Alejandro Alvarez-Melcon

Design of tapered leaky-wave antennas in hybrid waveguide-planar technology for millimeter waveband applications

Different types of waveguide leaky-wave antennas, asymmetrically perturbed with printed-circuits, are studied in this paper. The capability to modify the leakage constant of the excited leaky-wave mode, while maintaining unchanged its phase constant, is studied in detail for each design. Several slot and strip configurations are proposed, in which the width and the position of the planar printed circuit perturbation is modified along the length of the antenna to obtain different tapering topologies. All of them are mechanically flexible to realize, thus simplifying the manufacturing process for millimeter wavelengths applications. The two-dimensional method used for the analysis is accurate for any geometry of the planar perturbation, and also allows for the computation of the whole spectrum of modes in these open waveguides. This makes possible to predict the appearance of unwanted channel-guide modes, and also allows for the study of the coupling effect between the desired leaky-wave mode and them. A tapered design is performed and compared with full-wave three-dimension simulations to validate the proposed technology.

Graphene-Based Plasmonic Tunable Low-Pass Filters in the Terahertz Band

We propose the concept, synthesis, analysis, and design of graphene-based plasmonic tunable low-pass filters operating in the terahertz band. The proposed structure is composed of a graphene strip transferred onto a dielectric and a set of polysilicon dc gating pads located beneath it. This structure implements a stepped impedance low-pass filter for the propagating surface plasmons by adequately controlling the guiding properties of each strip section through graphenes field effect. A synthesis procedure is presented to design filters with desired specifications in terms of cutoff frequency, in-band performance, and rejection characteristics. The electromagnetic modeling of the structure is efficiently performed by combining an electrostatic scaling law to compute the guiding features of each strip section with a transmission line and transfer-matrix framework, approach further validated via full-wave simulations. The performance of the proposed filters is evaluated in practical scenarios, taking into account the presence of the gating bias and the influence of graphenes losses. These results, together with the high miniaturization associated with plasmonic propagation, are very promising for the future use and integration of the proposed filters with other graphene and silicon-based elements in innovative terahertz communication systems.

Analysis and design of periodic leaky-wave antennas for the millimeter waveband in hybrid waveguide-planar technology

This work presents a full-wave integral equation approach specifically conceived for the analysis and design of laterally-shielded rectangular dielectric waveguides, periodically loaded with planar perturbations of rectangular shape. This type of open periodic waveguide supports the propagation of leaky-wave modes, which can be used to build leaky-wave antennas which exhibit many desirable features for millimeter waveband applications. The particularities of the leaky-mode analysis theory are described in this paper, and comparisons with other methods are presented for validation purposes. Using this leaky-mode analysis method, a novel periodic leaky-wave antenna is presented and designed. This novel antenna shows some important improvements with respect to the features of previously proposed antennas. The results of the designed radiation patterns are validated with three-dimensional electromagnetic analysis using commercial software.

Spatially Dispersive Graphene Single and Parallel Plate Waveguides: Analysis and Circuit Model

The propagation of surface waves along spatially dispersive graphene-based 2-D waveguides is investigated in detail. Graphene is characterized using a full-kρ conductivity model under the relaxation-time approximation, which allows to obtain analytical and closed-formed expressions for the wavenumber of plasmons supported by sheets and parallel plate waveguides, respectively. Per unit length equivalent circuits are introduced to accurately characterize the propagation in different waveguides, and analytical relations between the effective TM-mode circuit lumped elements and graphene conductivity are derived. The proposed circuits allow identifying the different mechanisms involved in spatially dispersive plasmon propagation, explaining their connection with the intrinsic properties of graphene. Results demonstrate that spatial dispersion, which significantly decreases the confinement and the losses of slow surface plasmons, must be accurately assessed in the design of graphene-based plasmonic components at millimeter-waves and low terahertz frequencies.

Two techniques for the efficient numerical calculation of the Green's functions for planar shielded circuits and antennas

In this paper we present new contributions to the computation of the Greens functions arising in the analysis of multilayered shielded printed circuits and antennas. First the quasistatic term of the spectral domain Greens functions is extracted so that the convergence of the reminder dynamic modal series is enhanced. Moreover, it is shown that by extracting a second-order quasi-static term the convergence is further improved. In regard to the quasi-static terms, they are computed in the spatial domain by numerically evaluating the associated spatial images series. Then a new and efficient technique is developed for the summation of the slowly convergent modal series. The technique can be viewed as the application of the integration by parts technique to discrete sequences and greatly accelerates the convergence rate of the series involved. It is shown that the new algorithm is numerically very robust and leads to a drastic reduction in the computational effort and time usually required for the numerical evaluation of the shielded Greens functions.

Frequency Steerable Two Dimensional Focusing Using Rectilinear Leaky-Wave Lenses

The concept of frequency steerable two-dimensional electromagnetic focusing by using a tapered leaky-wave line source embedded in a parallel-plate medium is presented. Accurate expressions for analyzing the focusing pattern of a rectilinear leaky-wave lens (LWL) from its constituent leaky-mode tapered propagation constant are described. The influence of the main LWL structural parameters on the synthesis of the focusing pattern is discussed. The ability to generate frequency steerable focusing patterns has been demonstrated by means of an example involving a LWL in hybrid waveguide printed-circuit technology and the results are validated by a commercial full-wave solver.

Electrically and Magnetically Biased Graphene-Based Cylindrical Waveguides: Analysis and Applications as Reconfigurable Antennas

The propagation of surface waves along electrically and magnetically biased graphene-based cylindrical waveguides (GCWs) is investigated in detail. Analytical dispersion equations are derived for several GCW geometries, considering the presence of an inner metallic core and multiple (coaxial-like) graphene layers. The proposed formulation reveals a fundamental connection between surface plasmons found in GCWs/carbon nanotubes and planar graphene structures. Numerical results confirm the higher confinement of modes supported by GCWs compared with their planar counterparts, while keeping a similar level of losses. The proposed structure is applied to develop plasmonic reconfigurable dipole antennas in the low THz band, which provide higher radiation efficiency than current graphene-based radiators, without requiring the presence of bulky lenses. We envision that the proposed GCWs may find application in reconfigurable THz transceivers, near-field application, wireless interconnects, and sensing systems.

A Modified Pole-Zero Technique for the Synthesis of Waveguide Leaky-Wave Antennas Loaded With Dipole-Based FSS

An extension of the pole-zero matching method proposed by Stefano Maci for the analysis of electromagnetic bandgap (EBG) structures composed by lossless dipole-based frequency selective surfaces (FSS) printed on stratified dielectric media, is presented in this paper. With this novel expansion, the dipoles length appears as a variable in the analytical dispersion equation. Thus, modal dispersion curves as a function of the dipoles length can be easily obtained with the only restriction of single Floquet mode propagation. These geometry-dispersion curves are essential for the efficient analysis and design of practical EBG structures, such as waveguides loaded with artificial magnetic conductors (AMC) for miniaturization, or leaky-wave antennas (LWA) using partially reflective surfaces (PRS). These two practical examples are examined in this paper. Results are compared with full-wave 2D and 3D simulations showing excellent agreement, thus validating the proposed technique and illustrating its utility for practical designs.

Investigation on the Phenomenology of Impulse-Regime Metamaterial Transmission Lines

The phenomenology of impulse-regime propagation along metamaterial composite right/left-handed transmission lines (CRLH TL) is examined. For this purpose, a closed-form time-domain Greens function approach, based on the inverse Fourier transform of the 1D transmission line model, is applied. Since the CRLH TLs are electrically thin, this model is adequate to describe transient phenomena. The method is validated experimentally for the case of matched and mismatched CRLH impulse delay lines. Excellent agreement between theory and experiments is obtained. Various complex dispersive phenomena are investigated, including pulse delay as a function of the modulation frequency, pulse compression, and temporal Talbot pulse rate multiplication.

Design of Bandpass Transversal Filters Employing a Novel Hybrid Structure

A novel structure for the implementation of compact transversal bandpass filters is proposed in this paper. This new proposal consists of a hybrid structure, based on the combination of two different technologies: the waveguide and microstrip. It is shown that the novel hybrid microwave filter is able to implement both a second- and third-order filtering function with up to two or three transmission zeros, respectively. In this way, a practical implementation of a fully canonical transversal filter with a third degree response is for the first time directly achieved. In addition, the way to control the positions of transmission zeros with the new technology is treated in this paper. It is shown that symmetric or asymmetric responses and also filtering functions with a complex pair of transmission zeros can be easily obtained. To demonstrate the validity of this novel structure, several implementation examples are presented. A prototype has been designed, manufactured, and successfully tested, showing the practical validity of the new structure.