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Week 9: Helical Antennas (Total Marks = 20) Section I (All Question Carry Equal Marks = 1) Question 1

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Week 9: Helical Antennas (Total Marks = 20) Section I (All Question Carry Equal Marks = 1) Question 1 Week 9: Helical Antennas (Total Marks = 20) Section I (All question carry equal marks = 1) Question 1. A Helical Antenna can be operated in _____________ different modes. a) 1 b) 2 c) 3 d) 4 Question 2. In axial mode of helical antenna, if we increase the diameter of helix wire, its bandwidth will: a) Increase b) decrease c) remain almost constant d) first increase then decrease. Question 3. As the number of turns of an axial mode helical antenna decreases, its beamwidth: a) Increases b) decreases c) remains almost constant d) first increases then decreases. Question 4. The condition to obtain circular polarization in normal mode helical antenna is: 2 2 2 2 a) Cλ = 2Sλ b) Cλ = 2Sλ c) Cλ = √2Sλ d) Cλ = 2Sλ Question 5. In general, the approximate value of input resistance (in Ω) in case of normal mode helical antenna for very large ground plane is: a) < 1 b) 10 c) 100 d) > 100 Question 6. If we increase the wire radius in a normal mode helical antenna, then its bandwidth will: a) decrease b) increase c) remains almost same d) First increase then decrease Section II (All question carry equal marks = 2) Question 1. A Helical Antenna with pitch angle equal to 0° or 90° is similar to _______________ or _____________ antenna, respectively. a) Loop, Linear b) Loop, Slot c) Linear, Loop d) None of above Question2. Match the Following: Helical Antenna Mode Circumference (C) Polarization A. Normal I. ≈λ X. Linear B. Axial II. <<λ Y. Circular a) A-I-X, B-II-Y b) A-I-Y, B-II-X c) A-II-X, B-I-Y d) A-II-Y, B-I-X Question 3. For a given value of directivity, if number of helical antennas in an array increase then no. of turns per helix element and spacing between the helix array elements will, respectively a) increase, increase b) increase, decrease c) decrease, increase d) decrease, decrease Question 4. A Helical Antenna shown in Fig. 1 has diameter, D = 20 mm and spacing, S = 5 mm. The axial height and total wire length of the antenna for 4 turns helical antenna are: a) b) c) d) Question 5. An axial mode helical antenna has circumference = 75 mm, α = 13° and number of turns = 6. The input resistance for axial feed at a frequency of 4 GHz will be a) b) c) d) Question 6. What should be value of the axial ratio for the normal mode helical antenna with diameter of helix (D) = 0.012λ, axial length of helix (H) = 0.12λ and number of turns (n) = 6. a) b) c) d) Question 7. To operate helix in axial mode, the approximate pitch angle and minimum number of turns should be, respectively a) 11° and 3 b) 13° and 3 c) 11° and 5 d) 13° and 5 Week 10: Horn Antennas (Total Marks = 20) Section I (All question carry equal marks = 1) Question 1. The maximum aperture phase error (in degree) that can be tolerated in E and H-planes for optimum gain pyramidal horn antenna are respectively (as discussed in this course): a) 45, 90 (ans) b) 90, 45 c) 90, 135 d) 135, 90 Question 2. For pyramidal horn antenna, side lobe levels in E-plane are generally ____________ ascompared to those in H-plane. a) more(ans) b) less c) same Question 3. With increase in axial length (neck to mouth) of a pyramidal horn antenna for a given aperture, the efficiency of the horn will: a) Increase b) decrease c) first increase and then saturate d) remainalmost constant. Question 4. If the Horn antenna aperture increases with constant aspect ratio and fixed horn length, the directivity will a) Increase b) decrease c) first increase and then saturates d) first increase and then decrease. Question 5. The gain of a conical horn antenna is optimum for an aperture phase error (in degree)of approximately (as discussed in this course) a) 10 b) 60 c) 135 d) 180 Question6. In a corrugated horn antenna, number of teeth per wavelength should be between: a) 1-5 b) 1-10 c) 4-10 d) 10-20 Question 7. In a corrugated horn, depth of teeth (in terms of λ) should vary between: a) 0.1-0.25 b) 0.25-0.5 c) 0.5-1 d) greater than 1. Question 8. In a coaxial feed pyramidal horn antenna, the approximate distance between the short wall of the waveguide and the coaxial feed is: a) λg/8 b) λg/4 c) λg/2 d) λg Question 9. A coaxial feed pyramidal horn antenna has the following rectangular waveguide dimensions: Waveguide (WR340): a = 3.4” and b = 1.7”. What will be the cut-off frequency (in GHz) for TE10 mode operation? a) 0.174 b) 0.201 c) 1.737 (ans) d) 2.01 Section III (Marks =11) Question 1. A coaxial feed pyramidal horn antenna is to be operated at 10 GHz with following dimensions: Waveguide (WR90) = 2.3 cm × 1 cm, Aperture = 20 cm ×15 cm, Horn length from neck to mouth = 25 cm. 1.1 The maximum aperture phase error in E-plane is (4) a) 90 b) 126 c) 135 d) 212 1.2 The maximum aperture phase error in H-plane is (4) a) 90 b) 126 c) 135 d) 212 1.3 Assuming approximate efficiency of horn is 50%, then approximate gain of the antenna in dB will be equal to: (3) a) 24 (ans) b) 26 c) 48 d) 52 Week 11: Yagi-Uda and Log-periodic Antenna Arrays (Total Marks = 20) Section I (All question carry equal marks = 1) Question 1. As the number of elements in a Yagi-Uda antenna array increases, gain of the antenna: a) Increases b) decreases c) remains nearlyconstant d) First increases then saturates Question 2. A Log-periodic antenna array is generally used to increase: a) Gain b) Bandwidth c) Both a and b d) None of these Question 3. Spacing factor ‘σ’ of a Log-periodic dipole array with dipole length ‘Ln’ and spacing between the adjacent dipole elements ‘dn’ is given by: a) Ln/dn b) dn/Ln c) 2Ln/dn d) dn/ (2Ln) Question 4. Which of the following represents the radiation pattern of a Log-periodic antenna array. a) Broadside b) End-fire c) Bi-directional d) Omni-directional Question 5. In a Log-periodic antenna array, all the successive dipole elements are fed with: a) Equal phase b) Phase quadrature c) Out of phase by 180° d) Any arbitrary phase Question6. In a Log-Periodic antenna array, diameter of all the dipole elements should be: a) Equal b) vary with scaling factor ‘τ’ c) vary with a spacing factor ‘σ’ d) Vary with a factor ‘τσ’ Section II (All question carry equal marks = 2) Question 1. A Yagi-Uda antenna array with 3-elements (one reflector, one driven and one director) can provide an approximate gain (in dB) of: a) 2 b) 5 c) 7 d) 10 Question 2. In 3-elements Yagi-Uda antenna array, the lengths of reflector, driven and director should be in the range of (in terms of λ) respectively: a) 0.4-0.45, 0.45-0.49, 0.5-0.55 b) 0.45-0.49, 0.5-0.55, 0.4-0.45 c) 0.5-0.55, 0.4-0.45, 0.45-0.49 d) 0.5-0.55, 0.45-0.49, 0.4-0.45 Question 3. A Yagi-Uda antenna array generally provides _________ gain and ________ bandwidth compared to a Log-Periodic antenna array. a) better, better b) better, lower c) lower, better d) lower, lower Section III (All Questions carry equal marks = 4) Question 1. To design an optimum gain Log-periodic antenna array with σ opt = 0.16 and τopt = 0.86 over the frequency range from 700 MHz to 2500 MHz, then minimum number of dipole elements required is: (4) a) 8 b) 9 c) 10 (ans) d) 11 Question 2. A Yagi-Uda antenna is designed at 2.45 GHz. 2.1 What should be the approximate spacing between driven and reflector element? (2) a) b) c) (ans) d) 2.2 What should be the approximate spacing between driven and director element? (2) a) b) c) (ans) d) Week 12: Reflector Antenna Section I (All question carry equal marks = 1) Question 1. Cassegrain feed is generally used instead of prime focus feed with parabolic reflector to affect which of the following factor? a) Impedance b) Bandwidth c) Gain d) To place the feed at convenient place for high power transmitter Question 2. To increase the gain of a corner reflector antenna, the corner angle ‘α’ should be: a) Increased b) Decreased c) gain does not depend on ‘α’ d) None of above Question 3. To increase the taper efficiency of a parabolic reflector antenna, generally the feed antenna should have: a) High beamwidth b) Low beamwidth c) High Bandwidth d) Low Bandwidth Question 4. To increase the spill-over efficiency of a parabolic reflector antenna, generally the feed antenna should have: a) High beamwidth b) Low beamwidth c) High Bandwidth d) Low Bandwidth Question 5. If the aperture angle of a parabolic reflector antenna increases, then the aperture efficiency of the antenna will: a) Increase b) decrease c) remain almost constant d) First increase then decrease Question 6. To measure the gain and radiation pattern of a test antenna, the minimum distance (in terms of λ) required between test antenna and reference antenna is given by: a) 2d2/λ b) d2/λ c) 2d/λ2 d) 2d2 where d is maximum dimension of the antenna.
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