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Code No: R05310403 Set No. 1

III B.Tech I Semester Regular Examinations, November 2008ANTENNAS AND WAVE PROPAGATION

( Common to Electronics & Communication Engineering and Electronics &Telematics)

Time: 3 hours Max Marks: 80Answer any FIVE Questions

All Questions carry equal marks⋆ ⋆ ⋆ ⋆ ⋆

1. (a) Define and Explain: Directivity and power gain for an antenna. What is therelation between the two? Prove that the directivity of a λ/2 aerial is 0.39 dbmore than that of short dipole.

(b) What are principal planes? How the antenna beam width is defined in suchplanes? [10+6]

2. (a) Derive an expression for the Radiation Resistance of a Loop Antenna.

(b) Find the Effective Length of a Half Wave Dipole.

(c) A Small current element at 10MHz produces a radiated field in a directionmaking angle θ with the element. Assuming that the current is 10 Amp.Calculate the power radiated and radiation resistance of the Antenna.[6+6+4]

3. (a) Derive an expression for radiated power(Rp)of a BSA with ‘n’ vertical dipoles.Plot the Rp in vertical and horizontal planars for a BSA of 4 dipoles.

(b) Show the directivity of EFA(with increased directivity) is given by 1.789[4(L/λ)].[8+8]

4. (a) Derive an expression for electric field of a Non Resonant antenna of length ‘l’carrying current.

(b) Sketch and explain the constructional features of a Helical Antenna. [10+6]

5. (a) Describe the performance of paraboloids with respect to aperture blocking.

(b) Describe the constructional details of cassegrain antennas and sketch its radi-ation characteristics. [8+8]

6. (a) What is an Optimum Horn? Sketch and Explain its characteristics along withdimensional relations.

(b) Explain the first transmission formula and its applicability for antenna gainmeasurements. [8+8]

7. (a) Explain the following :

i. Ground Wave

ii. Space Wave

iii. Surface Wave.

(b) Discuss the characteristics of ionospheric layers. [8+8]

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8. (a) Space wave propagates between transmitting and receiving stations of heights‘h1’ and ‘h2’ respectively. Derive the expression for field strength.

(b) What are the different paths used for propagating radio waves from 300KHzand 300MHz. [8+8]

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1. (a) The Directivity

i. equal to number of point surfaces in the sky that the antenna can resolve.Justify this statement.

(b) The field pattern of an antenna is given by En=(Sin θ/θ) (Sin φ/φ)

i. Plot the Normalized power pattern.

ii. Estimate the main beam efficiency of the antenna. [8+8]

2. (a) Explain the terms ‘Radiation Field’, ‘Induction Field’ and ‘Electrostatic Field’by deriving the E Field of a current element through vector potential.

(b) What is Polarization? How many types of Polarizations are used in Antennas?Explain. [8+8]

3. (a) Find the radiation Pattern of 4 isotropic elements fed in phase, spaced λ/2apart by using pattern multiplication.

(b) What is uniform linear array? Discuss the application of linear array. [8+8]

4. (a) Write short notes on “Helical Antenna”.

(b) Derive an expression for radiated electric field strength of a traveling waveradiation of length ‘l’. [8+8]

5. (a) What is a parabolic element. How does a parasitic element act when lengthis greater than and smaller than λ/2.

(b) Distinguish between Spherical and Cylindrical paraboloids. Comment on theiraperture efficiency and applications. [8+8]

6. (a) What is the principle of equality of path length? How is it applicable to hornantenna?

(b) Discuss how the directivity of horn antenna can be measured. [8+8]

7. (a) What are different mechanisms of propagation of electromagnetic waves? Ex-plain.

(b) What is Critical Frequency? What is Virtual Height? Find the maximumdistance that can be covered if the virtual height of the ionospheric layer is250kms. [8+8]

8. (a) Establish the mathematical relation for

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i. Radio Horizon

ii. Radius of curvature of array path for LOS waves.

(b) Show that the field strength due to space wave given byE = (2E

0/d)sin(2πhthr/λd). [8+8]

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1. (a) Explain the following :

i. Normalized Field Pattern

ii. Beam Solid Angle

iii. Beam Efficiency

iv. Directivity.

(b) An antenna has a field pattern given by E (θ) =cos θ cos 2 θ for 00 <= θ <= 900.Find

i. the HPBW

ii. Beam Width between first nulls. (FNBW) [8+8]

2. (a) Show that for a slot in a conducting plane, the impedance Zslot is given byZdipole Zslot=Z0

2/4 where Z0 is the impedance of free space.

(b) The radiation intensity of a particular antenna is given by Ø(θ, Ø) = sin2θ.Calculate the directivity of the antenna.

(c) Define the effective aperture and calculate the effective aperture of 0.25λ di-pole. [8+4+4]

3. (a) Enumerate the advantages and disadvantages of linear arrays.

(b) Find the phasing required to steer a beam zenith to -400 for a 5 element arraywith 0.4λ inter element spacing. [8+8]

4. (a) What is meant by Non Resonant antenna? Derive from the first principle, thefield due to such antenna and sketch the field pattern for lengths λ and 2λ.

(b) Mention the merits and demerits of a traveling wave antenna. [8+8]

5. (a) A Paraboloid reflector of 1.8m diameter is used at 6GHz. Calculate the beamwidth between the nulls and gains in dBs.

(b) Design Yagi Uda antenna of six elements to provide a again of 12dB if theoperating frequency is 200MHz. [8+8]

6. (a) How is the field pattern of a receiving antenna experimentally determined.Explain with a neat diagram.

(b) Distinguish between Curved Surface Zoning and Plane Surface Zoning of lensantenna. Discuss their merits and demerits. [8+8]

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7. (a) What is Angle of Tilt? How does it affect the field strength at a distance fromthe transmitter?

(b) A 150m antenna transmitting at 1.2MHz by ground wave has an antennacurrent of 8A. What voltage is received by a receiving antenna 40kms away,with a height of 2meters? [8+8]

8. (a) A police radio transmitter operating at a frequency 1.69GHz is required toprovide a ground wave having a strength of 0.5mv/m at a distance of 16km.The transmitter antenna, having an efficiency of 50% produces a radiatingfield proportional of cosθ. The ground wave has σ= 5× 10−5 mho/cm and εr

= 15. Calculate the power transmitted.

(b) Derive the fundamental equation for free space propagation. [8+8]

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1. (a) Define and account for the presence of

i. Radial Power Flow

ii. Radiation resistance for a short dipole

iii. Uniform Current Distribution.

(b) Calculate the following for an antenna carrying 50A (rms) at 480 kHz havingeffective length of 60.96metres. Take loss resistance of the antenna= 5 Ω.

i. Radiation resistance

ii. Power radiated

iii. Antenna efficiency

iv. Directivity and power gain

v. HPBW and BWFN. [6+10]

2. (a) Define the following terms:

i. Directivity

ii. Gain

iii. Effective Aperture of Antennas.

(b) Show that the Directivity of an elementary Dipole (Current Element) is 1.5or 1.76dB.

(c) Calculate the power gain of a half wave Dipole whose ohmic losses and Direc-tive gain are 7.0 Ohms and 1.64 respectively. [6+6+4]

3. (a) Explain the concept of scanning arrays.

(b) Show that Dolph-Tchebyshelf distributions gives optimum distribution andminimum side lobe level for a given beam width of major lobe. [8+8]

4. (a) Distinguish between

i. Resonant and Non Resonant antennas.

ii. Narrow Band and Wide Band antennas.

(b) Show that a single wire excited with a traveling wave current is an end fireantenna with a sharp null along the direction of extension of the wire. [8+8]

5. (a) A paraboloid reflector has radiation characteristics whose HPBW is 50. Findout its null to null beam width and power gain.

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(b) Discuss the conditions under which a parasitic dipole placed near and parallelto a driven dipole can act as a reflector. [8+8]

6. (a) With a neat setup, explain the method of measurement of the beam width ofpyramidal horn antennas in E-plane. How does it differ from H-plane mea-surement?

(b) Explain the basic principle of operation of lens antenna. [8+8]

7. (a) Discuss the propagation characteristics of EM wave.

(b) Mention the salient features of Ground wave propagation. [8+8]

8. (a) Derive an expression for the curvature of the ray path due to the changingrefractive index in the troposphere.

(b) Explain the phenomenon of duct propagation. [8+8]

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