SUMMARY:
This proposal is, partially, the continuation of our previous research project (TEC2004-03214). In that project many tasks were part of certain research lines that are expected to have a natural prolongation in the present proposal. Thus, we pretend to continue with several of our traditional general research lines although moving to new directions involving more advanced problems and more practically oriented applications. In particular our investigation will focus on three general subjects:
The present proposal can be framed in the analysis of planar passive structures for microwave integrated circuits. It covers theoretical, numerical and experimental aspects of a variety of elements ranging from the basic printed circuit lines to more complex filters, periodic structures, and antennas. More specifically, three different topics will be dealt with in this research project, namely,
1. EXCITATION PROBLEMS IN PLANAR GUIDING SYSTEMS
The guidance of electromagnetic fields in microwave printed circuits has been a subject of considerable theoretical and experimental interest since long ago. A common simplified way of studying complex system, when possible, is by cascading the responses of individual segments of the systems, such as the guiding elements and discontinuities. Thus many CAD tools of transmission systems are based on approximate models (for instance, a quasi-TEM approach) or they assumed that the only effect of increasing the frequency comes from the dispersive nature of the modes. These approximations are certainly limited in frequency, and for that reason a rigorous treatment of the printed circuit requires the use of very powerful EM simulators, where the guiding elements are also included as part of the geometry to be solved. Clearly the inclusion of these "extended" metallic regions usually involves the use of enormous computational resources, and finally it will be only obtained some specific answer to our particular problem in terms of "numbers" and/or "plots", without too much information about the "physics" of the problem. Thus, it is very convenient to have a deep physical insight into the EM propagation phenomena in order to know the limitation of the approximate models as well as to get a more complete understanding of the EM propagation in planar guiding structures. Despite the maturity of this research area and the many efforts already carried out, there still persist some challenging topics that have not been sufficiently investigated, especially at relatively high frequencies; namely, those frequencies for which the ratio between the height of the line substrate and the free-space wavelength is about or beyond one tenth (see, for example [J. Zehentner at al., Proc. of IEEE MTT-S, pp. 507-510, 2004]). The Microwave Group of the University of Seville has been very active in this subject, mainly by means of a very fruitful collaboration with the Applied Electromagnetic Group of University of Houston, and in particular with Prof. David R. Jackson.
Our common approach to advance in this investigation was to include the effect on a realistic source in a translational-symmetry guiding system (for example, the inclusion of a delta-gap voltage source in an infinite microstrip line). This study has given many fruits, and in particular it should be emphasized that the translational nature of the guiding system has allowed us to deal the problem with quasi-analytical tools, which has made it possible the obtaining of many analytical results physically meaningful [Mesa et al, IEEE-MTT vol.47, pp. 207-215, 1999]. One of the most important contributions is the identification of two different in-nature EM fields excited by the source, namely, the discrete spectrum (DS)and the continuous spectrum (CS). The DS can be associated with the bound propagating modes of the guiding structure (for example the fundamental and higher-order modes of a microstrip line) whereas the CS can be associated with unintended but unavoidable radiation effects (leaky modes and the so-called residual waves).
Further interesting results product of this research line have been systematically presented in IEEE T-MTT and other relevant forums, and have constituted an important part of the investigation supported by previous proposals (see for example [F. Mesa and D.R. Jackson, Wiley Enc. of RF and Microw. Engin. vol.3, pp. 2268-2293, 2005] and references therein).
Nevertheless this subject is far from being closed, and thus there are very important practical issues that have been dealt with in the past and whose investigation should be continued in the future. As an example it could be highlighted the appearance of spurious effects due to the unavoidable excitation of the CS part of the total field [F. Mesa et al. IEEE-MTT vol.50, pp. 2267-2275, 2002]. The spurious effects can make the transmission system response be very different from that expected from standard transmission-line or waveguide theory, and they can take the form of unexpected decays in the energy transmission, unanticipated high levels of coupling with adjacent lines, changes in the expected input impedance, etc. These effects can be caused by the CS field itself and/or by its interference with the DS field. The existence of these effects makes that a proper design of the guiding systems should take into account the conditions that minimize these phenomena. Another topic that has recently been considered is how practical values of losses affect the DS and the CS [J. Bernal et al, Proc. of IEEE MTT-S, pp. 1307-1310, 2006]. It has been found that the CS component seems to be less affected by losses than its corresponding DS counterpart, thus causing that, far from the source, the propagation characteristics of the fields are dominated by the unwanted and "not well controlled" CS components.
Other important questions that are still open and that are proposed as parts of the future research supported by this proposal are the following:
1.1. Probe-fed excitation:
(A) Investigate the equivalent circuit representation for a probe-fed microstrip line (as was already done for the gap feed). This new situation can be more interesting because of the extended use of the probe feeding in printed-circuit lines.
(B) Extend the probe-fed microstrip investigation to the problem of a via interconnect that connects two microstrip lines on different sides of the ground plane (a very practical problem in packaging).
(C) Compare the relative level of high-frequency spurious effects from different types of feeds, such as the gap source, the via-probe feeding, the aperture-coupled feed, etc.
1.2. EMC problems and pulse propagation:
(A) Investigate time-domain current excitation on a lossy microstrip line (a combination of two previous topics dealt with by our group: the lossy microstrip line and time-domain pulse propagation [W. Langston et al, Proc. of IEEE MTT-S, pp. 1311-1314, 2006]). The loss might have interesting effects on the pulse shape far away form the source.
(B) Investigate pulse propagation on coupled lines. This is a problem of interest to the packaging community, who are interested in "far-end crosstalk" and "near-end crosstalk." The "eye diagram" should be obtained to study the level of signal distortion.
(C) Investigate the nature of the field surrounding a microstrip line or covered microstrip line when it is excited by a pulse. This could be extended to the field on a coupled line in order to get a physical insight into the crosstalk fields.
(D) Investigate the nature of the field in the cross section of a microstrip line (constant z) as z changes, to see how the nature of the field changes as the character of the current changes. This could be particularly interesting if loss is assumed, so that the bound mode decays for large distances, leaving eventually the RW current. What do the corresponding fields look like?.
1.3. Sources in complex structures:
(A) Investigate the effects of lateral sidewalls on the current excited by a gap voltage source, and also when there is a slotted upper metallic wall. This latter structure has been proposed as a very efficient leaky-wave antenna,and its rigorous study can bring more practical applications.
(B) Include a realistic source in a periodic 1D/2D structure. This situation is very challenging because of the inclusion of an aperiodic source in a periodic environment. In fact, this sort of problem has hardly been treated in the literature, probably because of its considerable theoretical complexity (in the past, most works dealt only with the computation of the Floquet modal solutions [P. Baccarelli et al., IEEE-MTT, vol. 54, pp. 1350-1362, 2006]).
Some relevant national and international groups related to the above topics are:
Another objective of the current proposal is the implementation of efficient numerical codes for the electromagnetic analysis of planar periodic structures embedded in multilayered dielectric media. In particular, our aim will be focused to implement specific numerical codes for the analysis of planar periodic structures with one-dimensional (1D) periodicity, and additional codes for the analysis of planar periodic structures with two dimensional (2D) periodicity.
2.1. 1-D periodic structures
The codes for the structures with 1D periodicity will be mainly oriented to the computation of the complex wavenumbers and the complex Bloch impedances of the unit cell of planar periodic transmission lines (PPTL) [F.J. Glandorf et al., IEEE-MTT, vol. 35, pp. 336-343, 1987; P. Baccarelli et al., IEEE-MTT, vol. 54, pp. 1350-1362, 2006]. The ABCD matrix of the unit cell can be obtained in terms of these wavenumbers and impedances, and once the ABCD matrix of the unit cell is known, one may easily compute the scattering matrix of a section of PPTL with a finite number of cells without requiring the electromagnetic analysis of the whole structure [C. Y. Ong et al., IEEE Microwave Wireless Components Lett., vol. 12, pp. 264-266, 2002; L. Zhu, IEEE-MTT, vol. 51, pp. 2133-2138, 2003]. Since PPTL show forbidden bands of propagation over certain frequency ranges (in fact, PPTL are a particular case of the so-called electromagnetic bandgap structures), the numerical codes for the characterization of finite sections of PPTL are intended to be used in the design, fabrication and measurement of band-stop and low-pass filters [V. Radisic et al., IEEE Microwave Guided Wave Lett., vol. 8, pp. 69-71, 1998; T. Kim et al., IEEE Microwave Guided Wave Lett., vol. 10, pp. 13-15, 2000]. The slow-wave characteristics exhibited by PPTL are also expected to be exploited to reduce the size of distributed circuit components fabricated with this type of lines [F. R. Yang et al., IEEE-MTT, vol. 47, pp. 1509-1514, 1999; J. Sor et al., IEEE-MTT, vol. 49, pp. 2336-2341, 2006]. Certain PPTL leak power along its length due to the n=-1 space harmonic, which makes them suitable for the fabrication of backward leaky-wave antennas [A. A. Oliner and R. C. Johnson, Leaky Wave Antennas, Antenna Engineering Handbook, 3rd ed. New York: McGraw-Hill, 1993, ch. 10]. These antennas show frequency scanning capability, high directivity and large radiation bandwidths, their fundamental radiation properties (main beam direction and beamwidth) being controlled by the values of the complex wavenumbers of the associated PPTL [K. Potharazu et al., IEEE-AP, vol. 40, pp. 950-958, 1992; A. A. Oliner and R. C. Johnson, Leaky Wave Antennas, Antenna Engineering Handbook, 3rd ed. New York: McGraw-Hill, 1993, ch. 10; J. L. Gómez-Tornero et al., IEEE-AP, vol. 53, pp. 2834-2842, 2005]. In accordance with this, the code for the analysis of PPTL is expected to be used for the design, fabrication and measurement of novel prototypes of planar periodic leaky-wave antennas.
It should be pointed out that the codes for the analysis of planar structures with 1D periodicity can be alternatively used for the analysis of frequency selective surfaces (FSS) that are infinite in one dimension and finite in the other. These FSS support array current surface waves that have not been detected in the analysis of FSS that are infinite in two dimensions [B. A. Munk et al., IEEE-AP, vol. 49, pp. 1782-1793, 2001].2.1 2D periodic structures The numerical codes for the structures with 2D periodicity will be mainly oriented to the determination of the reflection and transmission parameters of FSS that are infinite in two dimensions [R. Mittra et al., Proc. IEEE, vol. 76, pp. 1593-1615, 1988; B. A. Munk, "Frequency selective surfaces", Wiley Interscience, New York, 2000], and to the computation of the dispersion diagram of the modes supported by these structures [M. Bozzi et al., IEEE-AP, vol. 53, pp. 29-35, 2005]. Traditionally, FSS have been applied to the design of dichroic subreflectors of reflector antennas with different feeds working at different frequencies, to the design of hybrid radomes for military platforms, and to the design of polarizers [R. Mittra et al., Proc. IEEE, vol. 76, pp. 1593-1615, 1988]. However, in the last few years, FSS have found new applications in the field of antennas. For instance, conductor backed FSS behave as high-impedance surfaces (or artificial magnetic conductors) that can be used as ground planes of low-profile antennas in order to improve their radiation efficiency [C. R. Simovski et al., IEEE-AP, vol. 53, pp. 908-914, 2005; G. Goussetis et al., IEEE-AP, vol. 54, pp. 82-89, 2006]. Conductor backed FSS can also be used as substrates of printed antennas when the bandgap of the FSS coincides with the operating frequency band of the antennas, which reduces the excitation of surface waves and increases the gain of the antennas [R. Coccioli et al., IEEE-MTT, vol. 47, pp. 2131-2138, 1999], and to model the performance of planar reflectarrays [D.M. Pozar et al., IEEE-AP, vol. 45, pp. 287-296, 1997]. This latter type of antennas is a potential alternative to reflector antennas. In fact, reflectarrays are easier to manufacture than reflector antennas and present less distortion and cross-polarization at the cost of a narrower bandwidth. Bearing in mind the interest recently arisen in this type of antennas, our FSS code will be applied to explore new topologies of reflectarrays. In Spain, Prof. J. A. Encinar from the Polytechnic University of Madrid is a recognized expert in the field of reflectarrays [J. A. Encinar, IEEE-AP, vol. 49, pp. 1403-1410, 2001; J. A. Encinar et al., IEEE-AP, vol. 52, pp. 1138-1148, 2004], and it is expected to establish a collaboration with his group in the frame of this research project, which hopefully will help us to gain experience in the design, fabrication and measurement of reflectarrays.
Finally, it should be mentioned that the code for the analysis of FSS can be easily adapted to the computation of the dispersion diagram of rectangular waveguides periodically loaded with strips or slots. The fundamental mode of these periodically loaded waveguide presents lower and upper cutoff frequencies, which indicates that this type of periodic guiding structure shows potential band-pass properties [M. N. M. Kehn et al., IEEE-AP, vol. 54, pp. 2275-2282, 2006]. Our research group has experimentally verified this behaviour in the case of rectangular waveguides loaded with split ring resonators [R. Marqués et al., Phys. Rev. Lett., vol. 89, pp. 183901-1/4, 2002], and it is intended to check this result with the numerical code developed for the analysis of FSS.
Some research groups that have made relevant contributions in the electromagnetic analysis of 1D/2D planar periodic structures, leaky-wave antennas, and FSS are
Filters are essential components of communications systems. At RF and microwave frequencies, the implementation of the required filtering functions can be made by using both lumped parameters and distributed networks, and in particular in modern high-frequency devices, this filtering function is carried out by a wide variety of devices made in planar technology. However, the emerging applications of RF and microwave technologies are becoming more and more stringent about overall filter performance, size, weight and cost, which has pushed the microwave community to explore new ways of realizing the filtering function [J.S. Hong, M.J. Lancaster, Microstrip Filters for RF/Microwave Applications, John Wiley & Sons, New York, 2001]. To have an idea of the explosive increasing of this generic research topic in the last few years, one only needs to look over the recent scientific and technical literature about planar filters. Considering only a few of the leading journals in the microwave area, more than 350 papers have been published since year 2002 on different implementations of microstrip and coplanar waveguide filters to solve a large variety of practical problems. Lots of innovative ideas have recently been reported to improve the above mentioned desired features of many types of filters.
In the past, before the appearance in the market of the nowadays widespread used commercial electromagnetic simulators, filter designers were limited to use architectures and geometries for which tabulated data or analytical formulas were available. Now that this restriction has been removed, a world of new possibilities is open. Nevertheless, simulation capability is not the Holy Grail, and new concepts together with the development of simple circuit models are necessary. Moreover, these models and electromagnetic full-wave simulations can be combined with optimization algorithms to achieve the final goal of designing new competitive filters for the new applications of RF and microwave technologies.
The Microwaves Group of the University of Seville has been working for many years in the fundamental and applied problems behind the development of simulation tools, in particular, those especially conceived to deal with planar structures. Analytical and numerical techniques to characterize the behaviour of planar transmission lines, resonators, radiating patches, and full circuits have been provided in the past by members of this group, whose results can be consulted in more than one hundred of papers published in the most important journals in the field of microwave theories and techniques. These research lines have a place in the frame of the present proposal, as it has been exposed in previous sections. Nevertheless, some members of the group have also been involved in the design of innovative planar passive devices (including filters, of course), and it is expected to continue on this topic in the present proposal. Although commercial simulators will be necessarily used to develop this task, home-made codes will be also very useful for this purpose, as has been happening in recent years. The research field of planar filters is a very wide field, and thus our aim will focused on the following particular topics:
3.1. Compact filters
Miniaturized filters can be used in many applications where dissipative losses are not the bottle-neck of the system requirements (note that small-size microstrip filters used to be relatively lossy when compared with large size distributed circuit implementations). Reducing the size of a microstrip filter can be done in a rather trivial way just choosing a high dielectric constant substrate. However the choice of the substrate may be imposed by other considerations, and thus the circuit size reduction must then be achieved by using appropriately designed building block components. Small size resonators and sections have been proposed in the last few years by many authors (see, for instance, [P. Akkaraekthalin et al., ETRI Journal, vol. 28, n. 5, pp. 607-614, 2006; P.-H. Deng et al., IEEE-MTT, vol. 54, n. 2, pp. 533-539, 2006; M.K. Mandal et al., IEEE-MWCL, vol. 16, n. 1, pp. 46-48, 2006; H. Gan et al., IEEE-MWCL, vol. 16, n. 3, pp. 107-109, 2006] for bandpass filters or [W.-H. Tu et al., IEEE-MTT, vol. 54, n. 10, pp. 3786-3792, 2006]). It is expected to contribute to this research line with new designs improving filter selectivity, band-pass losses and stop-band rejection level and bandwidth.
3.2. Filters using the combination of microstrip and CPW structures
The use of the bottom side of the substrate (commonly used as ground plane) to insert filter components should allow us to increase the design flexibility in order to reduce the overall size of the filter or to eliminate spurious frequencies (the most recent paper using this idea is [P.-H. Deng et al., IEEE-MTT vol. 54, n. 10, pp. 3746-3750, 2006]). Quasi-elliptic filters with very good selectivity and wideband rejection can be fabricated using this methodology. Spurious bands suppression can also be attained in this way. In fact, researcher of the present proposals have already made some contributions in this field [M.C. Velázquez-Ahumada et al., IEEE-MTT, vol. 52, n. 3, pp. 1082-1086, 2004; IEEE-MTT, vol. 53, n. 5, pp. 1823-1828, 2005; IEEE-MTT vol. 55, n. 1, 2007, to appear]. It is believed that this line of reasoning can still give place to additional improvements in filter design.
3.3. Dual band filters
Dual-band filters are key components in dual-band wireless communication systems. Stepped impedance resonators based [Y.P. Zhang et al., IEEE-MTT, vol. 54, n. 10, pp. 3746-3750, 2006], compact dual-band resonator based [M.-L. Lai et al., IEEE-MTT, vol. 54, n. 1, pp. 160-168, 2006] and dual-band filters with embedded low-pass filters embedded [A. Manchec et al., IEEE-MWCL, vol. 16, n. 1, pp. 4-6, 2006] are recently proposed solutions. It is expected to explore in depth these lines of research making use of novel dual band resonators and employing double sided technology. Some selected groups working in the field: there are many groups making a very good work on the topic. A few selected national and international groups close to our interests are
In this section we will include those book chapters, journal and conference papers reporting original results of the project.
Abstract: The main goal of this contribution is to show that periodic arrangements of chiral scatterers can be the basis for the development of three dimensional and isotropic negative refractive index artificial media. Three dimensionality and isotropy are key issues in this context since only three dimensional structures can be properly viewed as a "medium," and only in isotropic media the refractive index is unambiguously defined. The proposed arrangements are cubic lattices of chiral split ring resonators conveniently designed to yield an isotropic behavior. The reported structures are shown to provide a significant frequency band of negative refraction with good matching to free space.
Abstract: The rational function fitting method has been found useful in the derivation of closed-form expressions of spatial-domain Green's functions for multilayered media. However, former implementations of the rational function fitting method lead to Green's functions expressions that are not accurate in the far field when this far field is dominated by the continuous spectrum instead of being dominated by surface waves (as it happens, for instance, in the case of lossy multilayered media). In this paper, the authors introduce a novel implementation of the rational function fitting method, which leads to Green's functions expressions that are accurate in the far field when this is dominated either by the continuous spectrum or by surface waves. In the new approach, the far-field contribution of the continuous spectrum to the Green's functions is numerically fitted in terms of functions with closed-form Hankel transforms, and this far-field contribution is explicitly added to the total least squares approximations of the Green's functions. The numerical results obtained for the Green's functions with the new approach have been compared with numerical results obtained via direct numerical integration of Sommerfeld integrals, and excellent agreement has been found despite the contribution - continuous spectrum or surface waves - dominating the far field.
Abstract: An efficient method is presented for the analysis of a vertical coaxial probe excitation of an infinite microstrip line. The novel feature of the method is that it uses a semianalytical Green's function that is derived for current sources in the presence of the infinite microstrip line. Hence, in a method of moments approach, unknown currents need only be placed on the conducting probe feed and not on the infinite strip surface. The method also utilizes an attachment mode at the contact point between the probe and line so that the correct Kirchhoff condition is automatically satisfied. Once the surface current density on the probe is known, the surface current density on the strip conductor can be readily obtained using the Green's function of the background grounded substrate. The method is valid even at high frequency, where simple transmission line theory fails to account for effects such as the continuous- spectrum current that is excited on the line. After validating the method with various commercial software simulation packages, results are presented to study the fundamental behavior of the input impedance, probe cu rrent, and current launched on the microstrip line, and to examine the high-frequency behavior of these currents.
Abstract: Extraordinary optical transmission of light or electromagnetic waves through metal plates periodically perforated with subwavelength holes has been exhaustively analyzed in the last ten years. The study of this phenomenon has attracted the attention of many scientists working in the fields of optics and condensed matter physics. This confluence of scientists has given rise to different theories, some of them controversial. The first theoretical explanation was based on the excitation of surface plasmons along the metal–air interfaces. However, since periodically perforated dielectric (and perfect conductor) slabs also exhibit extraordinary transmission, diffraction by a periodic array of scatterers was later considered as the underlying physical phenomenon. From a microwave engineering point of view, periodic structures exhibiting extraordinary optical transmission are very closely related to frequency- selective surfaces. In this paper, we use simple concepts from the theory of frequency-selective surfaces, waveguides, and transmission lines to explain extraordinary transmission for both thin and thick periodically perforated perfect conductor screens. It will be shown that a simple transmission-line equivalent circuit satisfactorily accounts for extraordinary transmission, explaining all of the details of the observed transmission spectra, and easily gives predictions on many features of the phenomenon. Although the equivalent circuit is developed for perfect conductor screens, its extension to dielectric perforated slabs and/or penetrable conductors at optical frequencies is almost straightforward. Our circuit model also predicts extraordinary transmission in nonperiodic systems for which this phenomenon has not yet been reported.
(This paper is included as a "Highly Cited Paper" in the Essential Science Indicators database of the ISI Web of Knowledge. It is also included in the research front "SUBWAVELENGTH HOLE ARRAYS; LIGHT TRANSMISSION; TERAHERTZ SURFACE PLASMON POLARITONS; EXTRAORDINARY OPTICAL TRANSMISSION; SURFACE PLASMON GENERATION" as one of the 25 core papers on the subject. These data were updated 05/12/2010).
Abstract: This paper presents an algorithm for the acceleration of the series involved in the computation of 2-D homogeneous Green's functions with 1-D and 2-D periodicities. The algorithm is based on an original implementation of the spectral Kummer-Poisson's method, and it can be applied to the efficient computation of a wide class of infinite series. In the algorithm the number of asymptotic terms retained in Kummer's transformation is externally controlled so that any of the series that has to be accelerated is split into one series with exponential convergence and another series with algebraic convergence of arbitrarily large order. Numerical simulations have shown that there is an optimum number of asymptotic terms retained in Kummer's transformation for which the CPU time needed in the summation of the series is minimized. The CPU times required by Ewald's method for the evaluation of 2-D Green's functions with 1-D and 2-D periodicities have been compared with those required by the present algorithm, and the algorithm has been found to be between 1.2 and 3 times faster than Ewald's method when working in optimum operation conditions.
Abstract: In this letter, the problem of extraordinary (ET) transmission of electromagnetic waves through opaque screens perforated with subwavelength holes is addressed from an analytical point of view. Our purpose was to find a closed-form expression for the transmission coefficient in a simple case in order to explore and clarify, as much as possible, the physical background of the phenomenon. The solution of this canonical example, apart from matching quite well with numerical simulations given by commercial solvers, has provided new insight in extraordinary transmission as well as Wood's anomaly. Thus, our analysis has revealed that one of the key factors behind ET is the continuous increase of excess electric energy around the holes as the frequency approaches the onset of some of the higher-order modes associated with the periodicity of the screen. The same analysis also helps to clarify the role of surface modes –or spoof plasmons in the onset of ET.
Abstract: The phenomenon of extraordinary optical transmission (EOT) through electrically small holes perforated on opaque metal screens has been a hot topic in the optics community for more than one decade. This experimentally observed frequency-selective enhanced transmission of electromagnetic power through holes, for which classical Bethe's theory predicts very poor transmission, later attracted the attention of engineers working on microwave engineering or applied electromagnetics. Extraordinary transmission was first linked to the plasma-like behavior of metals at optical frequencies. However, the primary role played by the periodicity of the distribution of holes was soon made evident, in such a way that extraordinary transmission was disconnected from the particular behavior of metals at optical frequencies. Indeed, the same phenomenon has been observed in the microwave and millimeter wave regime, for instance. Nowadays, the most commonly accepted theory explains EOT in terms of the interaction of the impinging plane wave with the surface plasmon-polariton-Bloch waves (SPP-Bloch) supported by the periodically perforated plate. The authors of this paper have recently proposed an alternative model whose details will be briefly summarized here. A parametric study of the predictions of the model and some new potential extensions will be reported to provide additional insight.
Abstract: This work analyzes the electromagnetic wave propagation through periodically stacked fishnets from zero frequency to the first Wood's anomaly. It is shown that, apart from Fabry-Perot resonances, these structures support two transmission bands that can be backward under the appropriate conditions. The first band starts at Wood's anomaly and is closely related to the well-known phenomena of extraordinary transmission through a single fishnet. The second band is related to the resonances of the fishnet holes. In both cases, the in-plane periodicity of the fishnet cannot be made electrically small, which prevents any attempt of homogenization of the structure along the fishnet planes. However, along the normal direction, even with very small periodicity transmission is still possible. An homogenization procedure can then be applied along this direction, thus making that the structure can behave as a backward-wave transmission line for such transmission bands. Closed-form design formulas will be provided by the analytical formulation here presented. These formulas have been carefully validated by intensive numerical computations.
Abstract: This letter presents a methodology to reproduce at microwave frequencies the physical behavior of certain diffraction structures that have been investigated in the optics community. The methodology will be exemplified with a microstrip circuit implementation of a transmission line system exhibiting an electromagnetic response that mimics the response of simple and compound diffraction gratings. The identification of such bridge circuit analogies could be very fruitful for the understanding and development of design strategies to devise practical components based on phenomena employed in the affine areas of optics and microwave engineering.
Abstract: This letter provides an experimental demonstration of extraordinary transmission in a closed waveguide system loaded with an electrically small diaphragm. This is a situation where the standard surface plasmon polariton (SPP) theory does not apply. The theoretical explanation is then based on the concept of impedance matching. This concept has previously been applied by some of the authors to account for enhanced transmission in situations where surface plasmon theory can be used: periodic arrays of small holes or slits in flat metal screens. The experiment in this letter supports the impedance matching model, valid for when SPPs are present or not.
(This paper is listed in Top 20 most downloaded papers corresponding to August 2009 issue of Applied Physics Letters).
Abstract: In the analysis of open planar structures, it is widely accepted that the spectral decomposition of the fields excited by a given source into its continuous and discrete spectrum components has an apparent physical meaning. The continuous spectrum is associated with radiation and/or reactive effects whereas the discrete spectrum corresponds to the bound fields guided by the structure. Although this interpretation is usually correct, the present work will show that it is not general. In particular it will be shown that a lossy grounded dielectric waveguide near the cutoff frequencies of its higher-order surface-wave modes is a simple and interesting counterexample in which the continuous and the discrete spectrum fail to bear their intended physical meaning. A careful analysis of the modal cutoff mechanism provides numerical results that fully support the above assertion.
Abstract: The current excited by a delta-gap source on a periodically loaded microstrip line is studied by means of the array scanning method. The method requires the efficient solution of a large number of auxiliary Floquet-periodic subproblems, cast here in the form of an electric-field integral equation (EFIE) that has been discretized with the method of moments (MoM) in the unit cell. Numerical results are provided for a specific structure in both bound and radiative frequency ranges and are validated through independent codes.
Abstract: Extraordinary transmission and other interesting related phenomena for 1D periodic arrays of slits (compound diffraction gratings) have recently been the object of intense research in the optics and solid state physics communities. This case should be differentiated from the extraordinary transmission through arrays of small apertures on metal screens, since small holes only support below-cutoff modes whereas slits can also support transverse electromagnetic modes without cutoff frequency. In this paper an equivalent-circuit approach is proposed to account for the most relevant details of the behavior of slit-based periodic structures: extraordinary transmission peaks, Fabry-Pérot resonances, and transmission dips observed in compound structures. The proposed equivalent-circuit model, based on well established concepts of waveguide and circuit theory, provides a simple and accurate description of the phenomenon that is appropriate for educational purposes as well as for the design of potential devices based on the behavior of the structures under study.
Abstract: In this work the total least squares algorithm (TLSA) is applied to the determination of the proper and improper poles of spectral domain multilayered Green's functions that are closer to the branch point, and to the determination of the residues at these poles. The introduction of an adequate transformation in the spectral domain permits that the TLSA provides accurate values of the poles and residues, regardless of the proximity of the poles to the branch point. It is shown that the poles and residues supplied by the TLSA can be used to write the far field of the spatial domain Green's functions in terms of closed-form expressions that are reliable in a wide variety of scenarios.
Abstract: The 3-D homogeneous Green's function with 1-D periodicity is commonly expressed as spatial and spectral infinite series that may show very slow convergence. In this work Kummer's transformation is applied to the spatial series in order to accelerate its convergence. By retaining a sufficiently large number of asymptotic terms in the application of Kummer's transformation, the spatial series is split into a set of series which can be accurately obtained with very low computational effort. The numerical results obtained show that, when the number of asymptotic terms retained in Kummer's transformation is large enough, the convergence acceleration method proposed in this work is always faster than existing acceleration methods such as the spectral Kummer-Poisson's method and Ewald's method.
Abstract: This paper presents an analysis of a slotline excited by a current source connected across the slot. The method of moments applied in the spectral domain is used. As a result, the voltage of a wave composed of bound and leaky modes as well as residual waves is calculated along the slot. Interesting behavior of the voltage wave in the neighborhood of the source is shown. Both numerical and measured results are given.
Abstract: In this paper, a folded stepped impedance resonator (SIR), modified by adding an inner quasi-lumped SIR stub, is used as a basis block for a new implementation of dual-band bandpass filters. The main advantage of the proposed filter is to make it possible to independently control the electrical features of the first and second bands. The behavior of the first band basically depends on the geometry of the outer folded SIR. The second band, however, is strongly influenced by the presence of the inner stub. Additional design flexibility is achieved by allowing the inner stub to be located at an arbitrary position along the high impedance line section of the main SIR. The position of the tapped input and output lines can be optimized in order to reach a reasonable matching of the filter at the central frequencies of both passbands. Some designs are reported to illustrate the possibilities of the proposed structure. Experimental verification has been included.
Abstract: This paper proposes an equivalent circuit model that uses lumped elements and transmission lines to explain the transmission of electromagnetic waves through a conducting screen periodically perforated with slits and sandwiched between two different dielectric slabs. The present model relies on the impedance-matching point of view, previously introduced by some of the authors, rather than on the surface plasmon polariton concept. Thus, the model constitutes a simple and insightful framework that easily leads to accurate qualitative and quantitative predictions about the nature of the transmission spectrum of such structures.
Abstract: Modified coplanar waveguide (CPW) edge-coupled bandpass filters with floating conductor strips printed below the coupled-line sections are proposed. These structures allow one to suppress the first undesired spurious pass band associated with the nonhomogeneous nature of the dielectric in CPW structures. Also, tight coupling levels can be easily achieved. An optimization code based on a fast quasi-TEM solver is used to optimize the dimensions of the filter sections, in such a way that final dimensions are obtained in less than one minute. A couple of filters have been built and measured to illustrate the theory.
Abstract: Extraordinary transmission through periodic distributions of sub-wavelength holes made in opaque screens has been demonstrated and exhaustively studied along the last decade. More recently, extraordinary transmission has also been predicted and experimentally observed through electrically small diaphragms located inside hollow pipe waveguides. This last phenomenon cannot be explained in terms of surface waves excited along the periodic system (the so-called surface plasmon polaritons). Transverse resonances can be invoked, however, as a sound explanation for extraordinary transmission in this kind of systems. In this paper, a simple waveguide system, exhibiting exactly the same behavior previously observed in periodic 2-D arrays of holes, is analyzed in depth. Analogies and differences with the periodic case are discussed. The theoretical and experimental results reported in this paper provide strong evidence in favor of the point of view emphasizing the concept of impedance matching versus surface wave excitation. The role of material losses is discussed as an important practical issue limiting the maximum achievable subwavelength transmission level. Most of our conclusions can be applied to both periodic arrays of holes and diaphragms in closed waveguides.
Abstract: A new type of miniaturized stepped impedance resonator (SIR) for bandpass filter applications is proposed in this paper. The new resonator incorporates a ground plane window with a floating conductor in the backside of the substrate. The ground plane window increase the characteristic impedance of the lines used to implement the inductive region of the quasi-lumped resonator, thus allowing some size reduction. Moreover, the presence of a floating conducting patch printed below the capacitive region of the resonator pushes up the first spurious band of the filter. A meaningful improvement of its out-of-band rejection level is then achieved. The coupling between adjacent resonators is also enhanced thus leading to wider achievable bandwidths. Some filter designs using the new resonator and other standard resonators are included for comparison purposes.
Abstract: This paper presents a simple analytical circuit-like model to study the transmission of electromagnetic waves through stacked two-dimensional (2-D) conducting meshes. When possible the application of this methodology is very convenient since it provides a straightforward rationale to understand the physical mechanisms behind measured and computed transmission spectra of complex geometries. Also, the disposal of closed-form expressions for the circuit parameters makes the computation effort required by this approach almost negligible. The model is tested by proper comparison with previously obtained numerical and experimental results. The experimental results are explained in terms of the behavior of a finite number of strongly coupled Fabry-Pérot resonators. The number of transmission peaks within a transmission band is equal to the number of resonators. The approximate resonance frequencies of the first and last transmission peaks are obtained from the analysis of an infinite structure of periodically stacked resonators, along with the analytical expressions for the lower and upper limits of the pass-band based on the circuit model.
Abstract: This paper studies the time-domain propagation and dispersion of a pulse propagating on a microstrip line with a metallic top cover. A gap voltage source is used to model a practical excitation on the line. High-frequency distortion effects are observed that cannot be accounted for by conventional transmission-line theory, since they are due to the simultaneous excitation of the bound mode and a strong leaky mode. The bound-mode and leaky-mode components of the pulse are identified and separately studied to aid in the physical interpretation of the pulse distortion. The excitation of a dominant leaky mode gives rise to an interesting pulse-splitting phenomenon, due to the different velocities of the bound mode and the leaky mode. The influence of dielectric and conductor losses on the pulse shape is also studied.
Abstract: Far-field closed-form expressions are derived for spatial domain multilayered Green's functions (GF). For the derivation of these far-field expressions, the spectral domain multilayered GF are approximated by means of the total least square algorithm (TLSA) in terms of the spectral variable u0=(krho2-k02)1/2, and uniform asymptotic expansions are determined for the Sommerfeld integrals of the resulting TLSA approximations. Numerical results show that the far-field asymptotic expressions are accurate within 0.1% for distances between source and field points larger than one free-space wavelength. Also, it is shown that the hybrid use of the discrete complex image method and the TLSA in terms of the spectral variable krho leads to near-field closed-form expressions of multilayered GF that are typically accurate within 0.1% for distances between source and field points smaller than one free-space wavelength. Therefore, the combination of the novel far-field asymptotic expressions and the well known near-field expressions makes it possible to compute multilayered GF with a great accuracy in the whole range of distances between source and field points, and in a wide range of frequencies.
Abstract: A semianalytical full-wave formulation is used to analyze a narrow gap in a microstrip line. The analysis assumes that the length of the gap is small compared to the strip width, but allows for an arbitrarily high frequency. The formulation accounts for radiation from the gap into space and into surface waves. Based on this formulation, the scattering parameters of the gap are obtained along with the complete equivalent circuit of the gap. The percentage of power lost due to gap radiation from an incident mode on the microstrip line is found, and the physics of the gap radiation are examined.
Abstract: This paper studies the excitation of a physical leaky mode in a covered microstrip structure at low frequencies. We calculate the current excited in the line by a delta-gap voltage source via a full wave analysis based on a mixed potential integral equation scheme. The current in the line is decomposed into its bound mode and continuous spectrum components. The bound mode component is associated with the propagation e?ects whereas the continuous spectrum component is associated with reactive and/or radiative effects and contains the contribution of the leaky mode. Our analysis also includes a detail study of the dispersion relations of the bound and leaky modes along with their corresponding electric fields. At low frequencies, in the covered microstrip structure with a low top cover height, we have found that the bound mode role is superseded by the leaky mode, in the sense that it is the leaky mode which partially or totally carries the signal energy. Therefore, the spurious e?ects associated with the excitation of a leaky mode, which usually appear at high frequencies in open
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