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WAVEGUIDES BY PRIYANKA GAUR ASSISTANT PROFESSOR (DEPT. OF ECE) ww.advanced.edu.in
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WAVEGUIDESBYPRIYANKA GAURASSISTANT PROFESSOR (DEPT. OF ECE)
www.advanced.edu.in
CONTENTS•Introduction
• waveguides and its Modes
•Cut-off frequency
•Usable Frequency Range
•Group Velocity and Phase Velocity
•Characteristic Impedance
•Guided Wavelength
•Impedance Matching
•Couplers
•Passive components
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Introduction•Microwaves have frequencies > 1 GHz approx.•Stray reactances are more important as frequency increases•Transmission line techniques must be applied to short conductors like circuit board traces•Device capacitance and transit time are important•Cable losses increase: waveguides often used instead
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WaveguidesPipe through which waves propagateCan have various cross sectionsRectangularCircularElliptical
Can be rigid or flexibleWaveguides have very low loss
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Waveguides
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Modes•Waves can propagate in various ways•Time taken to move down the guide varies with the mode•Each mode has a cutoff frequency below which it won’t propagate•Mode with lowest cutoff frequency is dominant mode
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Multimode propagation
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Mode Designations
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•TE: transverse electric•Electric field is at right angles to direction of travel
•TM: transverse magnetic•Magnetic field is at right angles to direction of travel
•TEM: transverse electromagnetic•Waves in free space are TEM
Rectangular Waveguides•Dominant mode is TE101 half cycle along long dimension (a)No half cycles along short dimension (b)Cutoff for a = c/2
•Modes with next higher cutoff frequency are TE01 and TE20Both have cutoff frequency twice that for TE10
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TE modes in Rectangular waveguide
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Cutoff Frequency•For TE10 mode in rectangular waveguide with a = 2 b
acfc 2
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Usable Frequency Range•Single mode propagation is highly desirable to reduce dispersion •This occurs between cutoff frequency for TE10 mode and twice that frequency•It’s not good to use guide at the extremes of this range
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Group Velocity•Waves propagate at speed of light c in guide•Waves don’t travel straight down guide•Speed at which signal moves down guide is the group velocity and is always less than c
2
1
ffcv c
g
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Variation of group velocity with frequency
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Phase Velocity•Not a real velocity (>c)•Apparent velocity of wave along wall •Used for calculating wavelength in guide•For impedance matching etc.
2
1
ff
cvc
p
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Variation of phase angle along a waveguide
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Characteristic Impedance•Z0 varies with frequency
20
1
377
ff
Zc
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Guide Wavelength•Longer than free-space wavelength at same frequency
2
1
ffc
g
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Impedance Matching•Same techniques as for coax can be used•Tuning screw can add capacitance or inductance
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Coupling Power to Guides•3 common methods
◦ Probe: at an E-field maximum◦ Loop: at an H-field maximum◦ Hole: at an E-field maximum
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Coupling power to a waveguide
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Directional Coupler•Launches or receives power in only 1 direction•Used to split some of power into a second guide•Can use probes or holes
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Two-hole directional coupler
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Passive Components•Bends
◦Called E-plane or H-Plane bends depending on the direction of bending
•Tees◦Also have E and H-plane varieties◦Hybrid or magic tee combines both and can be used for isolation
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Waveguide bends
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Waveguide tees
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Hybrid tees
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SUMMARY•The basic concept of transverse waves travelling in a waveguide .•Waveguide acts as a guide to any wave and it acts as a high-pass filter which allows frequencies higher than the cut-off frequency to pass through it.
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Contact Information
Priyanka Gaur
Assistant Professor (ECE Department)
College Address
Advanced Educational Institutions, 70 km Milestone , Mathura road, Palwal,
Aurangabad, Haryana 121105, Palwal
www.advanced.edu.in
