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Microwave Techniques
Objectives: The aim of this paper is to make students familiar with different microwave measurement techniques, microwave sources and methods to determine dielectric properties.
Course Outcome: On completion of the course, the student will be able to
• know about lab techniques in Microwaves
• understand the basic principles of measurements of different quantities
• understand the working of various microwave devices from a practical point of view .
• determine accuracy of measurements by different methods
• Select a suitable method for measurement of dielectric properties depending upon the sample.
a) Slotted Coaxial line techniques :
Coaxial line, coaxial slotted section, Characteristic Impedance, coaxial attenuator, matched termination, detector and indicating meter
Voltage Standing Wave Ratio, Measurement of VSWR for the unmodulated exciting wave by a crystal rectifier and Micro ammeter for the cases when VSWR is (i) very low (ii) moderate and (iii) high. Measurement of VSWR for the modulated exciting wave by crystal rectifier and VSWR meter.
b) Detector Characteristics:
Crystals as Detectors for Microwaves, Crystal mounting, Microwave Crystal parameters; Study of square law behavior and determination of operating range and detection efficiency of microwave crystal detector. Bolo meters and their mounting, Thermocouple
c) Slotted waveguide techniques:
Dominant modes in rectangular waveguides, Energizing a waveguide in a dominant mode; Group and Phase velocities; Characteristic wave impedance, Slotted waveguide section with a carriage and probe, Attenuators and Matched loads, short circuit terminations, Frequency meter. Characteristics of wave propagation in a waveguide and relationship between guide wavelength (λg) and free space wavelength (λ0 ), Wall currents.
Microwave sources used in the Laboratory:
(a) Reflex Klystron:
Its construction and operation, Transit time and mode number, Power-frequency characteristics, Mechanical and electrical tuning output efficiency, Modulation in Klystron, Power coupling, Basic Precautious, firing of the Reflex Klystron. Study of mode characteristics of a reflex Klystron
(b) Gunn Oscillator:
Gunn Effect, Gunn Oscillator, Modulation; I-V characteristics, Power and Frequency, Characteristics of a Gunn Oscillator, Modes of operation.
(a) Directional Coupler :
Characteristics of a Directional coupler, Directivity and coupling of a Directional coupler, Types of couplers: Bathe hole coupler, Double hole and multi-hole couplers Directional coupler parameters. Measurement of main line and auxiliary line VSWR; measurement of coupling and insertion loss
(b) Microwave Antennas :
E-H Horns, E-plane, H-plane and pyramidal horns, Radiation Pattern: Beam
width, Gain. Study of the E-plane and H-plane radiation pattern of a pyramidal horn antenna and estimation of beam width and directional gain of the antenna.
a) Attenuation Measurement:
Attenuators, Attenuator characteristics: Input VSWR, Insertion loss, and frequency sensitivity of attenuation. Attenuation Measurement: Direct or power measuring method, substitution method, Impedance measuring method
b) Power Measurement :
Methods of measuring microwave power for (i) high power (ii) low power (iii) very low power. Errors in power measurement, Peak and average power. Use of thermistor for power measurement, power meter.
a) Dielectric Measurement:
Dielectric constant, loss factor, Von Hipple or shorted waveguide method, Heston and Smythe method for low loss liquids, Surber method for medium and high loss liquids, Poly’s method for high loss liquids, Corfield Coaxial line method.
(b) Two point method (a) loss less dielectric (b) lossy dielectric
Measurement of dielectric constant by two point method of (a) low loss solid dielectrics (b) Liquids solutions; Measurement of dielectric constant of solids and liquids by Von-Hipple method (a) General sample length (b) Sample length in multiples of λd/2; Measurement of dielectric content of powders. Use of Impedance Bridge/Network Analyzer for dielectric measurement.
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2. O.B. Richard, “Laboratory Manual for Microwave Measurements” (Prentice Hall Incs., England Cliff, N.J. 1968)
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