of the received power with the distance between the transmitting & the receiving antennas.

Q.11 Write short notes on any TWO of the following:

(i) Baluns.
(ii) Bandwidth of antennas.
(iii) Diversity reception.
(iv) Yagi antennas. (7+7)





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