(SEM VII) THEORY EXAMINATION 2024-25 MICROWAVE & RADAR ENGINEERING

MICROWAVE & RADAR ENGINEERING (KEC074) – COMPLETE SOLVED PAPER

Time: 3 Hours  Max Marks: 100
Instructions: Attempt all Sections

SECTION A (2 × 10 = 20 Marks)

Attempt all questions in brief

a) Standing wave vs Standing Wave Ratio (SWR)

Standing wave: Pattern formed due to interference of incident and reflected waves in a waveguide.

SWR (or VSWR): Ratio of maximum to minimum voltage of a standing wave; indicates impedance matching.

b) Microstrip transmission line

A microstrip line is a planar transmission line consisting of a conducting strip separated from a ground plane by a dielectric substrate.

c) Isolator vs Circulator

Isolator: Allows microwave power to pass in one direction only.

Circulator: Multi-port device where power flows sequentially from one port to the next.

d) Directivity & Coupling factor (Directional coupler)

Directivity: Measure of isolation between forward and backward coupled ports.

Coupling factor: Ratio of input power to coupled power.

e) Limitations of conventional active devices at microwave frequencies

Transit-time effects                                      Parasitic capacitances

Reduced gain                                               Poor efficiency

f) Characteristics of Backward Wave Oscillator (BWO)

Wide frequency tunability                           Negative group velocity

Low noise                                                    Continuous wave operation

g) VSWR and Return Loss

VSWR: Indicates impedance mismatch in a transmission line.

Return Loss: Measure (in dB) of reflected power relative to incident power.

h) Dielectric constant

The dielectric constant is the ratio of permittivity of a material to free-space permittivity, affecting wave propagation velocity.

i) Delay line cancellers in MTI radar

They cancel stationary target echoes (clutter) by subtracting delayed signals, enhancing moving target detection.

j) Doppler effect in CW radar

Frequency shift occurs due to relative motion between radar and target, enabling velocity measurement.

SECTION B (10 × 3 = 30 Marks)

Attempt any three

a) Dominant mode in rectangular waveguide

The dominant mode is TE₁₀, having the lowest cutoff frequency. Field distribution shows maximum electric field across the wider dimension.

b) Directional coupler & 2-hole coupler

A directional coupler samples power in one direction.
In a 2-hole coupler, properly spaced holes ensure constructive interference in the forward direction and cancellation in reverse.
S-matrix shows coupling, isolation, and directivity parameters.

c) Faraday rotation isolator

Uses ferrite material and Faraday rotation to rotate polarization, allowing power flow in one direction only.
Applications: Microwave transmitters, radar systems.

d) Frequency measurement methods

Resonant cavity method                                     Wave meter

Heterodyne method                                           Frequency counter

e) CW radar block diagram & working

CW radar transmits continuous signal and detects Doppler shift in received signal to measure target velocity.

SECTION C (10 × 5 = 50 Marks)

Attempt one from each question

Q3(a) TE modes in circular waveguide

Field components are derived using Bessel functions.
TE modes have no longitudinal electric field (Ez = 0), and cutoff frequency depends on waveguide radius.

Q3(b) Transmission line equation

From Maxwell’s equations:                              d2Vdx2=γ2V\frac{d^2 V}{dx^2} = \gamma^2 Vdx2d2V​=γ2V

Solutions represent forward and backward traveling waves.

Q4(a) Short notes

i) Phase Shifters
Devices that change phase of microwave signals; used in phased array antennas.

ii) Attenuators
Reduce signal power without distortion; used for level control and impedance matching.

Q4(b) Microwave hybrid circuits

Hybrid circuits split power equally with phase difference.
Scattering matrix defines port relationships (e.g., magic tee, hybrid ring).

Q5(a) Traveling Wave Tube (TWT)

TWT amplifies microwaves using interaction between electron beam and RF wave.
Limitations: High cost, complexity, bulky size.

Q5(b) Backward Wave Oscillator (BWO)

Operates on slow-wave structure, where RF wave travels opposite to electron beam.
Provides wide tuning range.

Q6(a) Microwave measurements & VSWR meter

Microwave measurements include power, frequency, impedance, and noise.
VSWR meter measures standing wave ratio using detector and amplifier.

Q6(b) Short notes

i) Impedance measurement                Done using slotted line or network analyzer.

ii) Noise factor measurement             Determines degradation of signal-to-noise ratio by a device.

Q7(a) MTI radar block diagram & operation

MTI radar uses Doppler processing and delay line cancellers to suppress clutter and detect moving targets.

Q7(b) Radar range equation

Rmax⁡=(PtG2λ2σ(4π)3Smin⁡)1/4R_{\max} = \left( \frac{P_t G^2 \lambda^2 \sigma}{(4\pi)^3 S_{\min}} \right)^{1/4}Rmax​=((4π)3Smin​Pt​G2λ2σ​)1/4

Shows dependence of range on transmitted power, antenna gain, wavelength, and target cross-section.