In
electromagneticsElectromagnetism is the physics of the electromagnetic field, a field that exerts a force on particles with the property of electric charge and is reciprocally affected by the presence and motion of such particles....
and communications engineering, the term
waveguide may refer to any linear structure that conveys electromagnetic waves between its endpoints. However, the original and most common meaning is a hollow metal pipe used for this purpose.
A
dielectric waveguide employs a solid
dielectricA dielectric is a nonconducting substance, i.e. an insulator. The term was coined by William Whewell in response to a request from Michael Faraday...
rod rather than a hollow pipe. An optical fibre is a dielectric guide designed to work at optical frequencies. Transmission lines such as
microstripMicrostrip is a type of electrical transmission line which can be fabricated using printed circuit board [PCB] technology, and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate. Microwave...
, coplanar waveguide,
striplineStripline is a transverse electromagnetic transmission line medium, that was invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s.- Description :...
or
coaxialIn geometry, coaxial means that two or more forms share a common axis; it is the three-dimensional linear analogue of concentric.Coaxial cable, as a common example, has a wire conductor in the centre a circumferential outer conductor and an insulating medium called the dielectric separating...
may also be considered to be waveguides.
The electromagnetic waves in (metal-pipe) waveguide may be imagined as travelling down the guide in a zig-zag path, being repeatedly reflected between opposite walls of the guide. For the particular case of
rectangular waveguide, it is possible to base an exact analysis on this view. Propagation in dielectric waveguide may be viewed in the same way, with the waves confined to the dielectric by
total internal reflectionTotal internal reflection is an optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through...
at its surface. Some structures, such as nonradiative dielectric waveguide [NRD] and the Goubau line, use both metal walls and dielectric surfaces to confine the wave.
History
The first waveguide was proposed by
J. J. ThomsonSir Joseph John “J. J.” Thomson, OM, FRS was a British physicist and Nobel laureate, credited for the discovery of the electron and of isotopes, and the invention of the mass spectrometer...
in 1893 and experimentally verified by
Oliver Joseph LodgeSir Oliver Joseph Lodge, FRS was a physicist and writer involved in the development of key patents in wireless telegraphy . Lodge, in his Royal Institution lectures , coined the term "coherer." He gained the "syntonic" patent from the United States Patent Office in 1898...
in 1894; the mathematical analysis of the propagating modes within a hollow metal cylinder was first performed by Lord Rayleigh in 1897. (McLachan, 1947.)
Principles of operation
Depending on the frequency, waveguides can be constructed from either conductive or dielectric materials. Generally, the lower the frequency to be passed the larger the waveguide is. For example the natural waveguide the earth forms given by the dimensions between the conductive Ionosphere and the ground as well as the circumference at the median altitude of the earth is resonant at 7.83 Hz. This is also known as
Schumann resonanceThe Schumann resonances are a set of spectrum peaks in the extremely low frequency portion of the Earth's electromagnetic field spectrum...
. Waveguides can also be less than a millimeter in width. An example might be those that are used in
extremely high frequencyExtremely high frequency is the highest radio frequency band. EHF runs the range of frequencies from 30 to 300 gigahertz, above which electromagnetic radiation is considered to be low infrared light, also referred to as terahertz radiation...
(EHF)
SatelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
Communications(SATCOM).
Analysis
Electromagnetic waveguides are analyzed by solving
Maxwell's equationsMaxwell's equations are a set of four partial differential equations that relate the electric and magnetic fields to their sources, charge density and current density. These equations can be combined to show that light is an electromagnetic wave...
, or their reduced form, the
electromagnetic wave equationThe electromagnetic wave equation is a second-order partial differential equation that describes the propagation of electromagnetic waves through a medium or in a vacuum. The homogeneous form of the equation, written in terms of either the electric field E or the magnetic field B, takes the...
, with boundary conditions determined by the properties of the materials and their interfaces. These equations have multiple solutions, or modes, which are
eigenfunctionIn mathematics, an eigenfunction of a linear operator, A, defined on some function space is any non-zero function f in that space that returns from the operator exactly as is, except for a multiplicative scaling factor. More precisely, one has...
s of the equation system. Each mode is therefore characterized by an eigenvalue, which corresponds to a
cutoff frequencyIn physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.Typically in electronic systems such as filters and...
below which the mode cannot exist in the guide.
Waveguide propagation modes depend on the operating
wavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave – the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
and
polarizationPolarization is a property of waves that describes the orientation of their oscillations. This article primarily covers the polarization of electromagnetic waves such as light, although other types of wave also exhibit polarization....
and the shape and size of the guide. The
longitudinal modeA longitudinal mode of a resonant cavity is a particular standing wave pattern formed by waves confined in the cavity. The longitudinal modes correspond to the wavelengths of the wave which are reinforced by constructive interference after many reflections from the cavity's reflecting surfaces...
of a waveguide is a particular
standing waveA standing wave, also known as a stationary wave, is a wave that remains in a constant position. This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling in opposite...
pattern formed by waves confined in the cavity. The
transverse modeA transverse mode of a beam of electromagnetic radiation is a particular electromagnetic field pattern of radiation measured in a plane perpendicular to the propagation direction of the beam...
s are classified into different types:
- TE modes (Transverse Electric) have no electric field in the direction of propagation.
- TM modes (Transverse Magnetic) have no magnetic field in the direction of propagation.
- TEM modes (Transverse ElectroMagnetic) have no electric nor magnetic field in the direction of propagation.
- Hybrid modes are those which have both electric and magnetic field components in the direction of propagation.
In hollow waveguides (single conductor), TEM waves are not possible, since
Maxwell's EquationsMaxwell's equations are a set of four partial differential equations that relate the electric and magnetic fields to their sources, charge density and current density. These equations can be combined to show that light is an electromagnetic wave...
will give that the electric field must then have zero divergence and zero curl and be equal to zero at boundaries, resulting in a zero field. (or, equivalently, with boundary conditions guaranteeing only the trivial solution). However, TEM waves
can propagate in coaxial cable because there are two conductors.
The mode with the lowest cutoff frequency is termed the
dominant mode of the guide. It is usual to choose the size of the guide such that only this one mode can exist in the frequency band of operation. In rectangular and circular (hollow pipe) waveguides, the dominant modes are designated the TE
1,0 mode and TE
1,1 modes respectively.
Hollow metallic waveguides
In the
microwaveMicrowaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300MHz and 300 GHz. This is an extremely broad definition including both UHF and EHF , and various sources use different boundaries...
region of the
electromagnetic spectrumThe electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
, a waveguide normally consists of a hollow metallic conductor. Hollow waveguides must be one-half wavelength or more in diameter in order to support one or more transverse wave modes.
Waveguides are often pressurized to inhibit arcing/
multipactionMultipaction is a process in which a charged particle inside a gap in a vacuum environment , oscillates due to the influence of an externally applied oscillating electric field of large strength, and every time it hits the gap walls it causes other charged particles to be released...
, allowing higher power. Conversely, waveguides may be required to be evacuated as part of evacuated systems. (e.g. electron beam systems)
A
slotted waveguideA slotted waveguide is a waveguide that is used as an antenna in microwave radar applications. Prior to its use in surface search radar, such systems used a parabolic segment reflector....
is generally used for
radarRadar is an object detection system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in 1941 as an acronym for RAdio Detection And...
and other similar applications. The waveguide structure has the capability of confining and supporting the energy of an electromagnetic wave to a specific relatively narrow and controllable path.
A
closed waveguide is an electromagnetic waveguide (a) that is tubular, usually with a circular or rectangular cross section, (b) that has electrically conducting walls, (c) that may be hollow or filled with a
dielectricA dielectric is a nonconducting substance, i.e. an insulator. The term was coined by William Whewell in response to a request from Michael Faraday...
material, (d) that can support a large number of discrete propagating modes, though only a few may be practical, (e) in which each discrete mode defines the
propagation constantThe propagation constant of an electromagnetic wave is a measure of the change undergone by the amplitude of the wave as it propagates in a given direction. The quantity being measured can be the voltage or current in a circuit or a field vector such as electric field strength or flux density...
for that mode, (f) in which the
fieldIn physics, a field is a physical quantity associated to each point of spacetime. A field can be classified as a scalar field, a vector field, or a tensor field, according to whether the value of the field at each point is a scalar, a vector, or, more generally, a tensor, respectively...
at any point is describable in terms of the supported modes, (g) in which there is no
radiationIn physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body...
field, and (h) in which discontinuities and bends cause mode conversion but not radiation.
Hollow metallic waveguides are far narrower than the wavelength of operation. They can take the form of single conductors with or without a dielectric coating, e.g. the
Goubou lineA Goubau line, , or G-line for short, is a single wire transmission line intended for use at UHF and microwave wavelengths.-Description:The Goubau line itself consists of a single conductor coated with dielectric material...
and helical waveguides.
Voltage standing wave ratio (VSWR) measurements may be taken to ensure that a waveguide is contiguous and has no leaks or sharp bends. If such bends or holes in the waveguide surface are present, this may diminish the performance of both TX and RX equipment strings. Arcing may occur if there is a hole, if transmitting at high power (usually 200 watts or more). Waveguide plumbing
http://www.fnrf.science.cmu.ac.th/theory/waveguide/Waveguide%20theory%2012.html is crucial for proper waveguide performance. Reflected power may occur and damage equipment as well. Another cause for a bad VSWR in a waveguide is moisture build up which can typically be prevented with
silica gelSilica gel is a granular, vitreous, highly porous form of silica made synthetically from sodium silicate. Despite its name, silica gel is a solid....
, a
desiccantA desiccant is a hygroscopic substance that induces or sustains a state of dryness in its local vicinity in a moderately well-sealed container....
. Due to the negative effect of moisture buildup within the waveguide, desiccant silica gel canisters may be attached with screw-on nibs.
Dielectric rods for microwaves
Dielectric rod waveguides, in linear arrays of short transverse conductors, and planar resistive conductors use the same principle as optical waveguides.
These function via a
refractive indexThe refractive index of a medium is a measure of how much the speed of light is reduced inside the medium. For example, typical soda-lime glass has a refractive index close to 1.5, which means that in glass, light travels at 1 / 1.5 = 2/3 the speed of light in a vacuum...
effect where the waveguide slows the EM wave velocity below the free space velocity, continuously bending the relatively wide EM wavefronts towards the narrow waveguide and keeping them entrained. Helical waveguides and linear arrays of short conductors are used as part of "end-fire" antennas such as the
helical antennaA helical antenna is an antenna consisting of a conducting wire wound in the form of a helix. In most cases, helical antennas are mounted over a ground plane. Helical antennas can operate in one of two principal modes: normal mode or axial mode.In the normal mode, the dimensions of the helix are...
and
Yagi antennaA Yagi-Uda Antenna, commonly known simply as a Yagi antenna or Yagi, is a directional antenna system consisting of an array of a dipole and additional closely coupled parasitic elements . The dipole in the array is driven, and another element, typically 5% longer, effectively operates as a reflector...
. Planar resistive waveguides are used in
Over-The-Horizon radarOver-the-horizon radar, or OTH , is a design concept for radar systems to allow them to detect targets at very long ranges, typically up to thousands of kilometers...
and the
Ground Wave Emergency NetworkThe Ground Wave Emergency Network
, where the resistive surface of the Earth or ocean serves to slow the waves below free space velocity; entraining them and forcing them to follow the curvature of the Earth. Several waveguides based on entrainment of EM waves also exist.
Applications
Waveguides can be constructed to carry waves over a wide portion of the
electromagnetic spectrumThe electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
, but are especially useful in the
microwaveMicrowaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300MHz and 300 GHz. This is an extremely broad definition including both UHF and EHF , and various sources use different boundaries...
and
opticalOptics is the branch of physics which studies the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
frequency ranges. Waveguides are used for transferring both
powerIn physics, power is the rate at which work is performed or energy is converted. It is an energy per unit of time. As a rate of change of work done or the energy of a subsystem, power iswhere P is power, W is work and t is time....
and communication signals, usually for short distances. The main advantage of waveguides in comparison to other transmission lines is in high power applications.
Bell LabsBell Laboratories is the research and development organization of Alcatel-Lucent and previously of the American Telephone & Telegraph Company .Bell Laboratories has had its headquarters at Murray Hill, New Jersey, and it has research and development facilities...
in the 1970s built a waveguide line several miles long, to study possible use for intercity communication, but advances in
optical fiberAn optical fiber is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers...
disrupted the plan for good.
See also
- Angular misalignment loss
In waveguide design and construction, angular misalignment loss is power loss caused by the deviation from optimum angular alignment of the axes of source-to-waveguide, waveguide-to-waveguide, or waveguide-to-detector. The waveguide may be dielectric or metallic...
- Cavity resonator
- Cutoff frequency
In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.Typically in electronic systems such as filters and...
- Dielectric constant
The relative static permittivity of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux. It is the ratio of the amount of stored electrical energy when a potential is applied, relative to the permittivity of a vacuum...
- Electromagnetic radiation
Electromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
- Feedhorn
- Filled cable
In telecommunication, a filled cable is a cable that has a nonhygroscopic material, usually a gel called icky-pick, inside the jacket or sheath....
- Horn (telecommunications)
In telecommunications, the term horn has the following meanings:* In radio transmission, an open-ended waveguide, of increasing cross-sectional area, which radiates directly in a desired direction or feeds a reflector that forms a desired beam....
- Leaky mode
A leaky mode or tunneling mode in an optical fiber or other waveguide is a mode having an electric field that decays monotonically for a finite distance in the transverse direction but becomes oscillatory everywhere beyond that finite distance...
- List of telecommunications transmission terms
- List of antenna terms
- List of fiber optic terms
- Klystron tube
- Magic T
- Optical waveguide
- Radiation mode
For an optical fiber or waveguide, a radiation mode or unbound mode is a mode which is not confined by the fiber core. Such a mode has fields that are transversely oscillatory everywhere external to the waveguide, and exists even at the limit of zero wavelength.Specifically, a radiation mode is one...
- Radio wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
- Radio propagation
Radio propagation is a term used to explain how radio waves behave when they are transmitted, or are propagated from one point on the Earth to another. Like light waves, radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption and scattering.Radio propagation is...
- Transmission medium
A transmission medium is a material substance which can propagate energy waves. For example, the transmission medium for sound received by the ears is usually air, but solids and liquids may also act as transmission media for sound.The absence of a material medium can also be thought of as a...
- Wifi Cantenna
Further reading
- George Clark Southworth
George Clark Southworth , who published as G. C. Southworth, was a prominent American radio engineer best known for his role in the discovery of waveguides in the early 1930s.-Biography:...
, "Principles and applications of wave-guide transmission". New York, Van Nostrand [1950], xi, 689 p. illus. 24 cm. Bell Telephone Laboratories series. LCCN 50009834
External links
Patents
- Southworth, , "Wave guide electrotherapeutic system"
- Hopper, , "Wave guide frequency converter", September 10, 1957
Websites