Introduction to Waveguides

What is a Waveguide?
• Waveguides, like transmission lines, are
structures used to guide electromagnetic
waves from point to point.
• However, the fundamental characteristics of
waveguide and transmission line waves
(modes) are quite different.
• The differences in these modes result from
the basic differences in geometry for a
transmission line and a waveguide.
 Analogy with Transmission Lines

• A waveguide is a special form of transmission line consisting

of a hollow, metal tube. The tube wall provides distributed
inductance, while the empty space between the tube walls
provide distributed capacitance
 Where and Why to use?
• Waveguides are practical only for signals of extremely
high frequency, where the wavelength approaches the
cross-sectional dimensions of the waveguide. Below
such frequencies, waveguides are useless
• waveguides are considerably simpler than
two-conductor cables—especially coaxial cables—in
their manufacture and maintenance. With only a single
conductor (the waveguide’s “shell”)
• only dielectric in a waveguide is air. Moisture is not as
severe a problem in waveguides as it is within coaxial
cables, either, and so waveguides are often spared the
necessity of gas “filling.”
 Propagation
• because waveguides are single-conductor
elements, the propagation of electrical energy
down a waveguide is of a very different nature
than the propagation of electrical energy
down a two-conductor transmission line.
Wave paths in a waveguide at various frequencies

(a) At high
frequency

(b) At medium
frequency

( c ) At low
frequency

(d) At cutoff
frequency
PH0101 Unit 2 Lecture 4 7
Transverse Waves
Transverse Waves
TEM wave propagation through
parallel wire conductor
 Propagation in TL

• All electromagnetic waves consist of electric and magnetic fields propagating

in the same direction of travel, but perpendicular to each other. Along the
length of a normal transmission line, both electric and magnetic fields are
perpendicular (transverse) to the direction of wave travel. This is known as
the principal mode, or TEM (Transverse Electric and Magnetic) mode. This
mode of wave propagation can exist only where there are two conductors,
and it is the dominant mode of wave propagation where the cross-sectional
dimensions of the transmission line are small compared to the wavelength
of the signal.
 Limitations of Two Conductor Lines
• At some microwave signal frequencies (between 100
MHz and 300 GHz), two-conductor transmission lines
of any substantial length operating in standard TEM
mode become impractical.
• Lines small enough in cross-sectional dimension to
maintain TEM mode signal propagation for microwave
signals tend to have low voltage ratings, and suffer
from large, parasitic power losses due to conductor
“skin” and dielectric effects.
• Fortunately, though, at these short wavelengths there
exist other modes of propagation that are not as
“lossy,” if a conductive tube is used rather than two
parallel conductors. It is at these high frequencies that
waveguides become practical.
 Propagation in Waveguide

• When an electromagnetic wave propagates down a hollow tube, only

one of the fields—either electric or magnetic—will actually be
transverse to the wave’s direction of travel. The other field will “loop”
longitudinally to the direction of travel, but still be perpendicular to
the other field. Whichever field remains transverse to the direction of
travel determines whether the wave propagates in TE mode
(Transverse Electric) or TM (Transverse Magnetic) mode.
Propagation in Waveguide
 Advantage

• Waves propagate in all directions in open

space as spherical waves. The power of the
wave falls with the distance R from the source
as the square of the distance (inverse square
law). A waveguide confines the wave to
propagate in one dimension, so that, under
ideal conditions, the wave loses no power
while propagating. Due to total reflection at
the walls, waves are confined to the interior of
a waveguide.
Propagation Modes and Cutoff Frequencies
• A propagation mode in a waveguide is one solution
of the wave equations, or, in other words, the form
of the wave.
• Due to the constraints of the boundary conditions,
there are only limited frequencies and forms for the
wave function which can propagate in the
waveguide.
• The lowest frequency in which a certain mode can
propagate is the cutoff frequency of that mode.
• The mode with the lowest cutoff frequency is the
fundamental mode of the waveguide, and its cutoff
frequency is the waveguide cutoff frequency.
 Contd…
• The order of the mode refers to
the field configuration in the
guide, and is given by m and n
integer subscripts, TEmn and
TMmn.
– The m subscript corresponds
to the number of half-wave
variations of the field in the x
direction, and
– The n subscript is the
number of half-wave
variations in the y direction.
– Only a limited number of
different m, n modes can be
propagated along a
waveguide dependent upon
the waveguide dimensions
and format.
Modes in a waveguide
TE01 mode
 Go watch and remember to come
back!
• https://www.youtube.com/watch?v=XQ9LzbiI
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• https://www.youtube.com/watch?v=WPiSwl
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Modes in a Hollow Rectangular
Waveguide
 Comparison of Waveguides and Tlines
Transmission Line Waveguide
Two or more conductors separated by Metal waveguides are typically one
some insulating medium (two-wire, enclosed conductor filled with an insulating
coaxial, microstrip, etc. medium while a dielectric waveguide
consists of multiple dielectrics
Normal operating mode is the TEM or Operating modes are TE or TM modes
quasi-TEM mode (can support TE and TM (can not support a TEM mode)
modes but these modes are typically
undesirable.)
No cutoff frequency for the TEM mode. Must operate the waveguide at a
Tline can transmit signals from DC up to frequency above the respective TE or TM
high frequency mode cutoff frequency for that mode to
propogate
Significant signal attenuation at high Lower signal attenuation at high frequencies
frequencies
Small cross section line can transmit Can transmit high power levels
only low power levels
Large cross section tlines can transmit Large cross section waveguides are
high power levels. impractical due to large size and high
ELCT564 cost. 22
 Summary of Transmission Lines and Waveguides

Characteristic Coax Waveguide Stripline Microstrip

Modes: Preferred TEM TE10 TEM Quasi-TEM

Other TE, TM TM, TE TM,TE Hybrid TM, TE

Dispersion None Medium None Low

Bandwidth High Low High High

Loss Medium Low High High

Power Capacity Medium High Low Low

Physical Size Large Large Medium Small

Ease of Fabrication Medium Medium Easy Easy

Integration with Others Hard Hard Fair Easy

ELCT564 23