Abstract
The deep penetration of electromagnetic waves into lossy media can be obtained by properly generating inhomogeneous waves. In this work, for the very first time, we demonstrate the physical ...implementation and the practical relevance of this phenomenon. A thorough numerical investigation of the deep-penetration effects has been performed by designing and comparing three distinct practical radiators, emitting either homogeneous or inhomogeneous waves. As concerns the latter kind, a typical Menzel microstrip antenna is first used to radiate improper leaky waves. Then, a completely new approach based on an optimized 3-D horn TEM antenna applied to a lossy prism is described, which may find applications even at optical frequencies. The effectiveness of the proposed radiators is measured using different algorithms to consider distinct aspects of the propagation in lossy media. We finally demonstrate that the deep penetration is possible, by extending the ideal and theoretical evidence to practical relevance, and discuss both achievements and limits obtained through numerical simulations on the designed antennas.
This paper illustrates how the penetration of electromagnetic waves in lossy media strongly depends on the waveform and not only on the media involved. In particular, the so-called inhomogeneous ...plane waves are compared against homogeneous plane waves illustrating how the first ones can generate deep penetration effects. Moreover, the paper provides examples showing how such waves may be practically generated. The approach taken here is analytical and it concentrates on the deep penetration conditions obtained by means of incident inhomogeneous plane waves incoming from a lossless medium and impinging on a lossy medium. Both conditions and constraints that the waveforms need to possess to achieve deep penetration are analysed. Some results are finally validated through numerical computations. The theory presented here is of interest in view of a practical implementation of the deep penetration effect.
A planar antenna radiating twisted beams with different azimuthal order and a consistent beam angle is designed by employing a single uniform circular array embedded in a Fabry-Perot cavity. Circular ...phased arrays placed in free space are commonly employed to radiate conical beams carrying orbital angular momentum. However, the beam angle depends on both the array radius and the azimuthal order of the beam, thus requiring the use of multiple concentric circular arrays in order to produce beams with different azimuthal order and a common beam angle. In the proposed design, this is simply achieved by exciting higher-order cylindrical leaky waves through a single circular array feeding a Fabry-Perot cavity. Such waves radiate conical patterns whose beam angle is mainly determined by the relevant radial wavenumber and only weakly depends on the azimuthal order. In particular, we propose here an antenna design capable of radiating beams with azimuthal orders 0, ±1, ±2, and ±3 in the microwave range. The cavity is fed by an array of coaxial probes optimized for input matching through the inclusion of parasitic metal pins. Numerical full-wave simulations validate the effectiveness of the proposed design in terms of radiation patterns, passive input scattering parameters, and active input reflection coefficients.
The design of a K-band radial leaky-wave antenna is presented for polarization diversity applications. The antenna structure is constituted by an annular, radially periodic, and metallic strip ...grating printed on top of a single-layer grounded dielectric slab. The integrated feeding system is defined by a <inline-formula> <tex-math notation="LaTeX">2 \times 2 </tex-math></inline-formula> array of planar slot sources for cylindrical surface-wave excitation. By the addition of the grating, the surface wave is perturbed and enables cylindrical leaky-wave radiation by means of a fast <inline-formula> <tex-math notation="LaTeX">n = -1 </tex-math></inline-formula> space harmonic, whose behavior is characterized through a full-wave dispersive analysis. By proper phasing and spacing of the four independent TM feeds, positioned close to the center of the annular grating and on the ground plane, we demonstrate the possibility of radiating directive broadside beams offering linear, left- or right-handed circularly polarized radiation, and sum and delta patterns. Thus, we propose an original solution to flexibly control the polarization of a high-gain beam by means of a simple and low-cost feeding system, made by the minimum number of integrated array sources. To accurately assess the antenna features and performance, the role of a zeroth- and first-order cylindrical leaky waves propagating along the antenna aperture is also discussed. The proposed antenna design may be of interest for direction-of-arrival estimation by means of monopulse radars, as well as for a wide class of applications where flexible control of the polarization is desired, such as satellite and terrestrial point-to-point communication systems and earth observation.
The problem of improving the computational efficiency in the numerical analysis of planar periodic structures is investigated here using the mixed-potential integral-equation (MPIE) approach. A new ...regularization of the periodic Green's functions (PGFs) that are involved in the analysis of multilayered structures is introduced, based on the effective-medium concept. This regularization involves extracting the singularities of the PGFs up to second-order terms. The resulting regularized PGF is very smooth and amenable to interpolation. Thus, optimized interpolation procedures for the PGFs can be applied, resulting in a considerable reduction of computation time without any significant effect on the accuracy. Another benefit of the regularization is that it significantly enhances the convergence of the series for both the vector- and scalar-potential PGFs. The theoretical formulation is fully validated with various numerical results for both two-dimensional (2-D) and one-dimensional (1-D) layered-media periodic structures.
A planar and low-cost parallel-plate waveguide (PPW) launcher based on substrate integrated waveguide (SIW) technology is proposed. It can launch the fundamental TEM mode with a collimated wave front ...into a PPW structure while also being low profile and easy to fabricate. The launcher is implemented using a network of SIW transmission lines and its design is based on the leaky-wave (LW) theory. In our proposed structure, a selection of the SIW lines is made to leak power into the desired PPW region by means of a partially reflective surface, enabling planar wave front propagation of the TEM mode where efficiency values of over 90% are achieved. Measurements of a fabricated prototype are also reported at 15 GHz to demonstrate the design procedure and validate structure operation. When compared with similar co-axial and metallic waveguide launchers, our proposed SIW structure represents a compact, integrated, and unique design solution which may be useful to feed planar parallel-plate slot arrays and end-fire antennas. In addition, our novel and simple approach for PPW excitation can also be applied to other parallel-plate-based guides, low-cost transitions, and compact dividing/combining circuits for a new antenna and non-radiating LW feed systems at microwave and millimeter-wave frequencies.
Tunable THz antennas based on a single unpatterned graphene sheet placed inside a grounded dielectric multilayer are studied with the aim of characterizing their performance in terms of pattern ...reconfigurability, directivity, and radiation efficiency. The considered structures belong to the class of Fabry-Perot cavity (FPC) antennas, whose radiation mechanism relies on the excitation of cylindrical leaky waves with an ordinary (i.e., nonplasmonic) sinusoidal transverse modal profile. This allows for achieving radiation efficiencies considerably higher than those of alternative graphene-based radiators based on the excitation of surface-plasmon polaritons (SPPs) either in bound or leaky propagation regimes. A customized efficient circuit model has been employed in order to obtain all the radiation characteristics of such graphene FPC antennas, which have also been fully validated by means of a CAD tool. The role of the graphene quality is explicitly taken into account in this comprehensive investigation, proving that it plays a remarkable role in establishing the antenna performance. In particular, it is expected that the standard quality of graphene allows for designing low-efficiency reconfigurable THz antennas based on SPPs and, conversely, high-efficiency FPC antennas with slightly reduced reconfigurability.
Dispersion and radiation properties for bound and leaky modes supported by 1-D printed periodic structures are investigated. A new type of Brillouin diagram is presented that accounts for different ...types of physical leakage, namely, leakage into one or more surface waves or also simultaneously into space. This new Brillouin diagram not only provides a physical insight into the dispersive behavior of such periodic structures, but it also provides a simple and convenient way to correctly choose the integration paths that arise from a spectral-domain moment-method analysis. Numerical results illustrate the usefulness of this new Brillouin diagram in explaining the leakage and stopband behavior for these types of periodic structures.
Directive pencil beams scannable in both elevation and azimuth are obtained through a planar phased array placed inside a Fabry-Perot cavity. The key element of the proposed approach is the ...exploitation of a conical element pattern (EP) with high directivity in elevation, obtained through the excitation of a dominant, weakly attenuated cylindrical TM leaky wave of azimuthal order <inline-formula> <tex-math notation="LaTeX">n = 0 </tex-math></inline-formula> by means of a simple coaxial probe. Then, a highly reduced number <inline-formula> <tex-math notation="LaTeX">N </tex-math></inline-formula> of such sources are arranged to form a phased array radiating directive pencil beams. Beam-angle reconfigurability with continuous scanning both in azimuth and elevation inside a wide solid angular range is achieved by varying the array phasing and the operating frequency. An investigation on the features of truncated cylindrical leaky waves is first developed to properly characterize the EP. Then, conventional array theory is exploited to calculate the pattern of the entire array. The radiation efficiency is also evaluated accounting for the spurious surface wave related to the undesired excitation of the quasi-TEM mode. The proposed array design provides a simple and inexpensive innovative solution for obtaining a high-gain pencil beam continuously scanning in the 3-D space without suffering gain losses. In the presented implementation, the elevation angular scan, which is generally constrained by the wideband capability of the feeding system, by the requirements on the sidelobe level, and by the cutoff of the relevant leaky mode, ranges from about 21° to about 68°. Possible applications are envisaged for the next generation of wireless power transfer devices, for advanced radar and surveillance systems, earth observation, as well as for ceiling-mounted indoor localization and tracking.
A very efficient and accurate method is proposed to evaluate the lattice sums (LSs) for the analysis of leaky waves in 2-D periodic waveguides. The LSs are the series involving Hankel functions of ...arbitrary order, which are not convergent for complex wavenumbers. It is shown that by extending Ewald representations to higher order Hankel functions, the LSs can be expressed in terms of spatial and spectral series, granting Gaussian convergence even in the case of complex and leaky waves. The method allows for the appropriate choice of the spectral determination for each space harmonic of a given LS coefficient, thus permitting one to obtain modal solutions that may correspond to physical and nonphysical leaky-wave phenomena. First, the proposed LS calculation is exploited in the evaluation of the free-space 1-D periodic Green's function for 2-D structures. Then, the same procedure for the LSs is implemented in a cylindrical harmonic expansion method, based on the transition-matrix and the generalized reflection-matrix approach, for the full-wave analysis of leaky modes in 2-D electromagnetic band-gap waveguides formed by layered arrays of cylindrical inclusions. The presented LS formalism is numerically slim, very fast, and thus well suited for the analysis of a significant class of lossy periodic waveguides and leaky-wave antennas.
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