Leaky-wave Structure Research Articles

Modal Propagation and Excitation on a Wire-Medium Slab

A grounded wire-medium slab has recently been shown to support leaky modes with azimuthally independent propagation wavenumbers capable of radiating directive omnidirectional beams. In this paper, the analysis is generalized to wire-medium slabs in air, extending the omnidirectionality properties to even modes, performing a parametric analysis of leaky modes by varying the geometrical parameters of the wire-medium lattice, and showing that, in the long-wavelength regime, surface modes cannot be excited at the interface between the air and wire medium. The electric field excited at the air/wire-medium interface by a horizontal electric dipole parallel to the wires is also studied by deriving the relevant Green's function for the homogenized slab model. When the near field is dominated by a leaky mode, it is found to be azimuthally independent and almost perfectly linearly polarized. This result, which has not been previously observed in any other leaky-wave structure for a single leaky mode, is validated through full-wave moment-method simulations of an actual wire-medium slab with a finite size.

Half width leaky wave antennas

Leaky travelling wave antennas that use the first microstrip higher-order mode are analysed. The antennas investigated here are only half the width of previous leaky wave designs and utilise a structure that inherently suppresses the fundamental mode. Since mode purity is assured, the need for an elaborate feed structure is eliminated. A means for extracting the complex propagation constant of a leaky wave antenna is presented using the finite difference time domain (FDTD) method. Agreement is shown between the resulting propagation constant and another established theoretical method, the transverse resonance method. Representative measured far-field patterns are included, which are consistent with the computed propagation constants. The properties of curving the leaky wave structure are also studied.

Correction of Dielectric Losses in Practical Leaky-wave Antenna Designs

In this paper, the leaky-mode theory is applied to take into account for the dielectric losses in millimetre waveband inhomogeneous leaky-wave antennas. A practical dielectric-filled cosine-tapered periodic leaky-wave antenna working in the 45 GHz band is studied, showing how the desired sidelobes level and directivity are spoilt due to the effect of the losses. An iterative procedure is used to correct the negative effects of the losses in the radiation patterns of the leaky-wave structure. It is also shown the practical limits of the proposed correction approach. The leaky-mode theory is applied for the first time to compensate the losses in a practical leaky-wave antenna in hybrid waveguide printed circuit technology. This leaky-mode theory is validated with full-wave three-dimensional finite element method simulations of the designed antenna.

Periodic FDTD analysis of leaky-wave structures and applications to the analysis of negative-refractive-index leaky-wave antennas

The determination of the attenuation constants of periodic leaky-wave structures via the finite-difference time-domain (FDTD) method has been pursued so far via the simulation of a number of unit cells that is large enough to guarantee the convergence of the computed value. On the other hand, Brillouin diagrams of periodic structures can be readily extracted via the simulation of a single unit cell, terminated in periodic boundary conditions. This paper demonstrates a methodology that enables the concurrent extraction of leaky-wave attenuation constants and Brillouin diagrams of periodic structures, through the FDTD simulation of a unit cell. The proposed methodology is first validated and then employed to model leaky-wave radiation from a two-dimensional negative-refractive-index transmission-line (NRI-TL) medium. Apart from evaluating the characteristics of forward and backward leaky-wave radiation from such a medium, a lumped-element macro-model, with element values determined from the FDTD simulation, is extracted. The FDTD analysis, combined with this equivalent circuit, is used to investigate theoretically the possibility of the NRI-TL medium, as a leaky-wave antenna, to achieve continuous scanning from backward to forward end-fire and broadside radiation.

Finite element and perturbative study of buffered leaky planar waveguides

The effects of the presence of a high-index medium in the proximity of planar waveguiding structures that makes up buffered leaky waveguides, were studied using a finite element method (FEM) leaky mode solver and a perturbation method. Various phenomena observed in the FEM results were interpreted through the approximate analytical expressions derived using the perturbation method. The effect of the buffer layer thickness, the high-index medium refractive index, and the quasi-confinement of the modal field were investigated. The results show that the perturbation due to the high-index medium can lead to either an increase or a decrease of the real part of the effective index, and that the leakage loss of a TE-polarized mode is not always lower than TM-polarized mode of the same order. It was found that if the refractive index of the high-index medium goes to infinity, a leaky-wave structure evolves into a guided-wave structure.

Array factor approach of leaky-wave antennas and application to 1-D/2-D composite right/left-handed (CRLH) structures

An array factor approach of leaky-wave (LW) antennas is proposed for efficient and fast computation of the radiation patterns of complex metamaterial LW structures of arbitrary length from the analysis of a single periodic unit cell. This approach is demonstrated in one-dimensional and two-dimensional composite right/left-handed (CRLH) LW antennas.

Simple CAD model for a dielectric leaky-wave antenna

A simple model for a dielectric layer leaky-wave antenna is proposed; it represents the antenna as a leaky parallel-plate waveguide that consists of a grounded substrate layer with a shunt admittance screen on top. This simple model is then analyzed using a transverse equivalent network model in which the shunt admittance screen is modeled as a shunt load admittance in an equivalent circuit description. This equivalence allows for simple computer-aided design formulas to be obtained for all of the radiation properties of the dielectric layer leaky-wave antenna structure. Although derived in detail for a two-layer structure, the method can also be used for structures with multiple dielectric layers, which have been described in the literature as one-dimensional electromagnetic bandgap (EBG) structures.

Estimation of the radiation patterns of diverging/focusing type of leakywave antennas

AbstractIn this article, an extension of “field regions” to estimate simply and qualitatively the feature of the radiation patterns for the diverging/focusing systems with the inherent phase error on their aperture is discussed and applied to the curved leakywave structure. The result will be especially useful relating to the design of the curved leakywave structure in the microwave/mm‐wave applications. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 358–360, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10321

On a forming of cosecant square beam using a curved leakywave structure

A simple method of forming the cosecant square beam by using the curved leakywave structure based on the periodic structure is proposed. This method is a low-cost and easy design for beam shaping and is applicable as an antenna for relatively short-range communication systems at millimeter-wave band, including various wireless indoor LANs.

Efficient description of impedance and radiation features in printed-circuit leaky-wave structures-an unconventional scattering-matrix approach

This paper presents an original circuit model that furnishes an efficient description of the impedance and of the radiation performance for typical printed-circuit leaky-wave structures. In particular, referring to a junction between slot-coupled feeding and radiating microstrips, we have developed an unconventional equivalent transmission-line formulation, involving the propagation of the dominant mode for the feeding line and of the first higher mode for the radiating line, which is leaky. The quantification of the relevant scattering matrix, achieved with spectral-domain techniques, suitably summarizes the coupling effects and, consequently, the radiative features. Various antenna configurations have been tested, and the relevant results have been validated through comparisons with heavier full-wave numerical approaches. In addition to substantial computational advantages, this innovative approach gives the possibility of treating printed leaky-wave structures in a convenient fashion by means of a network formalism similar to that used in standard microwave circuits.

Analysis for a dielectrically filled parallel-plate waveguide with finite number of periodic slots in its upper wall as a leaky-wave antenna

A dielectrically filled parallel-plate waveguide (PPW) with finite number of periodic slots in its upper plate is analyzed as a leaky-wave antenna structure for E-polarization case. The characteristics of the finite periodic structure such as input impedance matching, antenna efficiency, gain, and radiation pattern are investigated. In particular, phenomena such as total reflection and transmission occurring in the practical finite periodic structure, associated with the coupling point and the real solution (propagation constant), respectively, in the fast wave region observed for the infinite open periodic structure case, are described in detail.

Diverging/focusing of electromagnetic waves by utilizing the curved leakywave structure: application to broad-beam antenna for radiating within specified wide-angle

A new "broad-beam antenna" for radiating electromagnetic energy in a specified wide angular region is proposed, utilizing the "curved leakywave structure". This structure consists of the leakywave structure curved physically and longitudinally into the form of an equi-angular spiral. The expression of the radiation field from this structure is given, the effects of various geometrical parameters on the radiation properties is discussed numerically for the design of the broad-beam antennas, and a simple experiment is also performed to verify the beam broadening by the proposed method. The concept in this paper may be easily applied to any leakywave structure such as a periodic dielectric antenna and a leaky nonradiative dielectric (NRD)-guide antenna for millimeter-wave application.

Radiation from Dielectric Leaky-Wave Antennas with Circular and Rectangular Apertures

ABSTRACT The dielectric leaky-wave antenna considered here consists of multiple dielectric layers above a ground plane, with a simple source such as a dipole inside the layers. The layers act as a guiding structure for the radiating leaky wave field, which propagates radially outward from the dipole source. Such an antenna may produce either a broadside beam or a scanned conical beam, depending on the chosen scan angle. For the practical case of a finite-size aperture, the radiation pattern is determined by integration of the tangential fields over the aperture. For narrow-beam structures the leaky-wave field on the aperture may extend to large distances, and hence it is important to have an efficient calculation of the radiation pattern for such cases. It is shown here how to efficiently calculate the radiation pattern for a dielectric leaky-wave antenna having a finite-size aperture that is either circular or rectangular in shape. The derived formulas apply for a dipole source inside any type of dielectric layer structure, and thus include the usual case of a microstrip antenna element on a single layer, as well as the more specific leaky-wave antenna structure considered here.

The nature of the spectral gap for leaky waves on a periodic strip-grating structure

The nature of the spectral gap at forward endfire for a periodic leaky-wave structure consisting of an infinite array of metallic strips on a lossless grounded dielectric layer is studied and compared with that for a simple grounded dielectric layer. The conclusions reached are expected to be valid for a general class of open periodic structures. One of the interesting features of the periodic structure is that a different branch choice is possible for each of the infinite number of space harmonics (although most of these will be nonphysical). This leads to an infinite number of steepest descent planes (SDP's) for the modal solutions, instead of only one as for the dielectric layer. As a result, one finds some interesting differences in the spectral-gap behavior, compared with that for the dielectric layer. One basic difference is that the nature of the spectral gap depends on whether or not a second space harmonic begins radiating before the main radiating harmonic is scanned to forward endfire. The spectral gap resembles that for a lossy dielectric layer when a second space harmonic is also radiating, and resembles that for a lossless dielectric layer otherwise. In addition, for the latter of these cases, the purely bound solution (which has a real propagation wavenumber and which occurs at the high-frequency end of the spectral gap) is physical only over a small frequency range, in contrast to the dielectric layer case, where that solution is present and physical at all higher frequencies. These behavioral differences are explained in detail in this paper.

Coupling between different leaky-mode types in stub-loaded leaky waveguides

Several leaky-wave antennas have been described in recent years that possess excellent properties, and they all have in common a parallel-plate stub guide of finite height as part of the cross section. For taller stubs and for larger leakage rates (wider radiated beams), some interesting new exotic interactions occur due to coupling between the desired leaky mode and another leaky mode, which is a modification of the "channel-guide" mode. Although we have described such coupling behavior briefly previously, this basic new feature (the coupling between two leaky (complex) modes) is not generally known, and is discussed here in more detail and in a more general context. In addition to a broad qualitative discussion, numerical results are presented for structures based on NRD guide and on stub-loaded rectangular guide. We believe that these effects are universal to all leaky-wave structures that possess a finite stub height in the cross section, so that the discussion here serves as a model for what to expect for other structures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Radiation characteristics of uniform and nonuniform dielectric leaky-wave antennas

An approach based on a waveguide model for analyzing the uniform and nonuniform dielectric leaky-wave structures is proposed for which the Galerkin solution method is applied. In the first part of the paper the far-field radiation of the uniform antenna is determined theoretically and experimentally in the Ku band and also the dependence of beamwidth and sidelobe level as function of strip width and frequency is demonstrated. In addition, an analysis of the antenna with a tapered-strip distribution-is presented. The second part describes a novel tapering procedure, performed by changing the width of the uniform antenna, for obtaining the radiation pattern with lower sidelobe levels. Sinusoidally and exponentially shaped dielectric leaky-wave structures are analyzed as nonuniform antennas. Finally, the radiation and scanning characteristics of these antennas are theoretically presented. >

An improved calculation procedure for the radiation pattern of a cylindrical leaky-wave antenna of finite size

A method is given for calculating the radiation pattern of a finite-size cylindrical leaky-wave antenna structure consisting of a single source near a planar surface that supports leaky-wave propagation. The method accounts for the finite radius of the planar surface by subtracting the radiation from that part of the leaky wave that lies beyond the physical surface, from the exact radiation pattern corresponding to an infinite planar surface. Experimental results obtained for a leaky-wave structure consisting of multiple dielectric layers above a ground plane confirm the improved accuracy of the method, particularly for the part of the radiation pattern away from the peak. >

The transition region between bound-wave and leaky-wave ranges for a partially dielectric-loaded open guiding structure

Most modes on partially dielectric-loaded open guiding structures are purely bound in some frequency range and leaky in another. The transition region between them is complicated and interesting, including a section where the dispersion curve doubles back because it connects a complex nonspectral (leaky-wave) solution with a real spectral (bound-wave or surface-wave) solution. The physical nature of this type of transition region is discussed qualitatively, where some anomalous features are considered; then numerical values for a specific example, a recently proposed novel leaky-wave antenna structure, are presented. >

Full-wave analysis of microstrip leaky-wave antenna

A full-wave analysis of a microstrip leaky-wave structure excited by the dominant LSM mode is presented. This approach is based on the waveguide model in which Galerkin&apos;s method is applied. Then, the far-field radiation can be computed from near fields using Fourier transforms.

New approach for computing radiation pattern of dielectric leaky-wave antenna

A novel approach based on a waveguide model is proposed for determining the complex propagation constant of a leaky wave structure by Galerkin's method. Then the far field radiation can be obtained in terms of the tangential electric fields in the Fourier transform domain. By this approach the undesirable sidelobe in the radiation pattern is shown and patterns with lower sidelobes can be obtained.