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Leaky-wave antenna (LWA) belong to the more general class of traveling wave antenna, that use a traveling wave on a guiding structure as the main radiating mechanism. Traveling-wave antenna fall into two general categories, slow-wave antennas and fast-wave antennas, which are usually referred to as leaky-wave antennas.
The traveling wave on a Leaky-Wave Antenna is 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.
A uniform structure has a cross section that is uniform (constant) along the length of the structure, usually in the form of a waveguide that has been partially opened to allow radiation to occur. The guided wave on the uniform structure is a fast wave, and thus radiates as it propagates.
A periodic leaky wave antenna structure is one that consists of a uniform structure that supports a slow (non radiating) wave that has been periodically modulated in some fashion. Since a slow wave radiates at discontinuities, the periodic modulations (discontinuities) cause the wave to radiate continuously along the length of the structure. From a more sophisticated point of view, the periodic modulation creates a guided wave that consists of an infinite number of space harmonics (Floquet modes). Although the main (n = 0) space harmonic is a slow wave, one of the space harmonics (usually the n = −1) is designed to be a fast wave, and this harmonic wave is the radiating wave.
A typical example of a uniform leaky-wave antenna is an air-filled rectangular waveguide with a longitudinal slot. This simple structure illustrates the basic properties common to all uniform leaky-wave antennas. The fundamental TE10 waveguide mode is a fast wave, with , where k0 is the vacuum wavenumber. The radiation causes the wavenumber kz of the propagating mode within the open waveguide structure to become complex. By means of an application of the stationary-phase principle, it can be found in fact that:
where θm is the angle of maximum radiation taken from broadside (x direction), and λ0 are the light velocity and the wavelength in vacuum, and λg is the guide wavelength. As is typical for a uniform LWA, the beam cannot be scanned too close to broadside (θm=0), since this corresponds to the cutoff frequency of the waveguide. In addition, the beam cannot be scanned too close to endfire (θm=90°,z direction) since this requires operation at frequencies significantly above cutoff, where higherorder modes can propagate, at least for an air-filled waveguide. Scanning is limited to the forward quadrant only (0<θm<Π/2), for a wave traveling in the positive z direction.
This one-dimensional (1D) leaky-wave aperture distribution results in a "fanbeam" having a narrow shape in the xz plane (H plane), and a broad shape in the cross-plane. A "pencil beam" can be created by using an array of such 1D radiators. Unlike the slow-wave structure, a very narrow beam can be created at any angle by choosing a sufficiently small value of α. A simple formula for the beam width, measured between half-power points (), is:
where L is the length of the leaky-wave antenna, and Δθ is expressed in radians. For 90% of the power radiated it can be assumed:
Since leakage occurs over the length of the slit in the waveguiding structure, the whole length constitutes the antenna's effective aperture unless the leakage rate is so great that the power has effectively leaked away before reaching the end of the slit. A large attenuation constant implies a short effective aperture, so that the radiated beam has a large beamwidth. Conversely, a low value of α results in a long effective aperture and a narrow beam, provided the physical aperture is sufficiently long. Since power is radiated continuously along the length, the aperture field of a leaky-wave antenna with strictly-uniform geometry has an exponential decay (usually slow), so that the sidelobe behavior is poor. The presence of the sidelobes is essentially because the structure is finite along z. When we change the cross-sectional geometry of the guiding structure to modify the value of α at some point z, however, it is likely that the value of β at that point is also modified slightly. However, since β must not be changed, the geometry must be further altered to restore the value of β, thereby changing α somewhat as well.
In practice, this difficulty may require a two-step process. The practice is then to vary the value of α slowly along the length in a specified way while maintaining β constant (that is the angle of maximum radiation), so as to adjust the amplitude of the aperture distribution A(z) to yield the desired sidelobe performance. We can divide uniform leaky-wave antennas into air-filled ones and partially dielectric-filled ones. In the first case, since the transverse wavenumber kt is then a constant with frequency, the beamwidth of the radiation remains exactly constant as the beam is scanned by varying the frequency. In fact, since:
where:
independent of frequency (λc is the cutoff wavelength). On the contrary, when the guiding structure is partly filled with dielectric, the transverse wavenumber kt is a function of frequency, so that Δθ changes as the beam is frequency scanned. On the other hand, with respect to frequency sensitivity, i.e., how quickly the beam angle scans as the frequency is varied, the partly dielectric-loaded structure can scan over a larger range of angles for the same frequency variation, as it is apparent in Fig. 2, and is therefore preferred.
In response to requirements at millimeter wavelengths, the new antennas were generally based on lower-loss open waveguides. One possible mechanism to obtain radiation is foreshortening a side. Consider for example the Non-Radiative Dielectric waveguide (NRD).
The spacing a between the metal plates is less than λ0/2 so that all junctions and discontinuities (also curves) that maintain symmetry become purely reactive, instead of possessing radiative content. When the vertical metal plates in the NRD guide are sufficiently long, the dominant-mode field is completely bound, since it has decayed to negligible values as it reaches the upper and lower open ends. If the upper portion of the plates is foreshortened, as in Fig. 3, a traveling-wave field of finite amplitude then exists at the upper open end, and if the dominant NRD guide mode is fast (it can be fast or slow depending on the frequency), power will be radiated away at an angle from this open end.
Another possible mechanism is asymmetry. In the asymmetrical NRD-guide antenna depicted in Fig. 4, the structure is first bisected horizontally with a metallic wall, to provide radiation from one end only; since the electric field is purely vertical at this midplane, the field structure in not altered by the bisection. An air gap is then introduced into the dielectric region to produce asymmetry. As a result, a small amount of net horizontal electric field is created, which produces a mode in the parallel-plate air region, which is a TEM mode, which propagates at an angle between the parallel plates until it reaches the open end and leaks away. It is necessary to maintain the parallel plates in the air region sufficiently long so that the vertical electric-field component of the original mode (represented in the parallel-plate guide by the below-cutoff TM1 mode) has decayed to negligible values at the open end. Then the TEM mode, with its horizontal electric field, is the only field left at the antenna aperture, and the field polarization is then essentially pure (the discontinuity at the open end does not introduce any cross-polarized field components).
Groove guide (shown in Fig. 5) is a low-loss open waveguide for millimeter waves, somewhat similar to the NRD guide: the dielectric central region is replaced by an air region of larger width (greater than λ0/2). The field again decays exponentially in the regions of narrower width above and below. The leaky-wave antenna is created by first bisecting the groove guide horizontally. It also resembles a stub-loaded rectangular waveguide.
When the stub is off-centered, the asymmetrical structure obtained will radiate. When the offset is increased, the attenuation constant α will increase and the beamwidth will increase too. When the stub is placed all the way to one end, the result is an L-shaped structure that radiates very strongly.
In addition, it is found that the value of β changes very little as the stub is moved, and α varies over a very large range. This feature allows to taper the antenna aperture to control sidelobes. The fact that the L-shaped structure strongly leaks may also be related to another leakage mechanism: the use of leaky higher modes. In particular, it may be found that all the groove-guide higher modes are leaky.
For example, consider the first higher antisymmetric mode. Because of the symmetry of the structure and the directions of the electric-field lines, the structure can be bisected twice to yield the L-shaped, as represented in Fig. 6.
The antenna may be analyzed using a transverse equivalent network based on a T-junction network. The expressions for the network elements are obtainable in simple closed forms and yet are very accurate. The resulting circuit is shown in Fig. 7.
Usually, the stub length needs only to be about half a wavelength or less if the stub is narrow.
To exploit the possibility of printed-circuit techniques, a printed-circuit version of the previous structure has been developed. In this way the fabrication process could make use of photolithography, and the taper design for sidelobe control could be handled automatically in the fabrication.
The structure is depicted in the inset of Fig. 8. The transverse equivalent network for this new antenna structure is slightly more complicated than the previous, and the expressions for the network elements must be appropriately modified to take the dielectric medium into account. Moreover, above the transformer, an additional susceptance appears.
The stub and main guides are no longer the same, so their wavenumbers and characteristic admittances are also different. Again,α can be varied by changing the slot location d, as it is apparent in Fig.8. However, it was found that a' is also a good parameter to change for this purpose, as shown in Fig.9.
An interesting variation of the previous structures has been developed and analyzed. It is based on a ridge or stepped waveguide rather than a rectangular waveguide. In the structures based on rectangular waveguide, the asymmetry was achieved by placing the stub guide, or locating the longitudinal slot, off-center on the top surface.
Here the top surface is symmetrical, and the asymmetry is created by having unequal lengths on each side under the main-guide portion, as shown in Fig. 10. The transverse equivalent networks, together with the associated expressions for the network elements, were adapted and extended to apply to these new structures. The equivalent circuit is represented in Fig. 11. An analysis of the antenna behavior indicates that this geometry effectively permits independent control of the angle of maximum radiation θm and of the beamwidth Δθ. Let us define two geometric parameters: the relative average arm length bm/a where bm = (bl+br)/2, and the relative unbalance Δb/bm where Δb=(bl+br)/2 . Figure 10: Stepped guide. It then turns out that by changing bm/a one can adjust the value of β/k0 without altering α/k0 much, and that by changing Δb/bm one can vary α/k0 over a large range without affecting β/k0 much.
The taper design for controlling the sidelobe level would therefore involve only the relative unbalance Δb/bm. The transverse equivalent network is slightly complicated by the presence of two additional changes in height of the waveguide, which can be modeled by means of shunt susceptances and ideal transformers. The ideal transformer accounts for the change in the characteristic impedance, while the storing of reactive energy is taken into account through the susceptance. Scanning arrays achieve scanning in two dimensions by creating a one-dimensional phased array of leaky-wave line-source antennas. The individual line sources are scanned in elevation by varying the frequency. Scanning in the cross plane, and therefore in azimuth, is produced by phase shifters arranged in the feed structure of the one-dimensional array of line sources. The radiation will therefore occur in pencil-beam form and will scan in both elevation and azimuth in a conical-scan manner. The spacing between the line sources is chosen such that no grating lobes occur, and accurate analyses show that no blind spots appear anywhere. The described arrays have been analyzed accurately by unit-cell approach that takes into account all mutual-coupling effects. Each unit cell incorporates an individual line-source antenna, but in the presence of all the others. The radiating termination on the unit cell modifies the transverse equivalent network. A key new feature of the array analysis is therefore the determination of the active admittance of the unit cell in the two-dimensional environment as a function of scan angle. If the values of β and α did not change with phase shift, the scan would be exactly conical. However, it is found that these values change only a little, so that the deviation from conical scan is small. We next consider whether or not blind spots are present. Blind spots refer to angles at which the array cannot radiate or receive any power; if a blind spot occurred at some angle, therefore, the value of α would rapidly go to zero at that scan angle. To check for blind spots, we would then look for any sharp dips in the curves of α/k0 as a function of scan angle. No such dips were ever found. Typical data of this type exhibit fairly flat behavior for α/k0 until the curves drop quickly to zero as they reach the end of the conical-scan range, where the beam hits the ground.
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.
The propagation constant of a sinusoidal electromagnetic wave is a measure of the change undergone by the amplitude and phase of the wave as it propagates in a given direction. The quantity being measured can be the voltage, the current in a circuit, or a field vector such as electric field strength or flux density. The propagation constant itself measures the change per unit length, but it is otherwise dimensionless. In the context of two-port networks and their cascades, propagation constant measures the change undergone by the source quantity as it propagates from one port to the next.
A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities decrease according to the inverse square law as they expand into three dimensional space.
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main advantage of a parabolic antenna is that it has high directivity. It functions similarly to a searchlight or flashlight reflector to direct the radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highest gains, meaning that they can produce the narrowest beamwidths, of any antenna type. In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the wavelength of the radio waves used, so parabolic antennas are used in the high frequency part of the radio spectrum, at UHF and microwave (SHF) frequencies, at which the wavelengths are small enough that conveniently-sized reflectors can be used.
In electromagnetics, an evanescent field, or evanescent wave, is an oscillating electric and/or magnetic field that does not propagate as an electromagnetic wave but whose energy is spatially concentrated in the vicinity of the source. Even when there is a propagating electromagnetic wave produced, one can still identify as an evanescent field the component of the electric or magnetic field that cannot be attributed to the propagating wave observed at a distance of many wavelengths.
A transverse mode of electromagnetic radiation is a particular electromagnetic field pattern of the radiation in the plane perpendicular to the radiation's propagation direction. Transverse modes occur in radio waves and microwaves confined to a waveguide, and also in light waves in an optical fiber and in a laser's optical resonator.
A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz. They are used as feed antennas for larger antenna structures such as parabolic antennas, as standard calibration antennas to measure the gain of other antennas, and as directive antennas for such devices as radar guns, automatic door openers, and microwave radiometers. Their advantages are moderate directivity, low standing wave ratio (SWR), broad bandwidth, and simple construction and adjustment.
A prism coupler is a prism designed to couple a substantial fraction of the power contained in a beam of light into a thin film to be used as a waveguide without the need for precision polishing of the edge of the film, without the need for sub-micrometer alignment precision of the beam and the edge of the film, and without the need for matching the numerical aperture of the beam to the film. Using a prism coupler, a beam coupled into a thin film can have a diameter hundreds of times the thickness of the film. Invention of the coupler contributed to the initiation of a field of study known as integrated optics.
In radio-frequency engineering and communications engineering, waveguide is a hollow metal pipe used to carry radio waves. This type of waveguide is used as a transmission line mostly at microwave frequencies, for such purposes as connecting microwave transmitters and receivers to their antennas, in equipment such as microwave ovens, radar sets, satellite communications, and microwave radio links.
Stripline is a transverse electromagnetic (TEM) transmission line medium invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s. Stripline is the earliest form of planar transmission line.
Radar engineering details are technical details pertaining to the components of a radar and their ability to detect the return energy from moving scatterers — determining an object's position or obstruction in the environment. This includes field of view in terms of solid angle and maximum unambiguous range and velocity, as well as angular, range and velocity resolution. Radar sensors are classified by application, architecture, radar mode, platform, and propagation window.
A dielectric resonator antenna (DRA) is a radio antenna mostly used at microwave frequencies and higher, that consists of a block of ceramic material of various shapes, the dielectric resonator, mounted on a metal surface, a ground plane. Radio waves are introduced into the inside of the resonator material from the transmitter circuit and bounce back and forth between the resonator walls, forming standing waves. The walls of the resonator are partially transparent to radio waves, allowing the radio power to radiate into space.
A Substrate integrated waveguide (SIW) is a synthetic rectangular electromagnetic waveguide formed in a dielectric substrate by densely arraying metallized posts or via-holes which connect the upper and lower metal plates of the substrate. The waveguide can be easily fabricated with low-cost mass-production using through-hole techniques where the post walls consists of via fences. SIW is known to have similar guided wave and mode characteristics to conventional rectangular waveguide with equivalent guide wavelength.
The non-radiative dielectric (NRD) waveguide has been introduced by Yoneyama in 1981. In Fig. 1 the cross section of NRD guide is shown: it consists of a dielectric rectangular slab of height a and width b, which is placed between two metallic parallel plates of suitable width. The structure is practically the same as the H waveguide, proposed by Tischer in 1953. Due to the dielectric slab, the electromagnetic field is confined in the vicinity of the dielectric region, whereas in the outside region, for suitable frequencies, the electromagnetic field decays exponentially. Therefore, if the metallic plates are sufficiently extended, the field is practically negligible at the end of the plates and therefore the situation does not greatly differ from the ideal case in which the plates are infinitely extended. The polarization of the electric field in the required mode is mainly parallel to the conductive walls. As it is known, if the electric field is parallel to the walls, the conduction losses decrease in the metallic walls at the increasing frequency, whereas, if the field is perpendicular to the walls, losses increase at the increasing frequency. Since the NRD waveguide has been devised for its implementation at millimeter waves, the selected polarization minimizes the ohmic losses in the metallic walls.
In radio and telecommunication, a traveling-wave antenna is a class of antenna that uses a traveling wave on a guiding structure as the main radiating mechanism. Its distinguishing feature is that the radio-frequency current that generates the radio waves travels through the antenna in one direction. This is in contrast to a resonant antenna, such as the monopole or dipole, in which the antenna acts as a resonator, with radio currents traveling in both directions, bouncing back and forth between the ends of the antenna. An advantage of traveling wave antennas is that since they are nonresonant they often have a wider bandwidth than resonant antennas. Common types of traveling wave antenna are the Beverage antenna, axial-mode helical antenna, and rhombic antenna.
An antenna array is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves. The individual antennas are usually connected to a single receiver or transmitter by feedlines that feed the power to the elements in a specific phase relationship. The radio waves radiated by each individual antenna combine and superpose, adding together to enhance the power radiated in desired directions, and cancelling to reduce the power radiated in other directions. Similarly, when used for receiving, the separate radio frequency currents from the individual antennas combine in the receiver with the correct phase relationship to enhance signals received from the desired directions and cancel signals from undesired directions. More sophisticated array antennas may have multiple transmitter or receiver modules, each connected to a separate antenna element or group of elements.
A frequency-selective surface (FSS) is any thin, repetitive surface designed to reflect, transmit or absorb electromagnetic fields based on the frequency of the field. In this sense, an FSS is a type of optical filter or metal-mesh optical filters in which the filtering is accomplished by virtue of the regular, periodic pattern on the surface of the FSS. Though not explicitly mentioned in the name, FSS's also have properties which vary with incidence angle and polarization as well - these are unavoidable consequences of the way in which FSS's are constructed. Frequency-selective surfaces have been most commonly used in the radio frequency region of the electromagnetic spectrum and find use in applications as diverse as the aforementioned microwave oven, antenna radomes and modern metamaterials. Sometimes frequency selective surfaces are referred to simply as periodic surfaces and are a 2-dimensional analog of the new periodic volumes known as photonic crystals.
A lens antenna is a microwave antenna that uses a shaped piece of microwave-transparent material to bend and focus the radio waves by refraction, as an optical lens does for light. Typically it consists of a small feed antenna such as a patch antenna or horn antenna which radiates radio waves, with a piece of dielectric or composite material in front which functions as a converging lens to collimate the radio waves into a beam. Conversely, in a receiving antenna the lens focuses the incoming radio waves onto the feed antenna, which converts them to electric currents which are delivered to a radio receiver. They can also be fed by an array of feed antennas, called a focal plane array (FPA), to create more complicated radiation patterns.
A transmitarray antenna is a phase-shifting surface (PSS), a structure capable of focusing electromagnetic radiation from a source antenna to produce a high-gain beam. Transmitarrays consist of an array of unit cells placed above a source (feeding) antenna. Phase shifts are applied to the unit cells, between elements on the receive and transmit surfaces, to focus the incident wavefronts from the feeding antenna. These thin surfaces can be used instead of a dielectric lens. Unlike phased arrays, transmitarrays do not require a feed network, so losses can be greatly reduced. Similarly, they have an advantage over reflectarrays in that feed blockage is avoided.
Arthur Aaron Oliner was an American physicist and electrical engineer, who was Professor Emeritus at Department of Electrical and Computer Engineering at New York University-Polytechnic. Best known for his contributions to engineering electromagnetics and antenna theory, he is regarded as a pioneer of leaky wave theory and leaky wave antennas.
