(SEM VI) THEORY EXAMINATION 2024-25 ANTENNA AND WAVE PROPAGATION

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

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=Vmin​Vmax​​ 

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=4πε0​1​rq​

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