Traveling wave antenna
Encyclopedia
Traveling-wave antenna is a class of antenna
that use a traveling wave on a guiding structure as the main radiating mechanism. Traveling-wave antennas fall into two general categories: slow-wave antennas and fast-wave antennas, which are
usually referred to as leaky-wave antennas.
vph that is less than the speed of light
in free space. Such a wave does not fundamentally radiate by its nature, and radiation
occurs only at discontinuities (typically the feed and the termination regions). The propagation wavenumber of the traveling wave is therefore a real number (ignoring conductor or other losses). Because the wave radiates only at the discontinuities, the
radiation pattern
physically arises from two equivalent sources, one at the beginning and one at the end of the structure. This makes it difficult to obtain highly-directive single-beam radiation patterns. However, moderately-directive patterns having a
main beam near endfire can be achieved, although with a significant sidelobe level. For these antennas there is an optimum length depending on the desired location of the main beam. Examples include wires in free space or over a ground plane, helixes, dielectric slabs or rods, corrugated conductors. An independent control of the beam angle and of the beam width is not possible.
(LWA) may be a fast wave, with a phase velocity greater than the speed of light. This type of wave radiates continuously along its length, and hence the propagation wavenumber kz is complex, consisting of both a phase and an attenuation constant. Highly-directive beams at an arbitrary specified angle can be achieved with this type of antenna, with a low sidelobe level. The phase constant β of the wave controls the beam angle (and this can be varied changing the frequency), while the attenuation constant α controls the beamwidth. The aperture distribution can also be easily tapered to control the sidelobe level or beam shape.
Leaky-wave antennas can be divided into two important categories, uniform and periodic, depending on the type of guiding structure.
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
that use a traveling wave on a guiding structure as the main radiating mechanism. Traveling-wave antennas fall into two general categories: slow-wave antennas and fast-wave antennas, which are
usually referred to as leaky-wave antennas.
Slow-wave antennas
In slow-wave antennas, the guided wave is a slow wave, meaning a wave that propagates with a phase velocityPhase velocity
The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the speed at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave will appear to travel at the phase velocity...
vph that is less than the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
in free space. Such a wave does not fundamentally radiate by its nature, and radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
occurs only at discontinuities (typically the feed and the termination regions). The propagation wavenumber of the traveling wave is therefore a real number (ignoring conductor or other losses). Because the wave radiates only at the discontinuities, the
radiation pattern
Radiation pattern
In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....
physically arises from two equivalent sources, one at the beginning and one at the end of the structure. This makes it difficult to obtain highly-directive single-beam radiation patterns. However, moderately-directive patterns having a
main beam near endfire can be achieved, although with a significant sidelobe level. For these antennas there is an optimum length depending on the desired location of the main beam. Examples include wires in free space or over a ground plane, helixes, dielectric slabs or rods, corrugated conductors. An independent control of the beam angle and of the beam width is not possible.
Leaky-wave antennas
By contrast, the wave on a Leaky wave antennaLeaky wave antenna
Leaky-Wave Antenna belong to the more general class of Traveling wave antenna, that use a traveling wave on a guiding structure as the main radiating mechanism...
(LWA) may be a fast wave, with a phase velocity greater than the speed of light. This type of wave radiates continuously along its length, and hence the propagation wavenumber kz is complex, consisting of both a phase and an attenuation constant. Highly-directive beams at an arbitrary specified angle can be achieved with this type of antenna, with a low sidelobe level. The phase constant β of the wave controls the beam angle (and this can be varied changing the frequency), while the attenuation constant α controls the beamwidth. The aperture distribution can also be easily tapered to control the sidelobe level or beam shape.
Leaky-wave antennas can be divided into two important categories, uniform and periodic, depending on the type of guiding structure.