Top Microwave Experiments Every Electronics and Communication Student Should Perform

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What Is a Microwave Engineering Lab and Why Does It Matter?

A microwave engineering lab is a practical training environment where ECE students work with actual microwave hardware like waveguide components, signal sources, detectors, and measurement instruments. Experiments performed here translate theoretical knowledge from subjects like Electromagnetic Field Theory and Antenna and Wave Propagation into measurable, repeatable results.

Microwave lab experiments help students develop four important industry relevant skills:

• Signal generation and source characterization
• Transmission line analysis and impedance matching
• Passive component measurement and verification
• Antenna characterization

Every microwave experiment in this guide helps students build one or more important practical skills.

Experiment 1 – Study of Gunn Diode Characteristics

Overview

Gunn diode is a solid state microwave signal source based on the transferred electron effect in gallium arsenide semiconductor material. When a DC bias voltage exceeding a threshold value is applied, the device exhibits negative resistance, a condition in which increasing voltage decreases current. This negative resistance property sustains microwave frequency oscillations within the associated waveguide cavity.

Gunn diode is compact, reliable, and serves as the primary signal source in most university microwave bench setups.

Experiment Objectives

• Plot V-I characteristics of Gunn diode
• Identify threshold voltage at which device begins exhibiting negative resistance
• Measure output power and oscillation frequency as a function of bias voltage

Lab Setup

The experiment uses a Gunn power supply, a Gunn oscillator mounted in a waveguide section, a PIN modulator, isolator, variable attenuator, frequency meter, slotted line, detector mount, and VSWR meter. Gunn power supply bias voltage is varied in steps, and corresponding current and output power readings are recorded.

Safety Note

Gunn diodes are thermally sensitive, bias voltage must not exceed 10V, and cooling fan must be operational at all times during experiment. Prolonged operation at high voltage without cooling causes permanent device damage.

Industry Relevance

Gunn diodes are used in radar proximity sensors, automotive collision avoidance systems, microwave communication links, and motion detection equipment. In the lab, the Gunn oscillator serves as a signal source for most subsequent experiments, making its characterization an essential starting point.

Experiment 2 – Reflex Klystron Characteristics

Overview

Reflex klystron is a vacuum tube based microwave oscillator. An electron beam emitted from a cathode is accelerated through a resonant cavity and then reflected back by a negatively biased electrode called repeller. Reflected electrons re enter cavity in bunches, delivering energy at the correct phase to sustain oscillations.

The frequency and power output of the klystron are controlled by adjusting the repeller voltage, which determines the transit time of electrons in the repeller space.

Experiment Objectives

• Draw a graph to show how output power changes when the repeller voltage is changed.
• Identify different oscillation modes and their corresponding repeller voltage ranges
• Calculate Electronic Tuning Sensitivity frequency change per unit change in repeller voltage

Lab Setup

klystron power supply provides beam voltage, heater voltage, and variable reflector voltage. klystron is mounted in a waveguide section connected to an isolator, attenuator, frequency meter, and detector. Beam voltage is set first, followed by variation of reflector voltage to observe mode transitions.

Industry Relevance

Reflex klystrons were used historically in radar receivers and microwave test equipment. While solid state sources have replaced them in many applications, klystrons remain relevant in high power applications such as satellite communication uplinks and particle accelerators. Studying this device provides a foundational understanding of velocity modulation, a principle also applied in traveling wave tubes  used in satellite transponders.

Experiment 3 – VSWR Measurement (Voltage Standing Wave Ratio)

Overview

When a microwave signal traveling along a transmission line encounters an impedance discontinuity, a portion of the signal reflects back toward the source. incident and reflected waves superpose to create a standing wave pattern along the line. Voltage Standing Wave Ratio quantifies severity of this mismatch.

VSWR is defined as ratio of maximum to minimum voltage amplitude along standing wave:

VSWR = V_max / V_min

A VSWR of 1 indicates perfect impedance matching with no reflections. Higher values indicate progressively worse mismatch and greater reflected power.

Experiment Objectives

• Measure VSWR for a given load using slotted line method
• Calculate reflection coefficient from VSWR value
• Apply double minimum method for loads with VSWR greater than 10

Lab Setup

The microwave bench consists of a signal source, isolator, variable attenuator, slotted waveguide section with a movable probe, detector mount, and VSWR meter. The probe is moved along a slotted section to locate positions of maximum and minimum signal amplitude. The ratio of these values gives VSWR directly.

For loads with very high VSWR, the minimum signal point becomes very sharp and hard to measure accurately. In this case, the double minimum method is used. The probe is moved to find two points on both sides of the minimum where the power becomes twice the minimum value. The distance between these two points is then used to calculate the VSWR.

Industry Relevance

VSWR measurement is one of the most fundamental diagnostic tools in RF and microwave engineering. Every transmission system, including antenna feeds, waveguide connections, coaxial assemblies, and PCB transmission lines, must be characterized for VSWR to ensure efficient power transfer. High VSWR in a radar or satellite system results in wasted transmit power and degraded signal quality. In 5G base station commissioning, VSWR verification is a mandatory step before a site is declared operational.

Experiment 4 – Frequency and Wavelength Measurement in a Rectangular Waveguide

Overview

A rectangular waveguide supports electromagnetic wave propagation in specific modes. Dominant mode is TE10 mode, where the electric field is entirely transverse to propagation direction with a single half wave variation across broad dimensions.

Inside a waveguide, guide wavelength differs from free space wavelength. These are related through cutoff wavelength:

1/λ0 = √1/λc² + 1/λg²

For TE10 mode: λc = 2a, where a is a broad dimension of waveguide.

Experiment Objectives

• Measure guide wavelength using slotted line
• Determine signal frequency using Frequency meter
• Verify theoretical relationship between guide wavelength, free space wavelength, and cutoff wavelength

Lab Setup

A Frequency meter is a precision resonant cavity coupled to a waveguide. When tuned to signal frequency, it absorbs power and creates a sharp dip on the VSWR meter. corresponding reading is directly read on frequency meter.

Guide wavelength is measured by locating two successive standing wave minima along a slotted line, distance between consecutive minima equals half guide wavelength.

Industry Relevance

Inside a waveguide, the speed and wavelength of a signal change depending on its frequency. Engineers must consider this while designing devices like filters, couplers, and feed systems. These parts are commonly used in satellite communication, radar systems, and long distance wireless communication networks.

Experiment 5 – Directional Coupler Characteristics

Overview

A directional coupler is a four port passive microwave component that extracts a controlled fraction of power traveling in a specified direction through a transmission line, without disturbing the main signal path significantly.

Four ports are: input, through, coupled and isolated. Most input power passes through the port. A small, precisely defined fraction is delivered to the coupled port. A signal traveling in reverse through the device is directed to isolated ports, with negligible power at coupled ports.

Key Parameters

• Coupling Factor – Ratio of coupled port power to input power. A 20 dB coupler delivers 1% of input power to the coupled port.
• Isolation – Attenuation of reverse signal at coupled port.
• Directivity – Difference between isolation and coupling factor, measuring coupler’s ability to discriminate between forward and reverse signals.

Lab Setup

A coupler is inserted into the waveguide bench between source and load. Detector and VSWR meter readings are taken at each port, with remaining ports terminated in matched loads. Port terminations are switched systematically to measure coupling factor, isolation, and through line insertion loss.

Industry Relevance

Directional couplers are essential components in power monitoring, reflectometers, automatic level control circuits, and network analyzers. Every vector network analyzer uses directional couplers internally to separate incident and reflected signal components, making this experiment directly relevant to understanding instruments that characterize virtually all RF and microwave components in professional settings.

Experiment 6 – Scattering Parameters of Magic Tee

Overview

Magic Tee is a four port waveguide junction that combines an E-plane T-junction and an H-plane T-junction shunt arm in a single structure. Its defining characteristic is mutual isolation between its E-arm and H-arm ports.

When H-arm is excited, the signal divides equally and in phase between two collinear ports, with no output at E-arm. When E-arm is excited, the signal divides equally but with a 180-degree phase difference between collinear ports, with no output at H-arm.

Experiment Objectives

• Measure scattering parameters of Magic Tee
• Verify mutual isolation between E-arm and H-arm
• Confirm equal power division and phase relationship at collinear ports

Lab Setup

Magic Tee is connected to a waveguide bench with each unused port terminated in a matched load. The signal source is applied to each port in turn, and power levels at remaining ports are measured. S-parameters are calculated from power ratios.

Industry Relevance

Magic Tees are used in radar duplexers to isolate transmitter from receiver while sharing a single antenna, in microwave balanced mixers and in antenna feed networks that require 180-degree hybrid junctions. The S-parameter framework introduced in this experiment is a universal standard for characterizing microwave device performance in simulation software, test instrumentation and component datasheets.

Experiment 7 – Attenuation Measurement

Overview

Attenuation is reduction in signal power as it passes through a component or transmission medium, expressed in decibels:

Attenuation (dB) = 10 log10 (P_input / P_output)

Fixed attenuators introduce a constant, specified loss. Variable attenuators allow attenuation to be adjusted by changing position of a resistive vane inside the waveguide. Since electric field is maximum at center of guide in TE10 mode, maximum absorption occurs when vane is centered, and attenuation decreases as vane moves toward the side wall.

Experiment Objectives

• Measure insertion loss of a fixed attenuator
• Characterize attenuation of a variable attenuator as a function of vane position
• Understand role of attenuators in signal level management and instrument protection

Lab Setup

Signal power levels are measured at detectors with and without attenuator in the waveguide path. The difference in decibels gives insertion loss. For variable attenuators, measurement is repeated at multiple vane positions to generate an attenuation versus position curve.

Industry Relevance

Attenuators are used throughout RF laboratories and production test environments to protect sensitive receivers from overload, establish calibrated signal levels and simulate transmission path losses. In automated RF test systems, programmable step attenuators characterize receiver dynamic range and measure minimum detectable signal thresholds.

Experiment 8 – Impedance Measurement Using Smith Chart

Overview

Impedance mismatch between source and load causes signal reflection and reduced power transfer. Smith Chart is a normalized circular graph that represents complex impedance values graphically, enabling reflection coefficient calculations, impedance transformations and matching network designs without complex algebraic computation.

The chart maps all possible passive impedance values onto a bounded circular region, with the center representing system characteristic impedance. Points on the outer circumference of the chart represent purely reactive impedances.

Experiment Objectives

• Measure VSWR and voltage minimum position for an unknown load
• Calculate reflection coefficient magnitude and phase from measurements
• Plot normalized impedance on Smith Chart to determine resistance and reactance

Lab Setup

An unknown load is connected to the waveguide bench. The slotted line measures VSWR and distance between load and nearest voltage minimum. Using Smith Chart construction procedure, normalized load impedance is determined from these two measured values.

Industry Relevance

Smith Chart analysis is a standard tool for antenna engineers, RF circuit designers, and system integration engineers. Input impedance matching for low noise amplifiers, filter design, transmission line stub calculations and antenna feed network design all rely on Smith Chart methods. Modern simulation platforms, including ADS and CST, incorporate Smith Chart displays, and the physical intuition developed through this experiment helps students use these tools effectively .

Experiment 9 – Circulator Characteristics

Overview

A circulator is a three port ferrite based microwave device with directional signal routing. A signal entering port 1 exits at port 2. A signal entering port 2 exits at port 3. A signal entering port 3 exits at port 1. Signals are strongly attenuated in reverse direction at each port.

This non reciprocal behavior results from interaction between the microwave field and a magnetized ferrite material inside the device. Ferrite exhibits different propagation constants for left hand and right hand circularly polarized waves, producing asymmetric transmission.

Key Parameters

• Insertion Loss – Attenuation in forward direction
• Isolation – Attenuation in reverse direction at each port

Lab Setup

The circulator is connected into a waveguide bench between source, a matched termination, and detector. Measurements are taken for each port combination to verify forward insertion loss and reverse isolation values.

Industry Relevance

Circulators are essential in radar systems that use a single antenna for both transmission and reception. The circulator routes high power transmitted pulse to antenna and directs low level received echo to receiver, while protecting sensitive receiver front end from transmitter output. Circulators are also used in satellite ground stations, microwave repeaters, and reflectometer test circuits.

Experiment 10 – Antenna Radiation Pattern Measurement

Overview

The radiation pattern of an antenna describes spatial distribution of radiated power as a function of direction. Measurements are taken in two principal planes, E-plane containing electric field vector and direction of maximum radiation and H-plane containing magnetic field vector and direction of maximum radiation.

Directional antennas such as horn antennas concentrate energy in a narrow beam, producing high gain in the main lobe direction. Half power beam width and side lobe level are primary parameters extracted from the radiation pattern.

Experiment Objectives

• Measure E-plane and H-plane radiation patterns of a horn antenna
• Determine half power beam width and first null beam width
• Compare measured pattern characteristics with standard horn antenna theory

Lab Setup

A transmitting horn antenna is connected to a microwave signal source. A receiving horn antenna is mounted on a rotating platform and positioned at the correct separation distance for far field measurement. The receiving antenna is rotated in fixed angular increments, and received signal power is recorded at each position, resulting data is plotted as a polar or Cartesian radiation pattern.

Industry Relevance

Antenna radiation pattern measurement is a mandatory step in design, production, and commissioning of every antenna system from cellular base station sector antennas to satellite dish feeds and radar arrays. Beam width and side lobe levels determine coverage area, interference rejection, and pointing accuracy. Engineers in wireless network planning, satellite engineering, and radar development perform these measurements as standard practice.

Experiment 11 – PIN Diode Modulator Characteristics

Overview

A PIN diode consists of a p-type layer, a wide intrinsic layer, and an n-type layer. At microwave frequencies, the intrinsic region acts as a variable resistor controlled by forward DC bias current. High bias current reduces resistance, allowing microwave signals to pass. Low bias blocks signal. This behavior makes the PIN diode an effective high speed microwave switch and amplitude modulator.

Experiment Objectives

• Study modulation of a microwave carrier signal using a PIN diode modulator
• Observe relationship between modulation input waveform and modulated microwave output
• Measure ON/OFF power ratio

Lab Setup

The PIN modulator is placed in the waveguide bench between Gunn oscillator and measurement section. Square wave pulses from Gunn power supply’s built in generator are applied to PIN modulator bias input. Modulated output is observed on an oscilloscope or measured with a VSWR meter.

Industry Relevance

PIN diode switches and phase shifters are fundamental building blocks in phased array antennas for 5G infrastructure, military radar, and electronic warfare systems. In a phased array, large numbers of PIN diode phase shifters operate in coordination to electronically steer the antenna beam without mechanical movement, enabling rapid beam scanning at microsecond timescales. The switching and modulation principles studied in this experiment form the conceptual foundation for that entire class of electronically steerable systems.

Essential Microwave Lab Equipment Reference

A clear understanding of each instrument and component on the bench is necessary before beginning any microwave experiment.

Gunn Power Supply – Provides regulated DC bias voltage for Gunn oscillator and includes a built in square wave generator for PIN modulation experiments. Voltage and current limits must be strictly observed.

VSWR Meter – A narrowband tuned amplifier and detector calibrated to display signal levels in VSWR units or decibels. input frequency range must be correctly set for accurate readings.

Slotted Waveguide Section – A waveguide section with a longitudinal slot along a broad wall, allowing a movable probe to sample internal electric field without significantly disturbing propagating signal. Used for standing wave analysis and VSWR measurement.

Frequency Meter (Cavity Wavemeter) – A precision resonant cavity coupled to waveguide. At resonance with signal frequency, it produces a power absorption dip on the VSWR meter. Frequency is read from a calibrated micrometer scale using the provided calibration chart.

Isolator – A two port ferrite device transmitting power in one direction only. Placed immediately after the signal source to prevent reflected power from pulling oscillator frequency.

Variable Attenuator – A waveguide component with an adjustable resistive vane used to set signal levels, protect instruments from overload, and perform insertion loss measurements.

Matched Termination – A waveguide termination that absorbs incident microwave power completely with no reflections. Used to terminate unused ports during measurements.

Best Practices for Microwave Lab Experiments

Identify signal path before energizing bench. Tracing each component from source to detector before switching on reduces setup errors and aids interpretation of results.

Allow adequate warm up time. Microwave sources and VSWR meters require a thermal stabilization period after power on. Early readings are affected by drift and are not reliable.

Maintain waveguide flange alignment. Flanged connections between waveguide sections must be clean and flush. Gaps or contamination at flanges introduce reflections that corrupt measurements.

Record data in a structured table during the experiment. Attempting to reconstruct readings from memory after a session is unreliable and introduces error.

Operate Gunn diodes within rated limits. The cooling fan must be running before bias voltage is applied. The diode reaches damaging temperatures rapidly at excessive bias without adequate cooling.

Follow the klystron power in sequence. For klystron setups, heater supply must be applied and allowed to stabilize before beam voltage is engaged. High tension (HT) switch is operated last. This sequence is documented in the lab manual and must not be bypassed.

Industry Applications of Microwave Lab Experiments

Measurement techniques and component knowledge developed in microwave labs correspond directly to professional engineering tasks across multiple industries.

Mobile and Wireless Communication – 4G LTE and 5G networks operate in microwave frequency bands. VSWR verification, antenna radiation pattern characterization and power level calibration are standard commissioning tasks for base station installations.

Radar Engineering – Weather radar, air traffic control systems, maritime navigation radar, and defense surveillance platforms are built on waveguide technology, circulators, directional couplers and precision impedance matching. Lab familiarity with these components provides direct preparation for radar system work.

Satellite Communication – Ground station antenna feeds, waveguide runs, low-noise amplifier input matching networks and uplink power chains all require measurement skills developed in microwave lab experiments. VSWR, gain and impedance characterization are routine tasks for satellite terminal engineers.

Defense and Aerospace Electronics – Electronic warfare receivers, phased array radar systems, and airborne communication equipment rely extensively on microwave components and measurement techniques covered in these experiments.

Medical Technology – Microwave ablation systems for tumor treatment and microwave based diagnostic imaging require precise power delivery under controlled impedance conditions, applying the same measurement principles practiced in the lab.

Conclusion

Microwave laboratory experiments are far more than just academic activities. They provide ECE students an understanding of practical communication and RF system operations in areas such as 5G, radar, satellite communication, aerospace and defence. Through these experiments students gain valuable hands-on experience in measuring a signal, impedance matching, testing an antenna and analyzing a microwave component.

Experiments range from the study of Gunn diodes and klystrons, through to the measurement of VSWR, radiation patterns and S-parameters, providing a good base for microwave engineering. Such skills are of great importance in RF engineering, wireless communications, embedded systems and other advanced electronics industries. Knowing the concepts of a microwave laboratory not only enhances the technical knowledge of the students but also equips them to face the engineering challenges in the real world.