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This paper is concerned with the electromagnetic theory of characteristic modes for general homogeneous material bodies using surface integral equations (SIEs), which are computationally more efficient than using volume integral equations (VIEs). However, the generalized eigenvalue equations (GEEs) based on SIEs are prone to generate spurious or extra modes, unlike their counterparts based on VIEs. In this work, several variant SIE-based GEEs are proposed to eliminate the spurious/extra modes and improve the numerical performances. These variant GEEs share a common point that the reactive power operator related to the material domain should be removed from the left-hand sides of GEEs by explicitly enforcing the boundary condition, because it seems sensitive to incur unwanted modes. These variant GEEs are diverse in choosing the active power operator related to the material domain on the right-hand sides to suit different medium parameters, which even can be dropped directly for lossless cases. Numerical results show that these variant GEEs do not generate spurious/extra resonant modes in observed frequency range, and at least one of the variants would have acceptable numerical accuracy.

[EN] A four-port wideband cavity-backed antenna is presented for indoor base stations applications. The antenna is composed of a square open cavity with an X-shaped isolating block and 4 feeding monopoles symmetrically and orthogonally arranged in the aperture of the cavity. A novel methodology based on Characteristic Modes Analysis (CMA) is used for identifying the modes which are contributing to the coupling. As a result of this analysis, an X-shaped isolating block placed at the center of the cavity is proposed for increasing the isolation between ports. A wide-band four-port antenna with unidirectional radiation patterns is obtained with a measured impedance bandwidth (S-11< 10 dB) ranging from 1.55 to 6 GHz (118%), covering most of the sub-6 GHz 5G bands. The proposed antenna provides four independent radiation patterns, with 16 dB of measured minimum isolation between ports and an efficiency higher than 84%. Multiple-input multiple-output (MIMO) compatibility is confirmed with a 4 x 4 MIMO simulated system with an envelope correlation coefficient (ECC), This work was supported by the Spanish Ministry of Science and Innovation (Ministerio Ciencia e Innovacion) under Project PID2019-107885GB-C32.

It is generally regarded that some properties of the characteristic modes of multiconductor transmission lines, such as per-unit-length impedance, admittance, or characteristic impedance, cannot be meaningfully computed analytically. This inability arises from the nature of the definition of the modes - sets of relative voltages and/or currents - which in the prevailing understanding may be arbitrarily scaled. Several methods with which to compute scaling factors have been proposed, but often - and as demonstrated in this work - the results of these processes do not agree with values determined directly from field quantities or experiments. This work begins by examining several facets of multiconductor-transmission-line modes under the approximation of ideal TEM propagation: firstly, that transmission-line modes are normal, and secondly, that as a result of possessing this property, it is postulated that total currents in the terminal domain may be directly equated to those in the modal domain. It is then shown that these relations allow the scaling factors to be determined to within a sign, and as a result, modal properties may be directly computed. This technique allows for the extraction of the modal transmission-line properties for any arbitrary system of conductors. Multiple examples are studied numerically, in which it is shown that the proposed process results in much stronger agreement with field solution than other proposed processes, and further validation of the proposed process is provided though experimentally obtained data.

This work addresses the use of the Characteristic Modes Analysis (CMA) for tailoring a Metasurface (MTS) in view of exploiting it in compact, low-profile, Circularly Polarized (CP) antennas. The investigated non-uniform MTS is exploited as a superstrate in the design of a novel compact and low-profile antenna for nanosatellite applications. The MTS consists of a $3\times 4$ array of unequal patches arranged in a rectangular lattice. The antenna parameters are carefully tailored by using the CMA to achieve a significant performance enhancement with respect to a uniform MTS in terms of both Axial Ratio (AR) bandwidth and aperture efficiency. The reliability of the CMA approach is verified by assessing the overall performance of the whole radiating structure comprising the stripline feeding excitation. The proposed MTS antenna is compact ( $0.068~\lambda 0$ ) provides a remarkable aperture efficiency going from 86 % up to 96 % and an AR coverage in the upper hemisphere greater than 84 % within the desired S-band (2.025 – 2.29 GHz).

A multimode antenna (MMA) can be an interesting alternative to a conventional phased antenna array for direction-of-arrival (DoA) estimation. By MMA, we mean a single physical radiator with multiple ports, which excite different characteristic modes. In contrast to phased arrays, a closed-form mathematical model of the antenna response, such as a steering vector, is not straightforward to define for MMAs. Instead, one has to rely on calibration measurement or electromagnetic field (EMF) simulation data, which is discrete. To perform DoA estimation, an array interpolation technique (AIT) and wavefield modeling (WM) are suggested as methods with inherent interpolation capabilities, fully taking antenna nonidealities like mutual coupling into account. We present a noncoherent DoA estimator for low-cost receivers and show how coherent DoA estimation and joint DoA and polarization estimation can be performed with MMAs. Utilizing these methods, we assess the DoA estimation performance of an MMA prototype in simulations for both the 2-D and 3-D cases. The results show that WM outperforms AIT for high signal-to-noise ratio. The coherent estimation is superior to noncoherent, especially in 3-D, because noncoherent suffers from estimation ambiguities. In conclusion, the DoA estimation with a single MMA is feasible and accurate.

An 8-port multiple-input multiple-output (MIMO) array operating in the long-term evolution (LTE) band 42 (3400-3600 MHz) based on the characteristic mode theory (CMT) is described. As designing a multi-antenna system with multiple significant modes is difficult for the CMT, a new method is, therefore, proposed to effectively provide useful physical insights and improve the envelope correlation coefficient (ECC). Although many of the same modes are excited by two antennas, the proposed method is still effective to improve the ECC of the two antennas. A detailed and reasonable framework adopting the CMT to analyze geometries with multi-significant modes is presented. The consideration of the mode number can be avoided by using this method. For an arbitrary structure, it is verified that the modal weighting coefficient (MWC) phases of the same modes excited by two ports are in phase or out of phase. Therefore, for the design of a multi-port MIMO array, the same modes excited by different ports can be divided into two types: equiphase modes and antiphase modes in the proposed method, and the tradeoff of the equiphase modes and the antiphase modes is first proposed and analyzed as an efficient way to improve the ECC. Furthermore, the proposed method can also be applied to asymmetric structures. In order to prove the effectiveness of the proposed method in the design of MIMO antennas with desirable ECC, an 8-port MIMO array for 5G mobile applications is implemented and measured.

The surface integral equation-based theory of characteristic modes (TCM) is presented for structures including perfect electric conductors (PECs) and lossy dielectric bodies. The formulation is a combination of the classical TCM formulation for PEC structures and a recently developed one for penetrable bodies. The potential of the methodology for practical antenna design problems is demonstrated by studying the coupling of the modes on a PEC plate (antenna ground plane) and an adjacent highly lossy dielectric block (user’s hand). This analysis reveals the existence of different types of modes having weak or strong interactions between PEC and dielectric parts.

In previously published papers about the metal-frame smartphone antennas, the screen is usually not included in the antenna model. As the big-screen, or even full-screen (also called all-screen or bezel-less) smartphones become the trend in recent and foreseeable future years, the effect of the very close screen to metal-frame antennas becomes prominent, e.g., the antenna's bandwidth is significantly narrowed. In this paper, a metal-frame antenna for the big-screen smartphone covering 824-960 MHz (GSM850/900), 1452-1496 MHz (LTE1500), 1710-2690 MHz (DCS, PCS, UMTS2100, LTE2300/2500), and 3400-3600 MHz (3.5 GHz 5G) is introduced. For the first time, the screen is fully considered, even designed as part of the antenna. The theory of characteristic modes (TCM) is applied in the design method. A prototype is made and measured for verifying the proposed methodology. Some key parameters of this antenna design are also discussed. This methodology helps to improve the degraded performance of smartphone antennas due to the big-screen industrial design.

This article belongs to the Special Issue Millimeter-wave and Terahertz Applications of Metamaterials. In this article, a novel metamaterial inspired UWB/multiple-input-multiple-output (MIMO) antenna is presented. The proposed antenna consists of a circular metallic part which formed the patch and a partial ground plane. Metamaterial structure is loaded at the top side of the patches for bandwidth improvement and mutual coupling reduction. The proposed antenna provides UWB mode of operation from 2.6-12 GHz. The characteristic mode theory is applied to examine each physical mode of the antenna aperture and access its many physical parameters without exciting the antenna. Mode 2 was the dominant mode among the three modes used. Considering the almost inevitable presence of mutual coupling effects within compact multiport antennas, we developed an additional decoupling technique in the form of perturbed stubs, which leads to a mutual coupling reduction of less than 20 dB. Finally, different performance parameters of the system, such as envelope correlation coefficient (ECC), channel capacity loss (CCL), diversity gain, total active reflection coefficient (TARC), mean effective gain (MEG), surface current, and radiation pattern, are presented. A prototype antenna is fabricated and measured for validation.

A novel metasurface is proposed as a superstrate for the design of circularly polarized antennas. A square loop is considered as the unit cell of the metasurface that has to be excited by a linearly polarized source. The use of stubs is investigated toward the excitation of orthogonal radiated fields with the necessary 90-degree phase shift without resorting to a different periodicity of the employed metasurface.

[EN] This paper presents a compact multimode antenna for GPS applications, where multiple modes are excited in order to achieve radiation pattern diversity. Based on the Characteristic Mode Analysis of a rectangular plate of 80 mm x 54 mm, a practical implementation of the excitation network is presented., This work has been supported by the Spanish Ministry of Economy and Competitiveness (Ministerio de Economía y Competitividad) under project TEC2016-78028-C3-3-P and Generalitat Valenciana under project AICO/2019/018.

A novel S-band antenna concept hosted on a 1U form factor CubeSat platform is designed by exploiting the Characteristic Modes Theory (CMT). The innovative strategy provides useful design guidelines to transform the external platform into an efficient radiator by stimulating an optimal current distribution on its conductive surface. The effect of the satellite platform on the radiated performances (efficiency, band, gain) is intrinsically taken into account and profitably exploited to realize an efficient radiation system. The minimally invasive radiators, strategically collocated on the platform thanks the CMT, allow achieving a great saving of space and an optimal modal current excitation able to provide excellent radiation performance.

The design of two antenna ports for a compact Y-shaped patch on top of a square ground plane is considered in this work. Specifically, characteristic mode analysis reveals candidate regions to locate two direct probe feeding ports to jointly excite two significant modes of the Y-shaped patch with low mutual coupling and correlation. The feed design procedure, based on both the amplitude and phase distributions of the characteristic electric near-fields, is validated in full-wave simulation with two example realizations of two-port designs.

Characteristic mode formalism is presented for closed surfaces modeled with the impedance boundary condition. The characteristic equations are expressed in terms of the electric, magnetic, and combined field integral operators (EFIO, MFIO, and CFIO). Similarly as for perfectly conducting (PEC) surfaces, the modal solutions based on EFIO (MFIO) are corrupted by spurious solutions, as the frequency coincides with the internal resonance frequency of a cavity with PEC (perfect magnetic conductor) walls, whereas the CFIO-based approach gives physically correct modal solutions.

We comprehensively study the characteristic-mode analysis (CMA) of arbitrarily shaped 3-D conducting objects embedded in a lossless planarly layered medium. The electric field integral equation (EFIE) with the layered medium Green’s function is applied as the core equation for numerical analysis. The modal behavior of an object in a layered medium is first comparatively studied with that in free space. The low-frequency breakdown problem of the EFIE-based CMA and its remedy is then elaborated. The asymptotic frequency dependence of the characteristic eigenvalues for the low-order modes is numerically investigated. A theoretical analysis is conducted by constructing a frequency-independent generalized eigenvalue problem and applying Taylor expansions. Finally, the modal expansion for scattering problems and parameter extraction for circuit problems are discussed, both in the context of a layered background. Since the electromagnetic property of an electrically small object is mainly governed by a few of low-order modes, this paper may provide an effective way of model order reduction for complicated structures residing in a layered medium.

A novel pattern-reconfigurable two-port multiple-input multiple-output (MIMO) antenna system at Wi-Fi band has been designed by resorting to characteristic modes analysis. The design method of the proposed MIMO antenna array is based on the selective excitation of three orthogonal characteristic modes. The selective excitation is achieved by integrating two pairs of capacitive coupling elements (CCEs) on the same plane with the ground. Two feeding networks related to two ports have been designed for achieving a good impedance matching and providing the feeding to the CCEs. By switching the p-i-n diodes in each feeding network, the selective excitation mode of the two ports can be varied, thus leading to the reconfigurable patterns. Three operating states can be obtained for the proposed antenna design. Simulation results demonstrate that the envelope correlation coefficients of the proposed MIMO antenna array are far less than 0.5 due to the orthogonality of radiation patterns between the characteristic modes in each of the three states. Measured S-parameters and the radiation patterns of a realized prototype are in good agreement with simulation ones confirming the reliability of the design method.

Basing on the characteristic mode theory, this paper develops the concepts of source-mode (SM) couplings and mode-mode (MM) couplings, which characterized by the modal weighting coefficients and the inter-modal coupling coefficients, respectively. It is found that SM couplings could be adjusted by MM couplings when feeding configuration remains unchanged. Therefore, just by properly adjusting mode-mode couplings, we can reduce Q factor of an antenna at its center frequency and hence its bandwidth could be extended. A new time-saving optimization method based on controlling mode–mode couplings is also proposed. Two examples (one is lossless and the other one is lossy) are given and analyzed in detail to show how to control mode–mode couplings in order to reduce Q factor at the center frequency and hence enhance −10 dB bandwidth.

Despite its long history, the theory of characteristic modes (TCMs) has only been utilized in antenna design for perfect electric conductors (PECs). This is due to computational problems associated with dielectric and magnetic materials. In particular, the symmetric form of the Poggio–Miller–Chan–Harrington–Wu–Tsai (PMCHWT) surface formulation for the method of moments (MoM) solves for both external (real) and internal (nonreal) resonances of a structure. The external resonances are the characteristic modes (CMs), whereas the internal resonances are not. This paper proposes a new postprocessing method capable of providing unique and real CMs in all physical mediums, including lossy magnetic and dielectric materials. The method removes the internal resonances of a structure by defining a minimum radiated power, which is found through utilizing the physical bounds of the structure. The CMs found using the proposed method are verified through the use of an MoM volume formulation, time domain antenna simulations, and experiments involving multiple antenna prototypes.

IEEE RADIO 2018, Radio and Antenna Days of the Indian Ocean, Flic en Flac, MAURICE, 15-/10/2018 - 18/10/2018; This paper discusses the problem of integrating metamaterial (MTM) structures with antennas. We present a new approach based on the computation of the energies of the surface current modes by means of the theory of characteristic modes (TCM). We also introduce new simple formulation to compute the stored energy using the impedance operation of the method of moment (MoM). This analysis can be done the antenna (driven element) and the inclusion (resonant element).

International audience; In this article, a technique to design a compact wideband antenna with the optimization of the input impedance is proposed. This technique is based on the use of two sets of characteristic modes. The first one is associated to an open circuit at the antenna input port while the second is associated to a short circuit at the antenna input port. The link of this analyze with a classical modal approach by a rational fraction approximation of the input impedance is demonstrated. This technique is firstly applied to a basic folded monopole antenna, then to a disc-loaded folded monopole antenna in the aim to optimize its bandwidth and radiation pattern while reducing its size. The disc-loaded folded monopole antenna is modeled and measured. Its VSWR performances are equivalent to thus of the Goubau antenna for equivalent dimensions. The radiation pattern is monopole-like on the full bandwidth.

A preliminary investigation of the modal excitation purity effect on a three dimensional platform is presented. A novel Balanced Inductive Exciter (BIE)is proposed for improving the modal excitation in the considered scenario. The Modal Weighing Coefficient (MWC)and radiated power (P r ) have been accurately investigated in order to assess the performance of the new exciters when compared to previously adopted solutions.

International audience; The recent development in communication systems requires more compact antennas to be integrated on a limited space in order to operate on the desired radio standards. Nevertheless, as the antenna size decreases with respect to the wavelength, it becomes limited by bandwidth, efficiency and directivity. Recently, several researches have focused on a new class of compact antennas with high directivity. Such antennas are called superdirective anetnnas. However, despite the high directivity achieved, the antennas suffer from very narrow bandwidth and low efficiency. This paper presents an approach to enhance the bandwidth of superdirective arrays based on characteristic modes analysis. By optimizing the modes in the unit element of the array, it is possible to obtain a wideband superdirective array. © 2018 International Union of Radio Science URSI.

© 2018 IEEE. Recently, researchers noticed that there are non-physical modes in characteristic modes of material bodies. In this paper, those non-physical modes are investigated. The reason for the non-physical modes is discussed. The influence of integral equations' internal resonance on the non-physical modes are studied. It is concluded that the internal resonance is not responsible for the non-physical modes. Meanwhile, based on the extinction theorem, the dependent relationship between the equivalent surface electric and magnetic currents are proposed, and it is observed that the non-physical modes do not obey the dependent relationship, whereas the physical modes are governed by the dependent relationship. Therefore, it can be concluded that the lack of the dependent relationship causes the non-physical modes. Besides, based on the dependent relationship, a novel post-processing method to remove the non-physical modes is proposed. Numerical results of a dielectric cylinder are presented to validate the discussions and new post-processing method.

[EN] Here, a detailed analysis of characteristic modes and fields of an infinite dielectric circular cylinder when computed through the Poggio-Miller-Chang-Harrington-Wu-Tsai formulation is carried out. The purpose is to determine their contribution to the total field, inside and outside the dielectric body and under two possible excitations: incident plane wave or electric line source within the cylinder. The study has been done analytically to provide necessary physical insight of the results obtained. New details about the so-called nonphysical modes are provided. It is found that these modes, that can be neglected outside the dielectric body, do have a significant contribution to the inner field when the excitation source is within the dielectric body. It is concluded that the terms physical and nonphysical characteristic modes should be replaced for radiating and nonradiating characteristic modes.

[EN] In this paper, the application of the Theory of Characteristic Modes for the design of narrow-in-width open slot antennas embedded in finite platforms is investigated. Purely magnetic and electric characteristic modes (CM) of a slot etched in both an infinite and a finite ground plane are analyzed, with the aim to provide a physical understanding of the slot behavior and its interaction with the metallic ground plane. Instead of traditional CM analysis, an alternative approach is proposed, which consists in dividing the design procedure into the separate and complementary analysis of purely magnetic and electric CM. Based on this analysis, some guidelines for the design of open slot antennas are provided, and a simple and compact narrow-in-width wideband open slot antenna is designed. Simulations and measurements are presented for the optimized slot antenna, showing an impedance bandwidth of 47.48% with a very stable radiation pattern., This work was supported by the Spanish Ministry of Economy and Competitiveness under Project TEC2016-78028-C3-3.

The Theory of Characteristic Modes (TCM) has recently been shown to be beneficial in solving a wide variety of complex electromagnetic problems. However, there are still open issues in using TCM to analyze objects which consist of simple or complex media. Either a volume integral equation (VIE) or a surface integral equation (SIE) is required to solve for the characteristic modes of these objects. Herein, we overview the important issue that the characteristic eigenvalues obtained from SIE formulations are not related to the modal reactive power, unlike the classical TCM definition. A recently proposed solution that restores the modal reactive power interpretation of characteristic eigenvalues is described. A numerical example is provided to demonstrate the differences between the SIE eigenvalues and the restored eigenvalues.

Although the classical Theory of Characteristic Modes allows an arbitrary structure to be analyzed prior to the implementation of physical feeds, structures containing dielectrics have so far received very little attention. Recently, a mesh perturbation method is proposed to remove internal resonances from the characteristic mode (CM) solution for lossy dielectrics obtained using the computationally efficient surface integral equation. Herein this method was applied to extract the CMs of a lossy structure consisting of a terminal chassis held in a user hand. These modes were then individually analyzed and a subset was chosen to design a MIMO antenna with not only very low correlation, but also low hand-induced losses.

International audience; A design methodology based on the combination of the Theory of Characteristic Modes (TCM) with an Optimization Algorithm, to design a wideband Electrically Small Antenna (ESA) is presented. Unlike other design techniques, loads or matching networks are not needed at the input of the antenna. A design example of an Inverted-L Antenna (ILA) is presented to validate the effectiveness of this technique. © 2017 Euraap.

International audience; Bandwidth and Efficiency enhancement by multiport loading of a double notched small antenna is presented in this paper. The technique used is based on the combination of the Network Characteristic Modes (NCM) with an optimization algorithm, in order to find the optimal load values at specified positions. The required loads are found to be simple capacitive loads.

[EN] Here, some aspects in the interpretation of the solutions of a PEC infinite circular cylinder with the Theory of Characteristic Modes are presented. First, natural resonances and characteristic mode resonances (CMRs) are introduced and compared. Second, characteristic eigenvalues are used to find those natural resonances considering complex ka values. Furthermore, by linking the standard and the generalized eigenvalue problems, a relation between natural resonances and characteristic mode eigenvalues is shown. Finally, the thesis stating that external CMR does not imply maximum field scattering is also demonstrated.

This Letter proposes the use of a modal pattern stability factor (MPSF) parameter to quantify the pattern stability associated to characteristic modes (CM). The proposed parameter can be used as a figure of merit for CM associated to arbitrary-shaped structures, aiding the antenna design process. This parameter can be used (together with other modal parameters) to optimise the geometry of a radiating element in order to improve the radiation pattern stability in a desired range of frequencies. Two examples are presented for analysis. First, MPSF for the CM of three ring antennas with different width are analysed, showing how pattern stability of CM depend on this geometrical parameter. Secondly, three metallic plates with different shapes are analysed in terms of MPSF, in order to assess the stability of the CM for each geometry., This work was supported by the Spanish Ministry of Economics and Competitiveness under project TEC2013-47360-C3-3-P, and by Generalitat Valenciana under project GV/2015/065.

For more than 30 years since it was first proposed by Harrington et al., the Theory of Characteristic Mode (TCM) has only been applied to perfect electric conductors (PEC), and more recently lossless dielectric materials. One key challenge in computing the characteristic modes (CMs) of non-PEC materials using the PMCHWT surface integral equation is the presence of internal resonances in the solution space, due to the required forcing of symmetry on the impedance matrix. In lossless dielectrics, it was shown that these non-real CMs can be removed in post-processing through far-field power analysis. However, this method breaks down in lossy materials as it relies on the assumption of no radiation losses. This paper proposes the use of basis function perturbation to isolate the non-real CMs from the real CMs when CM analysis is applied to lossy materials.

A novel pattern reconfigurable antenna operating at 2.45 GHz is designed for portable devices by exploiting the characteristic modes analysis (CMA). In particular, a null-steering antenna is proposed by exploiting the current modes distribution (J n ) over a ground plane of the dimensions of a mobile terminal. To achieve this goal, two current modes (J 4 and J s ) are asymmetrically excited by introducing a phase difference (Δθ) between the two capacitive coupler exciters (CCEs) hosted above the conductive plane. More in detail, the position of the CCEs allows exciting only Mode # 4 and Mode # S, related to the current modes J 4 and J s , respectively. The phase difference (Δθ) allows controlling the level of the excitation of these modes and obtaining the null-steering antenna in the y-z plane. As a proof of concept, a discrete phase shifter that employs several pin diodes has been designed in microstrip technology in order to obtain a subset of the phase differences. The radiation patterns prove that the proposed design guidelines are reliable and applicable to other kinds of structures.

In this paper, full- and sub-structure characteristic modes (CMs) are explored through the effect of coupling between the target and background structures. The eigenvalues and eigen-current distributions of the first few modes are compared for different coupling levels in the numerical example. Similarities and differences between the two types of CMs in specific conditions are revealed, paving the way for further feasibility studies of the sub- structure method for antenna design.