In this section we include those book chapters, journal papers and conference papers reporting results related to the project TEC2017-84724-P. It should be mentioned that a strong interaction with other national and international groups has been carried out during the development of this project, in such a way that several contributions are not exclusively associated with this specific project but are rather the result of those collaborations.
Abstract: A compact balanced-to-balanced diplexer composed of two balanced bandpass filters is proposed in this letter. The balanced filters are implemented using compact edge-coupled square split-ring resonators. The design methodology is based on the standard coupled-resonators filter synthesis procedure. First, each filter is independently designed. Then, they are connected to a common differential input port in order to achieve the desired diplexing operation, with the pertinent adjustments to take into account the loading effect of the second filter. Magnetic coupling inherently prevents common-mode transmission. An illustrative prototype example is provided with simulations and measurements to demonstrate the benefits of the proposed topology.
Abstract: Two balanced-to-balanced planar diplexers based on magnetically coupled microstrip resonators are proposed in this paper. For the first prototype, each channel/differential-output is composed of a second order single-band balanced bandpass filter based on open-loop resonators. For the second diplexer example, the filters composing the differential outputs are fourth order and are implemented by means of folded stepped-impedance resonators. The design procedure for the differential response is quite straightforward, since it is based on the use of the well-known external quality factor and coupling coefficients concepts. Common-mode is inherently rejected thanks to the benefits of magnetic coupling, which precludes common-mode transmission over a wide frequency range. The proposed structure also offers a high level of channel-to-channel isolation. To demonstrate the usefulness of the proposed idea, the two prototypes are simulated, fabricated, and measured. Good differential-mode and common-mode performance is observed in both examples. Simulations and measurements show good agreement.
Abstract: A fully analytical multimodal equivalent circuit is presented for the modeling of the scattering of an obliquely incident plane wave by a two-dimensional (2-D) periodic array of metallic patches (or apertures in a metallic screen) embedded in a layered medium. The topology of the equivalent network is rigorously derived in the analysis and all the network parameters are given in closed form. In contrast with the previously reported explicit circuit models, the proposed approach accounts for dynamical effects over a very wide frequency range, which enables the application of the model to a great variety of situations. The key advantages of the reported multimodal network representation are its analytical nature, its extremely low-computational cost and that the physical phenomena involved in the scattering can be easily understood in terms of transmission line and lumped circuital reasonings.
Abstract:
In the study of the propagation and scattering of electromagnetic waves, researchers have always
faced the dichotomy of dealing directly with Maxwell's equations and their complicated
analytical/numerical methods of solution or trying to find some simplified model (usually in terms
of an equivalent circuit) that can help to provide some physical insight into the involved
electromagnetic phenomena. The pursuit of analytical solutions for propagation, radiation, and
scattering problems in Microwave Engineering does not seem to be a very demanded task nowadays,
although it was only twenty years ago that many research papers on these topics used to include some
sort of analytic derivations. The impressive progress of computer hardware as well as Computational
Electromagnetic has made that most electromagnetic problems, either simple or very complex,
currently solvable by means of commercial simulators. This trend has the advantage of enabling the
analysis of many different complex electromagnetic scenarios by means of computational resources
affordable to most research laboratories. However, it has also brought up the unfortunate loss of
some fruitful skills, among them the ability to find out appropriate minimal-order models of complex
electromagnetic problems whose relevant parameters can be obtained in closed form. A particular type
of problems that was found to be very adequate to exploit the advantages of a minimal-order model
approach was the scattering of electromagnetic waves by discontinuities in waveguiding systems.
This paper has been awarded the "Best Paper Award" of the journal corresponding to year 2018
(awarded 2020).
Abstract: A critical discussion on the computation of the dispersion diagram of electromagnetic guiding/radiating structures with one-dimensional periodicity using general-purpose electromagnetic simulation software is presented in this work. In these methods, full-wave simulations of finite sections of the periodic structure are combined with appropriate simplifying network models. In particular, we analyze the advantages and limitations of two different combined methods, with emphasis on the determination of their range of validity. Our discussion is complemented with several selected numerical examples in order to show the most relevant aspects that a potential user of these methods should be aware of. Special attention is paid to the relevant role played by the high-order coupling between adjacent unit cells and between the two halves of unit cells exhibiting reflection, inversion, and glide symmetries.
Abstract: This letter shows an efficient hybrid method to study stacks of identical frequency selective surfaces (FSSs) separated by possibly different dielectrics. The method takes advantage of the ability of the equivalent circuit approach to create a Π-network for a pair of coupled FSSs and combines it with full-wave simulations involving a single FSS. This way, the solution of the complete problem of N stacked surfaces only involves full-wave simulations of a single periodic layer. Three scenarios have to be simulated where the surface is either standalone, or backed by perfect electric or perfect magnetic conductors. The proposed approach is validated through the analysis of several examples.
Abstract: This paper presents a study of the multipactor effect in a partially dielectric-loaded rectangular waveguide. To obtain the simulations presented in this paper, a detailed analysis of the dynamics of the electron inside this waveguide has been performed, taking into account the radio frequency electromagnetic fields propagating in the waveguide and the dc electric field that appears because of the charging of the dielectric layer. This electrostatic field is obtained by computing the electric potential produced by an arbitrary charge distribution on the dielectric layer in a dielectric-loaded waveguide. The electron trajectory is then found by numerically solving the equations of motion. The results obtained show that multipactor discharges do turn off by themselves under certain circumstances when they occur in such dielectric-loaded waveguide.
Abstract: A compact, balanced, dual-band bandpass filter with flexible and independent control of the differential-mode response is presented in this study. The filtering mechanism relies on the use of magnetically coupled embedded resonators. This provides two clear advantages: high level of miniaturisation and two differential-mode passbands that can be independently tuned. The response of the differential-mode is controlled by the external quality factor and coupling coefficients, whereas the common-mode is inherently rejected thanks to the use of a magnetic coupling mechanism. In order to illustrate the benefits of this structure, a prototype example is designed, fabricated and measured. Good differential- and common-mode performance has been obtained.
Abstract: The method of moments (MoM) in the spectral domain is used for the analysis of the scattering of a plane wave by a multilayered periodic structure containing conducting concentric split rings in the unit cell. Basis functions accounting for edge singularities are used in the approximation of the current density on the split rings, which makes it possible a fast convergence of MoM with respect to the number of basis functions. Since the 2-D Fourier transforms of the basis functions cannot be obtained in closed-form, judicious tricks (controlled truncation of infinite summations, interpolations, etc.) are used for the efficient numerical determination of these Fourier transforms. The implemented spectral domain MoM software has been used in the design of a circularly polarized reflectarray antenna based on split rings under the local periodicity condition. The antenna has been analyzed with our spectral domain MoM software, with CST and with HFSS, and good agreement has been found among all sets of results. Our software has proven to be around 27 times faster than CST and HFSS.
Abstract: This paper presents slow-wave transmission lines based on non-periodic reactive loading. Specifically, the loading elements are stepped impedance shunt stubs (SISS). By sacrificing periodicity using SISS tuned to different frequencies, multiple transmission zeros above the pass band arise, and the rejection level and bandwidth of the stop band is improved as compared with those of periodic structures. Through a proper design, it is possible to achieve compact lines, simultaneously providing the required electrical length and characteristic impedance at the design frequency (dictated by specifications), and efficiently filtering the response at higher frequencies. These lines are applied to the design of a compact power splitter with filtering capability in this work. The length of the splitter, based on a 35.35 Ω impedance inverter, is reduced by a factor of roughly two. Moreover, harmonic suppression better than 20 dB up to the fourth harmonic is achieved.
Abstract: Metasurfaces are thin two-dimensional metamaterial layers that allow or inhibit the propagation of electromagnetic waves in desired directions. For example, metasurfaces have been demonstrated to produce unusual scattering properties of incident plane waves or to guide and modulate surface waves to obtain desired radiation properties. These properties have been employed, for example, to create innovative wireless receivers and transmitters. In addition,metasurfaces have recently been proposed to confine electromagnetic waves, thereby avoiding undesired leakage of energy and increasing the overall efficiency of electromagnetic instruments and devices. The main advantages of metasurfaces with respect to the existing conventional technology include their low cost, low level of absorption in comparison with bulky metamaterials, and easy integration due to their thin profile. Due to these advantages, they are promising candidates for real-world solutions to overcome the challenges posed by the next generation of transmitters and receivers of future high-rate communication systems that require highly precise and efficient antennas, sensors, active components, filters, and integrated technologies. This Roadmap is aimed at binding together the experiences of prominent researchers in the field of metasurfaces, from which explanations for the physics behind the extraordinary properties of these structures shall be provided from viewpoints of diverse theoretical backgrounds. Other goals of this endeavour are to underline the advantages and limitations of metasurfaces, as well as to lay out guidelines for their use in present and future electromagnetic devices.
Abstract: In this study, a compact rat-race hybrid coupler with harmonic suppression based on slow-wave transmission lines (SW-TLs) is presented. Such artificial lines are implemented by periodic loading a host microstrip line with series meandered inductors and shunt patch capacitors. The presence of both loading elements has a twofold effect, i.e. phase velocity reduction (due to the enhancement of the effective inductance and capacitance of the periodic line), and the generation of a controllable stopband in the frequency response (due to the Bragg effect, inherent to periodicity). It is shown that by designing the unit cell of the periodic line with an electrical length of 45°, at least the first five harmonic bands of the rat-race coupler are efficiently suppressed, keeping the band of interest unaltered. Moreover, 79% size reduction, as compared to the ordinary coupler, is achieved in the reported SW-TL-based prototype.
Abstract: In this paper the Method of Moments (MoM) in the spectral domain is used for the analysis of multilayered structures containing periodic arrays of either patches or apertures. The patches and apertures may have many different geometries including complex surfaces limited by two parallel lines and two arbitrary curves, circular and elliptic rings, circular and elliptic arcs, and circular and elliptic sectors. Basis functions accounting for edge singularities are used in the approximation of the electric/magnetic current density on the patches/apertures, which enables a fast convergence of MoM with respect to the number of basis functions. Since the 2-D Fourier transforms of the basis functions cannot be obtained in closed-form, these Fourier transforms are efficiently computed by means of the nonuniform fast Fourier transform (NUFFT) algorithm. Results have been obtained for frequency-selective surfaces (FSSs), and for the elements used in the design of both reflectarray and metasurface antennas. The results obtained indicate that the software based on the NUFFT is only 15% slower than the standard spectral domain MoM software used for structures in which the 2-D Fourier transform of the basis functions is analytical, and between 50 and 80 times faster than CST.
Abstract: In this communication an efficient method of moments (MoM) code is used for the analysis of the extraordinary transmission (EOT) through a periodic array of rectangular slots in a conducting screen, in the case where the number of slots is finite in one direction and infinite in the orthogonal direction. The slots can be arbitrarily rotated within the periodic unit cell. Once the magnetic current density on the slots is obtained by means of MoM, both the transmission coefficient and the far-field radiated by the array of slots are computed. The onset of EOT turns out to be strongly dependent on the orientation of the slots with respect to the direction in which the array is infinite. If the slots are perpendicular to this direction, EOT appears for a single infinite chain of slots. However, tens of parallel chains of slots are required to reproduce the EOT response when the slots are aligned along the parallel chains direction. The obtained radiation patterns show the excitation of grating lobes as the number of slots grow in the direction where the arrays are finite.
Abstract: This paper studies wave propagation in a periodic parallel-plate waveguide with equilateral triangular holes. A mode-matching method is implemented to analyze the dispersion diagram of the structure possessing glide and mirror symmetries. Both structures present an unexpected high degree of isotropy, despite the triangle not being symmetric with respect to rotations of 90º. We give some physical insight on the matter by carrying out a modal decomposition of the total field on the hole and identifying the most significant modes. Additionally, we demonstrate that the electrical size of the triangular hole plays a fundamental role in the physical mechanism that causes that isotropic behavior. Finally, we characterize the influence of the different geometrical parameters that conform the unit cell (period, triangle size, hole depth, separation between metallic plates). The glide-symmetric configuration offers higher equivalent refractive indexes and widens the stopband compared to the mirror-symmetric configuration. We show that the stopband is wider as the triangle size is bigger, unlike holey structures composed of circular and elliptical holes where an optimal hole size exists.
Abstract: Mushroom-type electromagnetic bandgap (EBG) structures are known to operate as high-impedance surfaces at low-frequency bands. They are broadly used in the microwave regime. However, one of the main drawbacks of mushroom-type EBG structures is their narrow bandwidth. In this article, we propose a mushroom-type EBG structure with glide-symmetric edge vias to increase the operational bandwidth. This bandwidth increment is explained by the physical insight provided by an equivalent circuit model of the structure as well as the description of the field behavior. Simulation and measurement results show an improvement of approximately 67% over the case without glide symmetry in the structure. We conclude that applying glide symmetry to the mushroom-type EBG structures can improve their bandwidth without adding additional manufacturing costs.
Abstract: A new dual-band balanced bandpass filter based on magnetically coupled open-loop resonators in multilayer technology is proposed in this paper. The lower differential passband, centered at the Global Positioning System (GPS) L1 frequency, 1.575 GHz, was created by means of two coupled resonators etched in the middle layer of the structure, while the upper differential passband, centered at a Wi-Fi frequency of 2.4 GHz, was generated by coupling two resonators on the top layer. Magnetic coupling was used to design both passbands, leading to an intrinsic common-mode rejection of 39 dB within the lower passband and 33 dB within the upper passband. Simulation and measurement results are provided to verify the usefulness of the proposed dual-band differential bandpass filter.
Abstract: The equivalent circuit approach (ECA) is used in this work to analyze and design a previously proposed one-dimensional planar blazed grating of the resonant type. The analysis covers both the classical Littrow configuration, when the direction of the relevant diffracted order coincides with that of the incident wave (Bragg blazing), and when these directions are different (off-Bragg blazing). Once the scattering problem of the grating structure is posed as a discontinuity problem inside an equivalent generalized waveguide (corresponding to the unit cell of the original structure) and studied in terms of its equivalent circuit network, the possibility of transferring all the power of the incident plane wave into one single-diffraction order is seen as a simple impedance matching problem. An associated resonance phenomenon is also found to be implicitly associated with this matching condition. This simplifying and fruitful standpoint makes it possible to set up a systematic design procedure to find the specifications of the planar grating for either Bragg or non-Bragg blazing operation. Dielectric and strong skin-effect ohmic losses are easily implemented in the ECA and its effects in the practical design of structures are discussed.
Abstract: Transmission through seemingly opaque surfaces, so-called extraordinary transmission, provides an exciting platform for strong light-matter interaction, spectroscopy, optical trapping, and color filtering. Much of the effort has been devoted to understanding and exploiting TM extraordinary transmission, while TE anomalous extraordinary transmission has been largely omitted in the literature. This is regrettable from a practical point of view since the stronger dependence of the TE anomalous extraordinary transmission on the array's substrate provides additional design parameters for exploitation. To provide high-performance and cost-effective applications based on TE anomalous extraordinary transmission, a complete physical insight about the underlying mechanisms of the phenomenon must be first laid down. To this end, resorting to a combined methodology including quasi-optical terahertz (THz) time-domain measurements, full-wave simulations, and method of moments analysis, subwavelength slit arrays under s-polarized illumination are studied here, filling the void in the current literature. We believe this work unequivocally reveals the leaky-wave role of the grounded-dielectric slab mode mediating in TE anomalous extraordinary transmission and provides the necessary framework to design practical high-performance THz components and systems.
Abstract: A tapered vertical line source over the surface of the earth or ocean is explored as means of communication. Two types of tapers for the line source are explored. One is a "Zenneck-wave" taper and the other is an "optimum" taper. The Zenneck-wave taper is chosen to excite the Zenneck wave to the extent possible. The Zenneck wave is a "nonphysical wave" that propagates on a semi-infinite lossy earth. The wave is nonphysical in the sense that a simple source such as a dipole will not excite a field that resembles the Zenneck wave at large distances from the source. However, by using a tapered line source, a field that resembles the Zenneck wave can be produced along the surface of the earth out to some distance. The optimum taper is chosen to optimize the field at a given horizontal distance from the line source on the surface of the earth. The optimum line source can be used to produce a field along the surface of the earth that is nearly constant over a large range of distances, something that does not seem easily attainable with other types of sources.
Abstract: Extraordinarily transmitting arrays are promising candidates for quasi-optical (QO) components due to their high frequency selectivity and beam scanning capabilities owing to the leaky-wave mechanism involved. We show here how by breaking certain unit cell and lattice symmetries, one can achieve a rich family of transmission resonances associated with the leaky-wave dispersion along the surface of the array. By combining 2-D and 1-D periodic method of moments (MoM) calculations with QO terahertz (THz) time-domain measurements, we provide physical insights, numerical, and experimental demonstration of the different mechanisms involved in the resonances associated with the extraordinary transmission peaks and how these evolve with the number of slots. Thanks to the THz instrument used, we are also able to explore the time-dependent emission of the different frequency components involved.
Abstract: In this paper, we explore the excitation of magnetic current surface waves in truncated periodic arrays of slots in a conducting screen. A specialized Method of Moments (MoM) implementation is presented, which makes it possible to efficiently solve the scattering problem involving truncated arrays of several thousands of slots. By making use of the dispersion diagrams of surface waves propagating along infinite periodic arrays of slots, we are able to explain the absence of magnetic current surface waves in the arrays at frequencies in the neighborhood of the transmission peak associated to the slots natural resonances (length roughly equal to half the wavelength), while they are present when the arrays are excited under extraordinary transmission (EOT) conditions. In order to experimentally check this different behavior, an aluminium plate periodically perforated with slots has been fabricated and fed by means of a pyramidal horn, and the electric field behind the plate has been measured with a planar near-field system at a few centimeters from the plate. Our experimental results and MoM simulations agree, demonstrating the presence of a standing wave pattern of magnetic current surface waves at the EOT frequency, and the absence of surface waves at the slots natural resonant frequency.
Abstract: A mode-matching formulation is presented and used to analyze the dispersion properties of twist-symmetric transmission lines. The structures are coaxial lines periodically loaded with infinitely thin screens, which are rotated with respect to each other to possess twist symmetry. The results obtained using the proposed formulation are in good agreement with those of commercial simulators. Furthermore, using the presented mode-matching formulation, it is demonstrated that the propagation characteristics in the twist-symmetric structures are linked to the scattering and coupling of the higher order modes. The physical insight offered by this analysis is valuable for the design of various electromagnetic devices, such as filters, antennas, and phase-shifters.
Abstract: Here, we discuss the virtues of glide symmetry for designing low-frequency band-pass periodic filters in substrate integrated waveguide (SIW) technology based on complementary split-ring resonators (CSRRs). Conventional (non-glide) versions of these filters have a narrow passband, due to the fact that this band is below the cutoff frequency of the background waveguide. When glide symmetry is added to the filter configuration, the low-frequency passband is significantly widened, as well as the first stopband. The dispersion properties of both conventional and glide-symmetric periodically loaded waveguides are analyzed and compared with commercial software and an equivalent circuit model. Finally, two prototypes of the proposed glide-symmetric structure have been designed and built, illustrating the potential of this technique to widen the passband and reduce insertion losses of conventional subwavelength CSRR-loaded SIW filters.
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