JNTU
JNTU B.Tech III ANTENNAS AND WAVE PROPAGATION Previous Question Paper (March/April - 2021 - R16)
Answer any five questions. All questions carry equal marks.
Main
Explain the terms: i) Radiation resistance ii) Effective length iii) Radiation intensity.
Answer
i) Radiation Resistance: It is a fictitious resistance that, when substituted for the actual antenna, would dissipate the same amount of power as is actually radiated by the antenna for a given current at its terminals. It is defined as Rr = Prad / Irms2, where Prad is the total radiated power and Irms is the root-mean-square current at the feed point.
ii) Effective Length: For a wire antenna, the effective length (le) is the length of an equivalent linear antenna that would produce the same far-field electric field strength as the actual antenna when carrying a uniform current equal to the maximum current of the actual antenna. It relates the radiated field to the current distribution.
iii) Radiation Intensity: Radiation intensity (U) is defined as the power radiated from an antenna per unit solid angle. It is a far-field parameter independent of the distance from the antenna and is measured in Watts per steradian (W/sr). It is related to the radiation power density (W) by U = r2W.
Calculate D (θ, φ), the directivity for the unidirectional antenna with the power pattern U(θ, φ) = Umsinθsin2φ.
Answer
The directivity D is given by the ratio of maximum radiation intensity to the average radiation intensity: D = 4π Um / Prad. The total radiated power Prad is calculated by integrating the radiation intensity over the entire solid angle:
Evaluating the integrals:
- ∫0π sin2θ dθ = π/2
- ∫0π sin2φ dφ = π/2
Thus, Prad = Um (π/2) (π/2) = Um π2 / 4. Substituting this into the directivity formula:
Derive the field components and radiation resistance of a quarter wave monopole starting from Maxwell’s equations.
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Sketch and explain the construction, operation and design considerations for a Yagi-Uda antenna.
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Distinguish between sectoral, pyramidal and conical horns. Explain their utility.
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Draw the constructional features of microstrip antenna.
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Explain the importance of focal length to diameter ratio of a parabolic reflector.
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Calculate the directivity of a 20 turn helical antenna having α=12°, circumference equal to one wavelength.
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Demonstrate the constructional features of Cassegrain feed.
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Differentiate end-fire and broad side arrays.
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Calculate the directivity of a 10 element uniform linear end-fire array with improved directivity with a spacing of λ/4 between the elements.
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Explain the absolute method for measuring the gain of an antenna.
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What are the features of Binomial array? Explain its feeding concepts.
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What is Wave tilt? How does it affect the field strength at a distance from the transmitter?
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At what frequency a wave must propagate for the D-region to have an index of refraction 0.5? Assume that the Ionic density of D-layer is 400 electrons/cubic cm.
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Write short notes on: (i) Skip distance (ii) Day and night frequency.
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Find the range of LOS system when the receiver and transmitter antenna heights are 10m and 100m respectively. Take effective earth’s radius into consideration.
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