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In this paper, a characteristic mode (CM)-based design methodology is developed for the reconfigurable microwave metasurface absorbers (MMA) with using commercially available substrates. Theoretical intrinsic connection between modal behaviors and absorption properties is first formulated for the design of the structures. As a proof of the method, a broadband MMA consisting of butterfly elements is step-by-step designed. The positions and values of the impedance loading are determined based on the modal significances and modal currents for broadband operation. The absorption performance is evaluated by the formulated theoretical model and verified by full-wave simulation. After that, a flexible implementation a employed for free-form surfaces conformal. Experimentally, the proposed MMA is fabricated and measured. The absorption fractional bandwidth with reflectivity less than –10 dB is achieved from 3.6 to 11.6 GHz (105%). Good agreements between the simulation and measurement results for both planar and cylindrical cases validate the developed CM-based design methodology. The mimicry of time-varying scattering cross section (SCS) is then further realized through real-time coding absorption states, which is a promising candidate for practical electromagnetic systems.

This work presents a printed monopole antenna with a miniature size and enhanced wideband using characteristic mode analysis (CMA). The printed monopole antenna structure comprises one vertical and one horizontal monopole, and a ground plane with a vertical extension. By connecting the horizontal monopole to the ground plane extension through a via hole, a significant enhancement in bandwidth is achieved in both lower and high bands. A 105% increment in bandwidth is achieved with the via hole compared to without it. The influence of the via hole is analyzed using CMA to provide a physical insight into the design. The proposed antenna is compact with an electrical size of 0.01 λ2, leading to a miniaturized design. [ABSTRACT FROM AUTHOR]

In this research, a novel unit-cell design for a 2-bit reflective metasurface using the theory of characteristic modes is presented. The initial unit-cell consists of three sub-patches with 2-PIN diodes integrated within the gaps between sub-patches. The initial structure provides limited tuning of the phase curves with the change in switching states of PIN diodes. To provide the phase deviation required for the 2-bit operation, the symmetry of the modal currents is used to propose multiple slots in the sub-patches. Slots are introduced to lower the resonant frequencies, as well as provide tuning of the resonance of modes and consequently phase change between different switching states. For further resonance tuning and phase optimization, two vias are loaded in the middle patch to provide a 90° phase difference between each switching state. Simulated results show that the proposed unit-cell design with the aid of characteristic modes has a smaller size and provides 2-bit operation in the frequency range of 3.9–4.04 GHz. [ABSTRACT FROM AUTHOR]

This article presents a method of using modal analysis to characterize the circular polarization (CP) performance of a printed monopole antenna. The radiator and feedline are positioned on the edge of the substrate and fed accordingly at the edge, and a modified ground plane is employed. The complete antenna without excitation is analyzed using characteristic mode theory to provide physical insight into its CP operation. The modal analysis proves that several nearly orthogonal modes can be generated at certain frequencies with orthogonal phase differences as well. The axial ratio (AR) corresponds to the modal analysis which proves the validity of the modal analysis. An axial ratio bandwidth from 3.1 to 6.2 GHz (66. 7%) is achieved within an S11 bandwidth of 2.6–11 GHz (123.5%). The proposed antenna is compact with stable radiation patterns, easy to design, and fabricate without inserting parasitic patches or inserting slots on the antenna. [ABSTRACT FROM AUTHOR]

In this paper, modal analysis is used as a guide to design a circularly polarized open‐slot antenna. The design proposes an open wide slot that is fed electromagnetically by a modified feedline. Both open slot and feedline are asymmetrically placed to the edge of the substrate. The complete antenna without excitation is analyzed using characteristic mode theory to provide a physical insight into its circular polarization operation. The modal analysis proves that several nearly orthogonal modes can be generated at certain frequencies with 90° phase differences. The axial ratio (AR) corresponds to the modal analysis, which proves the validity of the modal analysis. An AR bandwidth from 3.2 to 6 GHz (61%) is achieved within an S11 bandwidth of 3.2–14 GHz (125%). The proposed antenna achieves a peak realized gain 4.3 dBi of within the AR band. [ABSTRACT FROM AUTHOR]

In this article, a novel double-face logarithmic spiral metamaterial (LSMTM) superstrate-inspired multiple-input multiple-output (MIMO) for fully enhanced circularly polarized (CP) antenna system is examined for 5G wireless communications. This novel double-face LSMTM superstrate acts as a planar concave-concave lens. Initially, the antenna is designed with a circular spiral patch to generate CP radiation in the frequency band of interest. Then, at a height of 6.5 mm (0.606 λo) above the MIMO antenna, which has a 0.8 mm (0.075 λo) edge-to-edge separation, the LSMTM superstrate is employed for isolation, gain, and bandwidth improvement. The proposed superstrate enhances the isolation, gain, and bandwidth of the antenna by about 32 dB, 3.47 dB, and 900 MHz, respectively. In contrast to the conventional technique of verifying operation with a simulated surface current distribution, characteristic mode analysis (CMA) is used to provide a better explanation of the proposed antenna's different modes and the creation of circular polarization. Additionally, the CMA supports the development of an effective technique that can predict whether or not the isolation can be further improved. The simulated results align with the measured results and are well adapted for 5G wireless communication devices. [ABSTRACT FROM AUTHOR]

The consistent interpretation of modes is crucial in characteristic mode analysis, which can usually be achieved by mode tracking algorithms. However, the problem of the eigenvalue crossing avoidance is still challenging to solve completely. In this paper, the possible causes of the eigenvalue crossing avoidance are discussed, and divided into two types. According to the characteristics of the modes with the cross avoidance, a comprehensive method for locating the frequency of crossing avoidance is first proposed. Since the crossing avoidance occurs in the case of degenerate mode coupling, the decoupling method is applied to eliminate the mode cross avoidance. Finally, the method's effectiveness is verified by dealing with crossing avoidance problems of a rectangular conductor, dipole, and skewed plate structure. The proposed method is only related to the mode number to be discussed, and its calculation is time-saving. [ABSTRACT FROM AUTHOR]

This work presents a bandwidth enhanced high gain rectangular patch antenna operating under two similar spectrally close modes created from a TM50‐like mode in the vicinity of another higher order mode of the patch. The patch is embedded with two wide slots and meandered linkages are used to maintain current continuity. The whole structure is studied using the theory of characteristic modes and the excitation of these higher order modes is controlled to enhance the bandwidth. One of the two similar modes is very weakly excited and does not affect the antenna's performance. The beam tilting at lower frequencies are compensated by properly choosing the capacitors as phase shifter elements on the meander linkages. The antenna size is miniaturized by properly choosing the dimensions of the meander linkages and the resonant length is reduced by λ/2 as compared to that of a conventional TM50 mode supporting patch. The patch size becomes 3.35 × 4.03λ/2 instead of 5 × 5λ/2 (λ is the substrate wavelength). The coaxial probe fed structure exhibits a good performance of impedance bandwidth of 10.5% around the center frequency of 5.7 GHz with a gain variation 10–12 dBi, which appear to be better than other antennas of similar size. [ABSTRACT FROM AUTHOR]

In this paper, a reconfigurable metal rim antenna with a small clearance and compact structure for 4G/5G smartphones is proposed. It consists of the system ground plane and a metal rim with two slits, in which a U-shaped slot of 1 mm (the smallest) clearance is realized between the system ground and metal rim. The theory of characteristic modes is applied to analyze the eigenmodes of the antenna so as to aid the optimum design. The characteristic modes (CMs) in the band of interest are studied, and an inductive coupling exciter is identified as the best since three CMs can therefore be generated in the lower frequency band (0.69-0.96 GHz), and an SP4T switch can be utilized to increase the bandwidth of the whole lower band. A prototype antenna of the optimized design was fabricated. A good agreement was obtained between the measured and simulated results. The 6-dB frequency bands are the lower band (0.69-0.96 GHz), the middle band (1.7- 2.7 GHz), and the higher 5G band (3.4- 3.6 GHz), respectively, and the total efficiencies are over 50% in all these frequency bands. They indicate that the antenna is a very good candidate for smartphone applications.

In this paper, the analysis of an UWB monopole antenna using the Theory of Characteristic Modes is presented. It starts from the modal study of an asymmetric dipole (structure 1), and then complements and compares it with a broadband monopole (structure 2), aiming to understand the operation principle of the proposed antenna. To achieve this, the electromagnetic properties of the structures are analyzed through the generation of its characteristic angles, modal radiation patterns, characteristic currents, and the power contribution percentage of each mode to the total radiated power, providing a vision on how the superposition of individual characteristic modes enables UWB operation. The prototype model is fabricated, and the measured results agree very well with the full-wave simulations results. The proposed prototype has a very low profile and operates in the frequency band from 2.8 GHz to exceed 10 GHz for a reflection coefficient S11 less than -10dB. [ABSTRACT FROM AUTHOR]

This paper describes the application of the theory of characteristic modes to excite a conducting cylinder representing the chassis of a rocket. Mode excitation is achieved by cutting H-shaped slots on the cylinder at specific locations where the maximum of modal current distribution occurs. The L-matching network is designed to match the impedance of the slots to the input coaxial cable. Finally, the proposed concept is verified during manufacturing and measurement. It is shown that the measured results are in excellent agreement with the simulation.

A systematic method based on the Theory of Characteristic Modes (TCM) for finding orthogonal radiation patterns of any electronically steerable parasitic array radiator (ESPAR) is described. This method can be useful for designing single-RF front-end multiple-input multiple-output (MIMO) systems in which orthogonal patterns can be utilized as a basis set for space modulation (SM) or full multiplexed MIMO systems. The method is based on the $N$ -port formulation of TCM and is not restricted to antenna type or array configuration. It can also provide closed formed solutions for the antenna element currents required to generate the orthogonal patterns. In addition an approach to finding the required load reactances in ESPAR for generating the orthogonal patterns is provided and is based on the quasi-Newton method utilizing a closed form expression for the initial approximate solution. Approximate estimation of the effective aerial degrees of freedom of the ESPARs is also discussed. Two simulation examples of ESPARs, a 4-element linear dipole array and an 8-element rectangular planar inverted-F antenna array, using SM as well as full multiplexed MIMO are provided, demonstrating the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper presents a compact 5G multiple‐input multiple‐output (MIMO) microstrip antenna with isolation enhancement based on a slotted complementary split‐ring resonator (SCSRR) and the theory of characteristic modes (TCMs). The metamaterial unit consists of three CSRR connected by extra slots. These added slots improve significantly the rejection response in terms of bandwidth and suppression. The dispersion diagram analysis is introduced to show the filtering characteristics of the band‐gap structure before and after adding these additional slots. The TCM is employed to investigate the behavior of this 5G MIMO antenna before and after adding the slotted CSRR. The TCM is also applied to the MIMO antenna system to build up a precise methodology that can foresee whether the isolation can be upgraded further or not. The slotted CSRR is inserted meticulously in specific locations to block the coupling modes and almost does not affect the results of the noncoupling modes to improve the isolation remarkably. With this slotted CSRR, a 27‐dB reduction in the mutual coupling between the two patch antennas is achieved. The whole design has been simulated utilizing the Microwave Studio CST ver. 18 simulator. The antenna being proposed is highly efficient and suitable for 5G wireless communication. [ABSTRACT FROM AUTHOR]

A hepta-band antenna with high efficiency, small clearance, and simple structure for metal-rimmed smartphones is proposed in this letter. The proposed antenna occupies only 71 × 5 mm2 of the total clearance, and it is mainly composed of a system ground, a metal rim with two gaps, and two shorting strips. The theory of characteristic modes is used to analyze the entire structure. All resonant characteristic modes (CMs) in the target frequency are traced, and their characteristic current distributions are analyzed. By using an L-shaped capacitive exciter, three CMs are excited at the lower bands, and an inverted L-shaped branch is utilized for improving the impedance matching at higher bands. The measured 6 dB impedance bandwidths were 800–1000 and 1700–2780 MHz, respectively, and they can cover seven operating bands. The measured efficiencies were 62%–85.7% at the lower band and 66.9%–91% at the higher band. [ABSTRACT FROM AUTHOR]

This paper describes the application of the theory of characteristic modes to excite a conducting cylinder representing the chassis of a rocket. Mode excitation is achieved by cutting H-shaped slots on the cylinder at specific locations where the maximum of modal current distribution occurs. The L-matching network is designed to match the impedance of the slots to the input coaxial cable. Finally, the proposed concept is verified during manufacturing and measurement. It is shown that the measured results are in excellent agreement with the simulation. [ABSTRACT FROM AUTHOR]

The Theory of Characteristic Modes (TCM) is proposed as a systematic approach to antenna design to achieve the goal of finding the antenna structure with optimum broadband behavior. This theory provides a physical insight to the radiating nature of microstrip patch antennas and reduces the design optimization time. In this work, the resonant behavior of the highly radiating structure of the single U-slot rectangular patch on a single-layer εr = 4.4 substrate is analyzed using TCM. Modal analysis of this single-layer structure with different single feed excitations concludes that VSWR ≤ 2 impedance bandwidth in the order of 96% can be achieved with a single T-probe feed. Experimental results, included here, show VSWR ≤ 2 impedance bandwidth in the order of 71%. [ABSTRACT FROM AUTHOR]

The theory of characteristic modes (TCM) for PEC bodies is showing great promise and application in the analysis, design, and placement of antennas. Its use for input impedance estimation, however, is still largely dependent on the idealized delta-gap model. Because TCM depends on all conductors, even feed structures, care must be taken for certain classes of antennas if estimating input impedance; probe-fed microstrip patch antennas (MSAs) are one example. In this paper, two different coaxial probe feed models are developed to estimate the input impedance of thin, arbitrarily shaped, air-dielectric MSAs using TCM. Their accuracy is compared with experimental results for a case study of an L-shaped MSA. Results show the importance of including all metal structures, including feeds, in TCM analyses. [ABSTRACT FROM AUTHOR]

In this letter, new metrics are proposed to provide physical insight into the behavior of metamaterial-based structures by means of the theory of characteristic modes. A relationship between the first eigenmode and an expression of the polarizability is proposed for a split-ring resonator. It thus allows us to describe a link between the artificial magnetic response of a split-ring resonator and modal characteristics. Using the new metrics proposed, a reactive impedance substrate based on metal-dielectric metamaterial excited by a parallel magnetic field is shown to exhibit artificial magnetism. These metrics are shown to propose good agreement with other analytical and numerical methods. This analysis should be useful when such metal-dielectric metamaterial structures are associated to radiating elements like antennas since it could be performed in the near-field region, unlike common approaches based on plane wave and far-field assumptions. [ABSTRACT FROM PUBLISHER]

In this letter, we propose a horizontally polarized antenna with omni-directivity in horizontal plane. Ripple coefficient of the radiation pattern in horizontal plane improves up to ±0.1 dB. The theory of characteristic modes is used to analyze and illustrate the reason for why the omni-directivity property of the proposed antenna improved. Finally, we verify the antenna performances by experiment. [ABSTRACT FROM AUTHOR]

The creation of a magnetic boundary condition into the plane of symmetry in a radiating ground plane of a portable device is proposed to enhance its radiation efficiency and bandwidth. This magnetic boundary condition is achieved by exciting the antenna through symmetrical feeding. A novel antenna consisting of a symmetrical folded dipole is presented in order to exemplify how a broad bandwidth and very good radiation properties can be obtained with the proposed technique. The Theory of Characteristic Modes is used to analyze and compare a folded dipole with the symmetrical folded dipole. This study shows how the magnetic boundary condition favors excitation of antenna modes in a broad bandwidth while avoiding excitation of transmission line modes. Two different prototype antennas, a monopole and a symmetrical folded dipole with an LC-balun, were implemented and measured. The measured -6 dB impedance bandwidth of the monopole extends from 0.95 GHz to 2.15 GHz. In turn, the bandwidth of the symmetrical folded dipole extends from 0.90 GHz to 1.96 GHz. The average of the measured total efficiencies at the aforementioned bandwidth is -1.6 dB for both prototypes. The measured maximum total gain at 0.95 GHz is 2.6 dBi for the monopole and 1.9 dBi for the symmetrical folded dipole. [ABSTRACT FROM AUTHOR]

A compact four element multi‐band multi‐input multi‐output (MIMO) antenna system for 4G/5G and IoT applications is presented in this paper. The proposed antenna is developed using the theory of characteristic modes helping in systematic design of MIMO antenna system. It consists of four L‐shaped planar inverted‐F antenna (PIFA) elements each operating at 3.5, 12.5, and 17 GHz bands with the bandwidth of 359 MHz, 1 GHz, and more than 3.7 GHz, respectively. The proposed antenna system is suitable for both 4G/5G and internet of things devices as it shows the satisfactory MIMO system performance. Good isolation characteristics are observed by implementing complimentary Metamaterial structure on the ground plane resulting in isolation level lower than −21 dB between the antenna elements. The proposed antenna is fabricated and experimental results are also presented and discussed. [ABSTRACT FROM AUTHOR]