ANTENNA AND WAVE PROPAGATION (KEC-603)
B.Tech Semester VI – Theory Examination 2021–22
ANTENNA-AND-WAVE-PROPAGATION-KE…
Antenna and Wave Propagation is a core subject in electronics and communication engineering that deals with the generation, transmission, radiation, and reception of electromagnetic waves, along with the behavior of these waves as they propagate through different media such as free space, atmosphere, and the ionosphere. This subject forms the foundation of wireless communication, radar systems, satellite communication, broadcasting, and navigation systems.
The uploaded question paper clearly shows that the examination emphasizes electromagnetic field theory, vector calculus, antenna fundamentals, radiation patterns, antenna parameters, array theory, and different modes of wave propagation including ground wave, sky wave, and space wave. To score well, answers must be written in continuous, well-structured paragraphs, supported by mathematical expressions, physical interpretation, and neat explanation of diagrams.
SECTION A – BASIC ELECTROMAGNETIC AND ANTENNA CONCEPTS
(Based on Section A on Page-1 of the paper)
An irrotational field is defined as a vector field whose curl is zero everywhere. Physically, this means that the field does not exhibit any rotational or swirling behavior, and mathematically it implies that the field can be expressed as the gradient of a scalar potential. Electrostatic fields are classic examples of irrotational fields.
Divergence is a scalar quantity that measures the net outward flow of a vector field from an infinitesimal volume. In electromagnetic theory, divergence provides information about the presence of sources or
sinks, such as electric charges in an electric field.
Electric field intensity is defined as the force experienced by a unit positive test charge placed in an electric field. It is a vector quantity and its direction is the same as the force acting on a positive charge. Electric field intensity plays a key role in antenna radiation and wave propagation.
The magnetic flux lines are always closed loops and never intersect each other. Unlike electric field lines, magnetic field lines do not begin or end on charges, indicating the absence of magnetic monopoles. Their density indicates the strength of the magnetic field.
The concept of solid angle and beam area is essential in antenna theory. A solid angle represents the three-dimensional angle subtended by a surface at a point, while beam area is the solid angle through which most of the antenna’s power is radiated.
The principal radiation pattern parameters include beam width, half-power beam width, side-lobe level, front-to-back ratio, directivity, and gain. These parameters collectively describe how an antenna radiates energy in space.
The log-periodic antenna is a frequency-independent antenna whose impedance and radiation characteristics repeat periodically with the logarithm of frequency. Its design involves scaling successive dipole lengths and spacings by a constant factor, making it suitable for wideband applications.
The folded dipole antenna offers higher input impedance compared to a simple dipole, making impedance matching easier. This is its major advantage and is widely used in TV and FM broadcasting.
The critical frequency is the maximum frequency that can be reflected by the ionosphere at vertical incidence. It depends on the maximum electron density of the ionosphere and is a crucial parameter in sky-wave communication.
Surface wave propagation refers to electromagnetic waves that travel along the surface of the earth. These waves follow the curvature of the earth and are significant at low frequencies, especially for AM broadcasting.
SECTION B – FIELD THEORY, ANTENNAS AND WAVE PROPAGATION
(Based on Section B on Page-1)
Stokes’ theorem and Divergence theorem are fundamental vector calculus theorems used extensively in electromagnetic field analysis. Stokes’ theorem relates a surface integral of curl to a line integral around the boundary, while the divergence theorem relates the volume integral of divergence to the flux through a closed surface. These theorems simplify Maxwell’s equations and help in field interpretation.
The magnetic field due to a finite line conductor carrying current is derived using the Biot–Savart law. The resulting expression shows that the magnetic field strength depends on the current, distance from the conductor, and the angles subtended by the conductor at the observation point.
Antenna temperature is a measure of noise power received by an antenna from its surroundings. It is directly related to the signal-to-noise ratio, as higher antenna temperature increases noise power and degrades SNR.
The analysis of horizontal antennas above a plane ground involves the concept of image theory. The ground is replaced by an image antenna, and the resulting radiation pattern is obtained by superposition of direct and reflected waves.
Skip distance and optimum frequency are important concepts in sky-wave propagation. Skip distance is the minimum distance between transmitter and receiver for sky-wave communication, while optimum frequency ensures reliable communication over long distances without excessive absorption or loss.
SECTION C – VECTOR CALCULUS AND BOUNDARY CONDITIONS
(Based on Section C on Page-1)
Line, surface, and volume integrals are mathematical tools used to describe electromagnetic quantities distributed over lines, surfaces, and volumes respectively. Line integrals are used for electric field work, surface integrals for flux, and volume integrals for charge or energy calculations.
The curl of a vector signifies the rotational tendency of a field. In electromagnetic theory, curl relates electric fields to time-varying magnetic fields and vice versa, forming the basis of Faraday’s law.
The boundary conditions for magnetic fields at dielectric–dielectric and dielectric–free space interfaces ensure continuity of tangential magnetic field intensity and normal magnetic flux density, which is essential for correct field behavior at material boundaries.
RADIATION, DIPOLES AND ANTENNA PARAMETERS
(Based on Page-2 of the paper)
The electric field due to a uniformly charged circular ring along its axis is derived using Coulomb’s law. The maximum electric field occurs at a specific distance from the center, and when the radius tends to zero, the field reduces to that of a point charge.
The Friis transmission formula relates transmitted power, received power, antenna gains, wavelength, and distance between antennas. It is fundamental in link budget analysis.
Fields from oscillating dipoles form the basis of antenna radiation. The relationship between directivity and gain shows that gain is the product of directivity and antenna efficiency.
The fields of a short dipole are derived by considering sinusoidal current distribution and show near-field and far-field components.
The radiation pattern of an array of isotropic sources is obtained using pattern multiplication, which combines individual element pattern and array factor.
WAVE PROPAGATION AND IONOSPHERE
(Based on Question 7 on Page-2)
Sky-wave propagation allows long-distance communication by reflection from the ionosphere. When the curvature of the earth is considered, the skip distance increases and affects coverage.
The refractive index of the ionosphere depends on electron density and operating frequency. The expression for critical frequency is derived from plasma theory and is essential for predicting ionospheric reflection.
HOW TO WRITE ANTENNA & WAVE PROPAGATION ANSWERS IN THE EXAM
In Antenna and Wave Propagation, never write answers in short bullet points. Always begin with a clear definition, followed by mathematical expression, physical interpretation, and practical relevance. Wherever diagrams are required, explain them in words along with equations. Examiners give high weightage to conceptual clarity, correct derivation, and logical explanation of electromagnetic behavior.
