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. Front to back ratio of an antenna can be increased by

(A) Sacrificing its gain.

(B) Increasing its size.

(C) Using material of high conductivity.

(D) None of the above.

b. Antennas commonly used for microwave links are

(A) Loop antennas. (B) Log periodic antennas.

(C) Rhombic antennas. (D) Paraboloidal dishes.

c. The troposphere extends from earth surface to a height of

(A) 5 Km. (B) 15 Km.

(C) 50 Km. (D) 90 Km.

d. The skip distance for radio wave increases with

(A) Increase in frequency. (B) Decrease in frequency.

(C) Atmospheric disturbances. (D) Temperature of atmosphere.

e. The directive gain may have a numerical value between

(A) 0 to 1. (B) 0 to 10.

(C) 0 to . (D) -1 to +1.

f. The quality factor of an antenna is

(A) Directly proportional to bandwidth.

(B) Directly proportional to square of the bandwidth.

(C) Inversely proportional to bandwidth.

(D) Inversely proportional to square of the bandwidth.

g. Polarization of electromagnetic wave is due to

(A) Reflection. (B) Refraction.

(C) Its longitudinal nature. (D) Its transverse nature.

h. The bandwidth of a folded dipole in comparison to that of an unfolded dipole is

(A) About 50% more. (B) About 10% more.

(C) About 50% less. (D) About 10% less.

PART I

Answer any THREE Questions. Each question carries 14 marks.

Q.2 a. Derive the relationship between E & H for a uniform plane wave and show that E & H are transverse. (6)

b. Compute

(i) the intrinsic impedance (ii) attenuation constant

(iii) phase shift constant (iv) the depth of penetration,

for a medium with at a frequency of 3 GHz. (8)

Q.3 a. Including relevant information draw the radiation pattern for a travelling wave antenna of 6 wavelengths long. (4)

b. Draw Radiation pattern of two non-directional radiators fed with equal currents with phase difference and with separation. (4)

c. Calculate the radiation resistance of wire dipole in free space. (2)

d. How directive gain, power gain, radiation intensity and effective area are related. (4)

Q.4 a. State and explain antenna theorems. (4)

b. Derive conditions required for an array to become an end-fire array or broadside array. Obtain parameters of radiation pattern for any one of these. (6)

c. Illustrate the vertical radiation pattern of resonant and non-resonant antennas for lengths L = , 2 and 4. (4)

Q.5 a. Classify the lens antennas in all possible ways. (6)

b. Illustrate and discuss the shapes, reflecting surfaces, feed methods and shadowing effects for lens antennas. (8)

Q.6 The general expressions for reflection coefficients for oblique incidence of horizontally and vertically polarized waves are

and

respectively. Using these relations derive expressions and illustrate variation with different parameters for for ground wave propagation. (14)

PART II

Answer any THREE Questions. Each question carries 14 marks.

Q.7 a. A plane electromagnetic wave having a frequency of 20 MHz has an average Poynting vector of 10 W/m². If the medium is lossless with relative permeability 4 and relative permittivity 5 find

(i) The velocity of propagation.

(ii) The wavelength.

(iii) The impedance of the medium and

(iv) The RMS electric field E. (10)

b. Define retarded potential and discuss its significance. (4)

Q.8 Differentiate between the following terms :

(i) Gain, Power gain and Directive gain. (3)

(ii) Effective aperture and collecting aperture. (3)

(iii) Establish relation between a current element and an electric dipole. (4)

(iv) Explain effect of ground on radiation patterns. (4)

Q.9 a. Draw a complete diagram of a helical antenna illustrating parameters involved therein. (6)

b. With necessary illustrations explain the transmission and radiation modes of helical antennas. Mention its possible applications. (8)

Q.10 a. List the advantages and disadvantages of discontinuous reflector surfaces. (5)

b. What is universal feed horn curve? Where and how it is used? (5)

c. Differentiate between radiating and non-radiating slots. Give one example of each using necessary illustrations. (4)

Q.11 a. Calculate the angular aperture for Paraboloidal reflector antenna for which aperture number is 0.3. Given that diameter of the reflector mouth is 20m, calculate the position of the focal point with reference to the reflector mouth. (7)

b. Describe the characteristics of electromagnetic waves in different ranges of frequencies in references to the variations in properties of earth, space and ionosphere. (7)