MICROWAVE ENGINEERING ; Module-1

Introduc on: RF and microwave spectrum, historical background, applica on of RF and Microwave
Impedance Matching–Unknown impedance measurement using shi in minima technique and
impedance matching using single and double stub matching.

Introduc on to RF and Microwave Spectrum

RF and Microwave Spectrum

 Radio Frequency (RF) generally covers the frequency range from 3 kHz to 300 MHz, while
Microwaves extend from 300 MHz to 300 GHz.

 These frequency ranges play a vital role in wireless communica on, radar, naviga on,
satellite systems, and industrial hea ng applica ons.

 Due to their short wavelengths, microwaves allow for compact antenna design, higher
bandwidth, and direc onal propaga on.

Historical Background

 James Clerk Maxwell (1864): Proposed electromagne c wave theory.

 Heinrich Hertz (1886): Experimentally demonstrated the existence of EM waves.

 Early 20th Century: Use of RF for radio and telegraphy.

 World War II: Microwave technology saw rapid growth due to radar development.

 Modern Era: Wide applica ons in mobile communica on, satellite broadcas ng, Wi-Fi,
medical imaging, and 5G/6G technologies.

Applica ons of RF and Microwave

1. Communica on Systems – Cellular, Wi-Fi, Satellite, Bluetooth, TV broadcas ng.

2. Radar Systems – Air traffic control, defense, weather monitoring.

3. Medical Applica ons – MRI, microwave imaging, diathermy.

4. Industrial Applica ons – Microwave ovens, material processing, remote sensing.

5. Scien fic Research – Par cle accelerators, radio astronomy.

Impedance Matching in RF and Microwave Systems

At high frequencies, power transfer efficiency is highly dependent on impedance matching between
transmission lines and loads. Poor matching causes reflec on, standing waves, and loss of power.

Unknown Impedance Measurement – Shi in Minima Technique

 Uses a slo ed line connected to a transmission line.

 By moving a probe along the line, the voltage standing wave minima posi ons are noted.
  The shi in minima posi on (compared to a matched condi on) gives the load impedance
using transmission line theory.

 This method is prac cal and widely used for measuring unknown complex impedances.

Stub Matching

Stub matching is a technique to cancel the reac ve part of load impedance using short-circuited
transmission line sec ons called stubs.

1. Single Stub Matching

o One stub (open/short-circuited) is placed at a specific loca on from the load.

o It cancels the load reactance, achieving impedance matching.

o Simple to implement but frequency sensi ve.

2. Double Stub Matching

o Two stubs are placed at fixed distances along the line.

o Provides more flexibility for matching arbitrary complex impedances.

o Commonly used in prac cal microwave circuits since stub loca ons are fixed and
only stub lengths are adjusted.

MODULE-2

Microwave Waveguides and Components

1. Waveguides
Waveguides are hollow conduc ng structures used to guide microwave signals.
Unlike transmission lines, they can handle high power with low loss and are
widely used in microwave communica on and radar systems.

1.1 Rectangular Waveguide

 Structure: Hollow rectangular metallic tube.
 Supports TE (Transverse Electric) and TM (Transverse Magne c) modes
(no TEM mode).
 Dominant mode: TE10.

 Cutoff frequency (fc):

fc= c (𝑚/𝑎)^2+(n/b)^2
2
where a = broad dimension, b = narrow dimension, m, n = mode
indices.
 Applica ons: Microwave ovens, radar, communica on links.
1.2 Circular Waveguide
 Structure: Hollow cylindrical conductor.
 Supports TE and TM modes.
 Dominant mode: TE11.
 Used in satellite feeds, antenna systems, and radar.
 Advantage: Lower a enua on compared to rectangular
waveguides.

2. Mode Structure
 TE mode: No electric field in direc on of propaga on (Ez = 0).
 TM mode: No magne c field in direc on of propaga on (Hz =
0).
 Dominant mode: Lowest cutoff frequency mode (TE10 for
rectangular, TE11 for circular).

3. Waveguide Parameters
 Cutoff Frequency: Below this frequency, wave cannot
propagate.
 Wall Current: Flow of surface current on waveguide walls due to
EM fields.
  A enua on: Loss of signal power due to conductor and
dielectric losses.

4. Microwave Cavi es
 A cavity resonator is a hollow metallic enclosure that stores
microwave energy.
 Rectangular Cavity Resonator: Resonates at discrete
frequencies based on cavity dimensions.
 Q-Factor (Quality Factor):
Q= 2π Energy Stored
Energy Lost per cycle
o High Q → Low loss and narrow bandwidth.
o Used in filters, oscillators, and klystrons.

5. Microwave Components
Power Divider
 Splits input power into mul ple outputs.
 Example: Wilkinson power divider (provides isola on and
matching).
Sca ering Matrix (S-Matrix)
 Describes rela onship between incident and reflected waves at
microwave ports.
 For an N-port device:
[b]=[S][a]
where a = incident wave vector, b = reflected wave vector.
Transmission Matrix (T-Matrix)
 Relates input and output voltages and currents.
 Useful for cascaded networks.

6. Passive Components
A enuator
 Reduces signal strength without distor on.
 Types: Fixed, variable, and waveguide a enuators.
Phase Shi er
 Alters phase of microwave signal without affec ng amplitude.
 Used in phased-array radars, beam steering.
Direc onal Coupler
 Couples a fixed amount of power from main line to auxiliary
line.
 Applica ons: Signal monitoring, feedback.
Bethe Hole Coupler
 Small circular hole couples fields from one waveguide to
another.
 Used for weak coupling.
Magic Tee
 Hybrid junc on combining E-plane and H-plane tees.
 Proper es: Isola on between certain ports.
 Used in mixers, balanced circuits.
Hybrid Ring (Rat-Race Coupler)
  4-port device with 360° ring structure.
 Provides power division and phase shi ing.
Circulator
 3-port or 4-port non-reciprocal device.
 Power enters port 1 → exits port 2 → enters port 2 → exits port
3, and so on.
Isolator
 2-port device allowing power to flow only in one direc on.
 Protects sources from reflected power.

7. Ferrite Devices
 Ferrites: Non-conduc ve magne c materials with anisotropic
proper es.
 Microwave ferrite devices use Faraday rota on and non-
reciprocal behavior.
 Examples: Isolators, Circulators, Phase shi ers.

MODULE-3

Planar Structures, Microwave Tubes and Semiconductor Microwave

Devices

1. Planar Transmission Line Structures

1.1 Stripline
  Consists of a conduc ng strip placed between two parallel
ground planes with dielectric in between.
 Fields are completely confined → TEM mode propaga on.
 Advantages: Low radia on loss, wide bandwidth.
 Used in filters, couplers, and microwave ICs.
1.2 Microstrip Line
 Consists of a conduc ng strip separated from a single ground
plane by a dielectric substrate.
 Supports quasi-TEM mode.
 Advantages: Easy fabrica on, compact size, low cost.
 Widely used in antennas, RFICs, PCB circuits.
1.3 Coplanar Waveguide (CPW)
 Signal conductor and ground conductors lie in the same plane
on dielectric substrate.
 Supports quasi-TEM mode.
 Advantages: Easy integra on with ac ve devices, no via-holes
required, wideband performance.
 Applica ons: MMICs, microwave circuits, and antennas.

2. Microwave Tubes
Limita ons of Conven onal Tubes
 At microwave frequencies, conven onal vacuum tubes fail due
to:
o Transit- me effect (electrons cannot respond fast).
o High losses and low efficiency.
 o Parasi c capacitance and inductance of electrodes.
 Solu on → Use of velocity modula on and slow-wave
structures.

2.1 Mul cavity Klystron

 A linear-beam tube using velocity modula on.
 Mul ple cavi es: Buncher → Dri Space → Catcher.
 Used as microwave amplifiers (radar, satellite).
 Advantages: High gain, moderate bandwidth.
2.2 Reflex Klystron
 Single cavity tube with reflector electrode.
 Provides oscilla ons due to bunching of electrons reflected
back.
 Used as a microwave oscillator in lab measurements.
2.3 Magnetron
 Cross-field device (E ⊥ B).
 Electrons follow curved paths and interact with cavity
resonators.
 Produces high-power microwave oscilla ons.
 Applica ons: Radar, microwave ovens, industrial hea ng.
2.4 Travelling Wave Tube (TWT)
 Linear beam tube with helix slow-wave structure.
 Con nuous interac on between electron beam and RF signal.
 High gain, wide bandwidth.
 Used in satellite communica on.
2.5 Backward Wave Oscillator (BWO)
 Similar to TWT but wave travels opposite to electron beam.
 Provides tunable microwave oscilla ons.
 Used in sweep generators, ECM (Electronic Counter Measure).

3. Semiconductor Microwave Devices

3.1 Tunnel Diode
 Nega ve resistance device due to quantum tunneling.
 Very high switching speed.
 Used as oscillator and amplifier in microwave circuits.
3.2 Gunn Diode
 Bulk semiconductor device using transferred electron effect.
 Generates microwave oscilla ons without junc on.
 Applica ons: Radar speed guns, oscillators in microwave links.
Waveguide Mounts
 Special fixtures that allow diodes to be inserted in waveguide
circuits for proper opera on and cooling.
 Provide impedance matching, biasing, and thermal stability.

MODULE-4

Avalanche Diodes, Microwave Transistors and Applica ons of

Microwaves
1. Avalanche Diodes
1.1 IMPATT Diode (Impact Avalanche Transit-Time Diode)
 Operates using avalanche breakdown and transit- me effect.
 Generates microwave oscilla ons due to a phase delay
between voltage and current.
 Frequency range: 3 – 100 GHz.
 Advantages: High power output.
 Limita on: High noise.
 Applica ons: Radar, communica on transmi ers.
1.2 TRAPATT Diode (Trapped Plasma Avalanche Triggered Transit
Diode)
 Uses avalanche breakdown with plasma forma on.
 Provides high efficiency (up to 60%) compared to IMPATT.
 Generates power at microwave frequencies.
 Applica ons: High-power microwave sources.

2. Microwave Bipolar Transistors

2.1 Microwave Bipolar Junc on Transistor (BJT)
 Operates at microwave frequencies with short transit mes.
 Limita ons: Lower efficiency due to minority carrier storage.
 Applica ons: Amplifiers in lower microwave region.
2.2 Heterojunc on Bipolar Transistor (HBT)
 Uses different semiconductor materials for emi er and base
(e.g., GaAs/AlGaAs).
 Reduces base resistance and transit me.
  Higher cutoff frequency and gain.
 Applica ons: High-speed microwave amplifiers, mobile
communica on.

3. Microwave Field Effect Transistors (FETs)

3.1 JFET (Junc on FET)
 Voltage-controlled device.
 Limited frequency response (not suitable for very high
microwave range).
 Used in low-noise amplifiers.
3.2 MOSFET (Metal Oxide Semiconductor FET)
 High input impedance.
 Used in RF and lower microwave frequency amplifiers.
 Limita on: Gate capacitance reduces efficiency at higher
microwave frequencies.
3.3 MESFET (Metal Semiconductor FET)
 Uses Scho ky metal–semiconductor junc on as gate.
 Provides high frequency opera on (up to tens of GHz).
 Low noise and high gain.
 Widely used in microwave ICs, radar, and satellite systems.

4. Applica ons of Microwaves

4.1 Industrial Applica ons
1. Hea ng and Drying – Food processing, tex le drying, wood
curing.
 2. Material Processing – Plas c welding, ceramic sintering.
3. Microwave Ovens – Domes c and industrial cooking.
4. Remote Sensing – Non-destruc ve tes ng of materials.
5. Plasma Genera on – Industrial plasma processes.
4.2 Other Applica ons
 Communica ons – Satellite, radar, mobile networks.
 Medical – Diathermy, cancer treatment, MRI.
 Scien fic Research – Par cle accelerators, radio astronomy.

MODULE-5
Microwave Measurement and Equivalent RF Circuit Parameters

1. Microwave Measurements
1.1 VSWR (Voltage Standing Wave Ra o) Measurement
 VSWR gives informa on about the degree of mismatch
between load and transmission line.
 Method:
o Use a slo ed line sec on to measure maxima and minima
of standing waves.
o Calculate VSWR:
VSWR=Vmax/Vmin
 Applica ons: To find reflec on coefficient and impedance
matching.

1.2 Power Measurement

  Bolometer Method:
o A bolometer (temperature-sensi ve resistor) absorbs
microwave power → resistance change is measured.
o Used for average power measurement.
 Calorimetric Method:
o Measures rise in temperature of absorbing medium due
to microwave hea ng.
o Suitable for high power measurement.

1.3 Impedance Measurement

 Performed using slo ed line method or network analyzers.
 Shi in minima technique used to determine unknown
complex impedance.
 Impedance is expressed as:
ZL=R+JX
where R = resistance, X = reactance.

1.4 Frequency Measurement

 Cavity wavemeter is commonly used.
 Principle: A resonant cavity absorbs maximum power at its
resonant frequency.
 By adjus ng cavity dimensions (or using calibra on chart), the
frequency is measured accurately.

2. Equivalent RF Circuit Parameters

2.1 Low Pass Filter (LPF)
 Passes low frequency/RF signals and a enuates high frequency
signals.
 Applica ons: Harmonic suppression, noise reduc on.
2.2 High Pass Filter (HPF)
 Passes high frequency signals and blocks low frequency signals.
 Applica ons: Removing low-frequency interference, signal
condi oning.
2.3 Band Pass Filter (BPF)
 Passes signals within a specific frequency band while
a enua ng frequencies outside the band.
 Applica ons: Channel selec on in receivers, communica on
systems.
2.4 RF Amplifier
 Used to amplify weak RF signals while maintaining low noise.
 Parameters: Gain, bandwidth, noise figure, linearity.
 Applica ons: Communica on receivers, transmi ers, radar
systems.