SECTION A – Short Answers (2 Marks Each)
(a) Losses present in waveguides
Waveguide losses include conductor loss due to finite conductivity of walls, dielectric loss due to imperfect dielectric filling, radiation loss at bends and discontinuities, and leakage loss at joints.
(b) Characteristic impedance
Characteristic impedance is the ratio of voltage to current of a travelling wave on a transmission line or waveguide under matched conditions.
(c) Directivity and coupling factor
Directivity is the ability of a directional coupler to distinguish between forward and backward waves.
Coupling factor is the ratio of input power to coupled power, expressed in dB.
(d) Properties of S-matrix
S-matrix is square and symmetric for reciprocal networks, unitary for lossless networks, and elements are bounded between 0 and 1.
(e) Limitations of conventional tubes at microwave frequencies
At microwave frequencies, conventional tubes suffer from transit time effects, parasitic capacitance and inductance, high losses, and reduced efficiency.
(f) Use of slow-wave structure in TWT
Slow-wave structures reduce phase velocity of RF waves to match electron beam velocity, enabling continuous interaction and amplification.
(g) VSWR
Voltage Standing Wave Ratio (VSWR) is the ratio of maximum to minimum voltage on a transmission line, indicating impedance mismatch.
(h) Dielectric constant
Dielectric constant is the ratio of permittivity of a material to the permittivity of free space.
(i) Low-angle tracking in RADAR
Low-angle tracking is used to accurately track targets flying close to the ground by minimizing multipath interference.
(j) Pulse repetition frequency (PRF)
PRF is the number of radar pulses transmitted per second.
SECTION B – Long Answers (10 Marks Each)
(a) TE₁₀ mode in rectangular waveguide
The TE₁₀ mode is the dominant mode in rectangular waveguides. Electric field has no longitudinal component, and magnetic field has a longitudinal component.
TE₀₁ and TM₁₀ modes do not exist because boundary conditions cannot be satisfied simultaneously.
(b) Faraday rotation isolator
A Faraday rotation isolator uses ferrite material under magnetic bias to rotate plane of polarization. It allows wave propagation in one direction and absorbs reflected waves in the reverse direction.
Applications include protection of microwave sources and radar systems.
(c) TWT vs Klystron amplifier
Klystrons provide high gain but narrow bandwidth, while TWTs provide wide bandwidth and moderate gain.
In TWT, continuous interaction between electron beam and RF wave causes amplification.
(d) Microwave measurements and VSWR meter
Microwave measurements involve power, frequency, impedance, and VSWR.
A VSWR meter measures standing wave ratio using a detector and calibrated scale to indicate mismatch.
(e) CW radar with non-zero IF
CW radar with non-zero IF uses frequency offset to avoid leakage problems. Doppler frequency shift provides target velocity information.
SECTION C – Long Answers (10 Marks Each)
3(a) TE mode in circular waveguide
Field components are derived using Maxwell’s equations in cylindrical coordinates. TE modes have no longitudinal electric field, and cutoff frequency depends on Bessel functions.
3(b) Numerical: TE₁₁ mode
For a circular waveguide of diameter 12 cm:
Cut-off frequency
fc=1.841cπdf_c = \frac{1.841c}{\pi d}fc=πd1.841c
Guide wavelength and wave impedance are calculated using standard equations at 2.5 GHz.
4(a) Four-port circulator
A circulator allows power to flow sequentially from one port to the next.
For an ideal circulator, S-matrix is cyclic with zero reflection and unit transmission.
4(b) Short notes
Phase shifters: Control phase of microwave signals.
Attenuators: Reduce signal power without distortion.
5(a) Backward Wave Oscillator (BWO)
BWO operates on backward wave interaction between electron beam and slow-wave structure. Frequency is tunable by beam voltage. Used in microwave sources.
5(b) Two-cavity klystron numerical
Using given parameters, gap voltage for maximum output, voltage gain, and efficiency are calculated neglecting beam loading.
6(a) Short notes
Power meters: Measure microwave power using bolometers or thermistors.
Microwave amplifiers: Amplify weak microwave signals using tubes or solid-state devices.
6(b) Short notes
Insertion loss: Power loss due to device insertion.
Return loss: Measure of reflected power due to mismatch.
7(a) Radar range equation
Rmax=(PtG2λ2σ(4π)3Pmin)1/4R_{max} = \left( \frac{P_t G^2 \lambda^2 \sigma}{(4\pi)^3 P_{min}} \right)^{1/4}Rmax=((4π)3PminPtG2λ2σ)1/4
It relates maximum detectable range to radar parameters.
7(b) Pulse radar numerical
Duty cycle = Ton / Tr
Maximum unambiguous range = cTr / 2
Average power = Peak power × duty cycle
