About this previous year paper
BEC603 – ANTENNA AND WAVE PROPAGATION
Time: 3 Hours | Max Marks: 70
SECTION A – Short Answer Questions
(2 × 7 = 14 marks | Attempt ALL)
Write definition + diagram where applicable
a. End Fire Array
An end fire array is a linear antenna array in which maximum radiation occurs along the axis of the array.
Radiation Pattern: Main lobe directed along the array axis (θ = 0° or 180°).
b. Applications of Helical Antenna Satellite communication
GPS systems Spacecraft telemetry
Circularly polarized communication
c. Conduction Current vs Displacement Current
| Conduction Current | Displacement Current |
|---|---|
| Due to flow of charges | Due to time-varying electric field |
| Exists in conductors | Exists in dielectrics/vacuum |
| Given by J = σE | Given by J = ε ∂E/∂t |
d. Why Magnetic Dipole Does Not Exist?
Magnetic monopoles (isolated north or south poles) do not exist in nature; magnetic fields are always produced by current loops or dipoles.
e. VSWR (Voltage Standing Wave Ratio)
VSWR is the ratio of maximum voltage to minimum voltage on a transmission line.
VSWR=VmaxVminVSWR = \frac{V_{max}}{V_{min}}VSWR=VminVmax
f. Beam Solid Angle & Antenna Beam Efficiency
Beam Solid Angle (ΩA): Solid angle subtended by main lobe
Beam Efficiency: Ratio of power radiated in main lobe to total power radiated
g. Modes of Wave Propagation Ground wave propagation
Sky wave propagation Space wave propagation
SECTION B – Medium Answer Questions
(7 × 3 = 21 marks | Attempt ANY THREE)
Write statement → explanation → diagram/example
a. Divergence Theorem
The divergence theorem states that the flux of a vector field through a closed surface equals the volume integral of its divergence.
∭(∇⋅A⃗)dV=∬A⃗⋅dS⃗\iiint (\nabla \cdot \vec{A}) dV = \iint \vec{A} \cdot d\vec{S}∭(∇⋅A)dV=∬A⋅dS
Application: Electrostatic flux through closed surfaces.
b. End Fire Array of N Sources Equal amplitude and spacing
Progressive phase shift applied Main beam along array axis
Direction of Pattern Minima:
Occurs when phase difference causes destructive interference.
c. Log-Periodic Antenna
Structure:
Multiple dipoles of varying lengths arranged logarithmically.
Why Frequency Independent?
Its characteristics repeat periodically with the logarithm of frequency, making it suitable for wideband
applications.
d. Vector Calculus Problem Given on paper:
Find
(i) Gradient of V
(ii) Divergence of Q
(iii) Curl of P
Use standard vector calculus operators (∇).
e. Electric Potential Due to Point Charge Electric potential at distance r from charge q:
V=14πε0qrV = \frac{1}{4\pi \varepsilon_0}\frac{q}{r}V=4πε01rq
Derived using work done against electric field.
SECTION C – Attempt ANY ONE
(7 marks)
a. 3-Element Yagi-Uda Antenna
Elements: Reflector (≈ 0.55λ) Driven element (≈ 0.5λ) Director (≈ 0.45λ)
Spacing: Reflector–Driven: 0.2λ
Driven–Director: 0.15λ
Draw neat labelled diagram.
b. Short Notes (Any Two) Physical significance of divergence and curl
Displacement current Physical significance of Maxwell’s IV equation