Metasurface Cloaks to Decouple Closely Spaced Printed Dipole Antenna Arrays Fed by a Microstrip-to-Balanced Transmission-Line Transition

In this work, we present a numerical study of 1D and 2D closely spaced antenna arrays of microstrip dipole antennas covered by a metasurface in order to properly cloak and decouple the antenna arrays operating at neighboring frequencies. We show that the two strongly coupled arrays fed by a microstrip-to-balanced transmission-line transition are effectively decoupled in 1D and 2D array scenarios by covering the dipole antenna elements with an elliptically shaped metasurface. The metasurface comprises sub-wavelength periodic metallic strips printed on an elliptically shaped dielectric cover around the dipole antennas and integrated with the substrate. We present a practical design of cloaked printed dipole arrays placed in close proximity of each other and demonstrate that the arrays are decoupled in the near field and operate independently in the far field with their original radiation characteristics as if they were isolated

Graphene-Metal Metasurface for Cloaking of Cylindrical Objects at Low-Terahertz Frequencies

Most of the theoretical studies involving graphene, typically assume an ultra-high mobility and/or high value of Fermi energy to achieve exciting functionalities such as polarization conversion, absorption, cloaking, among others. In practice, however, graphene with such high mobility is very difficult to realize, on account of the numerous flaws and impurities that inevitably are introduced during the graphene growth and transfer process. This severely compromises graphene\u27s practical performance, despite the appeal of theoretical predictions. A novel design was presented in \u27X. Wang et al., IEEE Trans. Antenna Propagat., vol. 67, pp. 2452-2461, 2019\u27, wherein a graphene metasurface was devised to accomplish perfect absorption with excellent electrical tunability, for a wide span of practical mobility values for graphene (2,000 to 20,000 cm2V-1s-1). Motivated by this design, in this paper, we introduce a graphene-metal metasurface structure, which is engineered to facilitate the realization of practical cloaking of conducting cylindrical objects. Though the framework for our cloak is ostensibly similar to the metasurface structure in the aforementioned reference, the phenomenon that our structure is based on, is radically different; consequently, the functionality and the numerical analysis differs. When this specific metasurface is enveloped around the cylindrical objects, their scattering width reduces noticeably, thereby making them \u27invisible\u27 to the impinging plane wave. Our design demonstrates cloaking even for the most practical low-mobility graphene, utilizing extremely low Fermi energy values, thereby making our construct desirable not only in theory, but also feasible for practical applications

Effects of Spatial Dispersion on Reflection from Mushroom-type Artificial Impedance Surfaces

Several recent works have emphasized the role of spatial dispersion in wire media, and demonstrated that arrays of parallel metallic wires may behave very differently from a uniaxial local material with negative permittivity. Here, we investigate using local and non-local homogenization methods the effect of spatial dispersion on reflection from the mushroom structure introduced by Sievenpiper. The objective of the paper is to clarify the role of spatial dispersion in the mushroom structure and demonstrate that under some conditions it is suppressed. The metamaterial substrate, or metasurface, is modeled as a wire medium covered with an impedance surface. Surprisingly, it is found that in such configuration the effects of spatial dispersion may be nearly suppressed when the slab is electrically thin, and that the wire medium can be modeled very accurately using a local model. This result paves the way for the design of artificial surfaces that exploit the plasmonic-type response of the wire medium slab.Comment: submitted for publication, under revie

Exceptional Points of Degeneracy and Branch Points for Transmission-Line Problems - Linear Algebra and Bifurcation Theory Perspectives

We demonstrate several new aspects of exceptional points of degeneracy (EPD) pertaining to propagation in two uniform coupled transmission-line structures. We describe an EPD using two different approaches - by solving an eigenvalue problem based on the system matrix, and as a singular point from bifurcation theory, and the link between these two disparate viewpoints. Cast as an eigenvalue problem, we show that eigenvalue degeneracies are always coincident with eigenvector degeneracies, so that all eigenvalue degeneracies are implicitly EPDs in two uniform coupled transmission lines. Furthermore, we discuss in some detail the fact that EPDs define branch points (BPs) in the complex-frequency plane; we provide simple formulas for these points, and show that parity-time (PT) symmetry leads to real-valued EPDs occurring on the real-frequency axis. We discuss the connection of the linear algebra approach to previous waveguide analysis based on singular points from bifurcation theory, which provides a complementary viewpoint of EPD phenomena, showing that EPDs are singular points of the dispersion function associated with the fold bifurcation. This provides an important connection of various modal interaction phenomena known in guided-wave structures with recent interesting effects observed in quantum mechanics, photonics, and metamaterials systems described in terms of the EPD formalism

Transmission through stacked 2D periodic distributions of square conducting patches

In this paper, we study the transmissivity of electromagnetic waves through stacked two-dimensional printed periodic arrays of square conducting patches. An analytical circuit-like model is used for the analysis. The model accounts for the details of the transmission spectrum provided that the period of the unit cell of each patterned layer is well below the wavelength in the dielectric slabs separating the printed surfaces. In particular, we analyze the low-pass band and rejection band behavior of the multilayer structure, and the results are validated by comparison with a computationally intensive finite element commercial electromagnetic solver. The limiting case of an infinite periodic structure is analytically solved and the corresponding band structure is used to explain the passband/stopband behavior of finite structures. In addition, we study in depth the elementary unit cell consisting of a single dielectric slab coated by two metal patch arrays, and its resonance behavior is explained in terms of Fabry-Pérot resonances when the electrical thickness of the slab is large enough. In such case, the concept of equivalent thickness of the equivalent ideal Fabry-Pérot resonator is introduced. For electrically thinner slabs it is also shown that the analytical model is still valid, and its corresponding first transmission peak is explained in terms of a lumped-circuit LC resonance.Ministerio de Ciencia e Innovación TEC2010-16948, CSD2008-00066Junta de Andalucía P09-TIC-459

Analytical circuit model for stacked slit gratings

This work presents a rigorous circuit model to compute the transmission/reflection properties of a finite number of stacked slit gratings printed on dielectric slabs of arbitrary thickness. A key aspect of the present approach is that the circuit model itself leads us to find fully analytical expressions for the finite stacked-grating structure. An analytical model to obtain the Brillouin diagram for the fully periodic structure (infinite number of identical unit cells) is also provided.Ministerio de Economía y Competitividad TEC20iO-16948, CSD2008-00066Junta de Andalucía P12-TIC-143

Study of forward and backward modes in double-sided dielectric-filled corrugated waveguides

This work studies the propagation characteristics of a rectangular waveguide with aligned/ misaligned double-sided dielectric-filled metallic corrugations. Two modes are found to propagate in the proposed double-sided configuration below the hollow-waveguide cutoff frequency: a quasi resonant mode and a backward mode. This is in contrast to the single-sided configuration, which only allows for backward propagation. Moreover, the double-sided configuration can be of interest for waveguide miniaturization on account of the broader band of its backward mode. The width of the stopband between the quasi-resonant and backward modes can be controlled by the misalignment of the top and bottom corrugations, being null for the glide-symmetric case. The previous study is complemented with numerical results showing the impact of the height of the corrugations, as well as the filling dielectric permittivity, on the bandwidth and location of the appearing negative effective-permeability band. The multi-modal transmission-matrix method has also been employed to estimate the rejection level and material losses in the structure and to determine which port modes are associated with the quasi-resonant and backward modes. Finally, it is shown that glide symmetry can advantageously be used to reduce the dispersion and broadens the operating band of the modesEuropean Union COST Action SyMat CA18223Ministerio de Ciencia, Innovación y Universidades TEC2017-84724-