R5310405-antennas And Wave Propagation

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Code No: R5310405

III B.Tech I Semester(R05) Supplementary Examinations, May 2009 ANTENNAS AND WAVE PROPAGATION (Electronics & Communication Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Define Directivity and Gain of an antenna. Bring out the relation between Directivity and Gain. (b) The Normalized field pattern of an end fine array is given by En=Sin(π/2N)Sin(nψ/2)/Sin(ψ/2) where ψ =dr (cosφ -1)- π/n dr= π/2 n=10. i. Calculate the Gain G ii. Calculate the Approximate Gain. [8+8] 2. (a) From the expressions for E and H due to radiating element, show the average power radiated is given by Pav = Errms / 2n w/m2 (b) Explain how an antenna radiates power into free space and list out the properties of an antenna. [6+10] 3. (a) Derive the expressions for width of principal beam for i. broadside array ii. end fire array of n isotropic elements of spacing distance ‘d’ between successive elements. (b) Describe Binomial array and state its merits and demerits. [10+6] 4. (a) Explain the construction of Rhombic Antenna with balloon and termination. Draw the radiation pattern of rhombic antenna i. over perfectly conducting plane. ii. in the presence of earth. (b) Derive electric field expression for Non Resonant antenna. [8+8] 5. (a) Explain how the radiation pattern of folded dipole will be modified with the addition of a reflector and two directors parasitic elements. (b) Bring out the differences between Active and Passive Corner Reflectors. What are Retro Reflectors? [8+8] 6. (a) Explain why the beam widths in the two perpendicular planes of radiation of a square pyramidal horn will be different. Derive gain formula. (b) What is an antenna test range? What is minimum distance at which test should be taken for an antenna 5m in diameter and working at 6GHz? Derive the necessary expression. [8+8] 7. (a) How are Ionospheric layers formed and how does it allow radio propagation? Explain. (b) Derive an expression for critical frequency and MUF for an ionospheric layers. [8+8] 8. (a) Find the maximum distance that can be covered by a space wave when antenna heights are 80m and 50m. ‘ (b) Explain the concept of Super Refraction. [8+8] ?????

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Code No: R5310405

III B.Tech I Semester(R05) Supplementary Examinations, May 2009 ANTENNAS AND WAVE PROPAGATION (Electronics & Communication Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Write short notes on Gain of an antenna. (b) Sketch the radiation pattern of a half wave dipoles on i. Horizontal plane. ii. Vertical plane.

[8+8]

2. (a) Show that the radiation resistance of a small loop is equal to 320π 4(A/λ2 ) ohms where A is loop area. (b) What is Folded Dipole ? Find its Radiation Resistance.

[8+8]

3. (a) Explain the concept of scanning arrays. (b) Show that Dolph-Tchebyshelf distributions gives optimum distribution and minimum side lobe level for a given beam width of major lobe. [8+8] 4. (a) Describe the construction and properties of Rhombic Antenna. (b) Describe the characteristics of long wire traveling wave antenna. Sketch their pattern for lengths of i. λ/2 ii. 5 λ iii. 20 λ.

[10+6]

5. (a) With reference to paraboloids, explain i. Aperture Blocking. ii. FID ratio. iii. Spill over. (b) Evaluate the power gain directing and the required diameter of a paraboloid having a null beam width of 100 at 3GHz. [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 lens antenna. Discuss their merits and demerits. [8+8] 7. It is defined to establish short wave communication between two points on earths surface(assumed flat) separated by 1200km. Calculate MUF and angle of take off of the transmitted wave from the following: Highest signal frequency returned to earth after vertically upward propagation=7.2MHz. Virtual height of the ionized layer=200kms. Deduce the working formula. [16] 8. (a) Explain how tropospheric ducts are formed. Discuss the mechanism of wave propagation inside the ducts. (b) Explain how are the problems of severe fading partly occur. ?????

[8+8]

3

Code No: R5310405

III B.Tech I Semester(R05) Supplementary Examinations, May 2009 ANTENNAS AND WAVE PROPAGATION (Electronics & Communication Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) What is Radiation Resistance of an antenna. (b) An antenna has effective height of 80m, with a current at base being 200amps(rms) at 25Hz. Find the antenna efficiency. (c) Define Antenna beam width and Directivity and obtain the relation between them. [4+8+4] 2. (a) Obtain the Expression for the Radiation Fields of Half Dipole and Draw the pattern in two planes. (b) Explain the effect of Earth in evaluating radiating fields from Antennas close to Earth. [8+8] 3. (a) Explain what are meant by Collinear, End fire and Broadside disposition of antenna arrays. (b) Prove that the directivity of a BSA of two identical isotropic in phase point sources spaced distance ‘d’ apart is given by D(θ, Ø) = 2/[1 + Sinßd/ßd]. [8+8] 4. (a) Explain the construction of Rhombic Antenna with balloon and termination. Draw the radiation pattern of rhombic antenna i. over perfectly conducting plane. ii. in the presence of earth. (b) Derive electric field expression for Non Resonant antenna.

[8+8]

5. (a) Compute the gain, principle beam width and HPBW of a 10m diameter parabolic dish with a half wave length dipole feed in focus at 10GHz. (b) Explain the Cassegrain mechanism in transmission mode. List out the advantages and disadvantages of Cassegrain feed. [8+8] 6. (a) With a neat setup, explain the method of measurement of the beam width of pyramidal horn antennas in E-plane. How does it differ from H-plane measurement? (b) Explain the basic principle of operation of lens antenna.

[8+8]

7. (a) Discuss the features that lead to fading and attenuation in ionospheric propagation. (b) What is Wave Tilt? How does it effect the field strength received at a distance from the transmitter? [8+8] 8. (a) Give brief account of atmospheric effects in space wave propagation. (b) Explain the mechanism of Troposcatter. ?????

[8+8]

4

Code No: R5310405

III B.Tech I Semester(R05) Supplementary Examinations, May 2009 ANTENNAS AND WAVE PROPAGATION (Electronics & Communication Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Discuss the current distribution on a thin wire antenna. (b) Define the following parameters related to antenna i. Main and Side lobe ii. Beam Area iii. Gain and Directivity.

[8+8]

2. (a) Define i. Effective Length ii. Effective Aperture. (b) Calculate the relative amplitudes of Radiation, Induction and Electrostatic Fields at a distance of 2λ from the short current element. (c) Prove that if Power radiated (Pr) is expressed in terms of capacitance and voltage and if the voltage is held constant, that the radiated power will then proportional to the fourth power of frequency for a Hertzian Di-Pole. [4+6+6] 3. (a) Define Broadside and End fire arrays. What are the conditions for a linear array of N isotropic elements to radiate in end fire and broadside arrays. (b) State the advantages of Dolph-Tchebyshelf distribution. (c) Find the first null beam width of BSA and EFA when array length is 20λ and number of elements is 20. [8+4+4] 4. (a) Compute the optimum direction of V antenna with leg lengths of l= 2 λ and l=3 λ. (b) Show that input impedance of helical antenna operating in axial mode is 50 ohms. [8+8] 5. (a) Explain the working of Yagi Antenna. (b) Compare Corner reflector and Parabolic reflector.

[8+8]

6. (a) Describe the method of measuring the gain and radiation pattern of an antenna. (b) A standard gain horn antenna with a power gain of 12.5, is used to measure the gain of a large directional antenna by comparison method. The test antenna is connected to the receiver and an attenuator adjusted to 23dB in order to have the same receiver output. Find out the gain of the large antenna. [8+8] 7. (a) Explain the following: i. Ray Path ii. Skip Distance iii. MUF. (b) What is meant by critical frequency? Describe a method to measure critical frequency. [8+8] 8. (a) At what equal heights would each of two UHF antennas have to be placed for each to be on the radio horizon of the other, if the distance between the two is 100 kms. (b) Explain the terms LOS and Radio Horizon. [8+8] ?????

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