Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech III ANTENNAS AND WAVE PROPAGATION Previous Question Paper (September - 2021 - R13)
Answer any five questions. All questions carry equal marks.
Main
Obtain an expression for maximum effective aperture for λ/2 Antenna.
Answer
The effective aperture Ae of an antenna is related to its directivity D by the fundamental relation Ae = (λ2 / 4π) * D. For a half-wave dipole antenna, the maximum directivity D is approximately 1.64 (or 10/π). Substituting this value, the maximum effective aperture is:
This represents the area over which the antenna effectively captures power from an incident electromagnetic wave.
Show that the radiation resistance of λ/2 antenna is 73 ohm.
Answer
The radiation resistance Rr is defined by the relation Prad = I02 * Rr / 2, where I0 is the peak current at the input terminals. For a half-wave dipole, the current distribution is sinusoidal: I(z) = I0 sin(k(l - |z|)). The radiated power is calculated by integrating the Poynting vector over a far-field sphere:
Substituting the far-field electric field expression E(θ) = (η I0 / 2πr) * [cos((π/2)cosθ) / sinθ], the integral becomes:
The integral evaluates to the cosine integral Ci(x), specifically 0.577 + ln(2π) - Ci(2π) ≈ 1.219. Thus, Prad = (η I02 / 4π) * 1.219. Given η ≈ 120π Ω, we find:
Rounding to the standard value, the radiation resistance is 73 Ω.
Describe the construction and working of helical antenna.
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Draw the structure of pyramidal Horn Antenna and derive the equation for gain of it.
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Describe the construction and working of microstrip antennas.
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Mention the advantages and disadvantages of lens antenna.
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Compare the performance of parabolic reflector and corner reflector.
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Explain geometry of rectangular patch antenna.
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For an end-fire array antenna consisting of 4 isotropic sources separated by λ/2 and fed with equal amplitude, find the expression for distant field, array pattern, and plot the radiation pattern.
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Describe the method of measuring gain of an antenna by 3-Antenna method.
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Find power density at a distance 3 km from an isotropic source if power density at a distance 2 km is 10 m watt/square units.
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What is zoning? What are the advantages in terms of antenna parameters?
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Explain the term wave tilt of surface waves.
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Describe the factors affecting ground wave propagation.
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A distance of 1500 km one is to be covered along earth surface using communication link of reflection region of ionosphere has fc is 6 MHz and fMUF is 7.5 MHz, calculate the height of the region.
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Discuss the duct wave propagation with neat diagrams.
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Obtain an expression for refractive index of the ionosphere in case of ionospheric wave propagation.
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