Characteristic Mode Analysis for Antennas with Waveport Problems

Characteristic mode (CM) analysis of multi-scale and complex targets has attracted much attention in computational electromagnetics. However, it is hard to calculate the multi-scale problems in terms of desired accuracy and stability by using conventional methods. What is more, it is a challenging work to generate a good quality mesh for the multi-scale structures including very fine features. In this paper, we present Discontinuous Galerkin (DG) surface integral equations-based theory to analyze the CMs. The multi-scale objects are firstly divided into several subdomains, and we generate the meshes of every parts independently. HRWG basis functions are adopted in our work and the characteristic currents are expanded by HRWG basis functions. In many applications, only characteristic currents with small eigenvalues are desired because they are corresponding to the modes that radiate efficiently and have large modal coefficients. Special singularity extraction schemes are chosen to treat the infinite term in the resulting integral equations since HRWG basis functions are not divergence-conforming and there is divergence operator in electric field integral equation (EFIF) and magnetic field integral equation (MFIE). In DG method, they are no requirements for normal continuity of currents across the contour of between every two subdomains, hence, the proposed method can handle both non-conformal mesh and conformal mesh. Numerical results are presented to validate the propose formulation. This work may offer great flexibility to CM analysis of multi-scale and complex structures.

The integration of electric field enhancement structures (EFESs) with Rydberg atomic sensors (RASs) has garnered considerable interest due to their potential to enhance detection sensitivity in quantum measurement systems. Despite this, there is a dearth of research on the directional response of EFES, and the analysis of the three-dimensional (3D) patterns of RAS remains a formidable challenge. RASs are employed in non-destructive measurement techniques, and are responsive to electric fields, primarily serving as reception devices. However, analyzing their reception patterns is a complex task that requires a sophisticated approach. To address this, we adopt characteristic mode (CM) analysis to illustrate the omnidirectional performance of RAS. According to the CM theory, the reception pattern can be calculated by a series of modal currents and their corresponding coefficients. The analytical representation of these coefficients negates the need for time-consuming full-wave (FW) numerical simulations, which are typically required to generate EFES patterns due to the necessity of scanning numerous angle parameters. This approach significantly reduces the complexity of solving EFES patterns, and provides insightful guidance for the design process. To validate the efficacy of our proposed method, we construct three prototypes. The results indicate that the final model resonates at 1.96 GHz, achieving an electric field gain of 25 dB and an out-of-roundness of 2.4 dB. These findings underscore the effectiveness of our method in analyzing EFES patterns, highlighting its potential for future applications in the field.

A single-layer broadband vertically polarized (VP) omnidirectional metasurface antenna is proposed in this communication. To guide the optimization and design of the antenna, characteristic mode (CM) analysis (CMA) is performed to investigate the intrinsic modal behaviors of the metasurface. A mode with VP omnidirectional radiation is first identified by analyzing an array of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4 \times 4$ </tex-math></inline-formula> squared patches. To excite this mode, a complex feeding network is required, which leads to gain loss and increased structural complexity. Furthermore, the mode currents change with the frequency. Consequently, this mode cannot be excited efficiently in a wide bandwidth using a fixed feeding structure. To solve these problems, the modal current is analyzed at different frequencies. The sizes of the four corner patches and the other four edge patches are adjusted to optimize the modal behaviors. After optimization, the mode becomes much more desirable and can be efficiently excited in a wide bandwidth using a single probe. Therefore, the proposed antenna can be designed on a single-layer substrate and excited by a simple feeding structure with broadband characteristics. To verify the design, the proposed metasurface antenna is simulated, fabricated, and evaluated. The results show that the proposed metasurface antenna can achieve a broad bandwidth of 64.2%.

Characteristic mode analysis has proven useful in the design of multi‐input multi‐output antennas. By identifying characteristic current modes associated with a conductive structure, the designer can place ports to replicate those current modes, which should generate orthogonal far fields with high isolation from one another. An algorithm that uses characteristic mode analysis is proposed to synthesise multi‐input multi‐output antennas from a conductive structure by determining port placement and excitations without visual inspection. Our algorithm uses a correlation metric to determine the proper excitation from a particular port, and uses a global optimisation algorithm to determine the optimal port placement. Given a particular structure and a domain of possible port locations, our algorithm places ports and generates excitations to synthesise the desired multi‐input multi‐output antenna. Our algorithm is used to synthesise and simulate a simple 3D cell phone model with a metal ground plane, metal rim, and dielectric. The result is an antenna with the desired six isolated channels at 2.5 GHz.

A characteristic mode eigenanalysis exploiting Finite Element Methofi (FEM) features is introduced. In the typical characteristic mode eigenanalysis the Method of Moment (MoM) is utilized (llarringion R. F.. and Mautz J. R., IEEE Trans, on Ant. and Propagat., AP-19, 1971). Moreover, to solve composite structures including both metallic as well as material-dielectric loading, a modified surface integral equation of MoM is demanded (Shafai, IEEE Trans, on Ant. and Propagat., 34, 666–673, 1986). Equivalent electric and magnetic currents are defined in order to take into consideration the interaction between the different homogeneous parts. However, it is proved thai the linear system that occurs, should be solved only in terms of the equivalent electric current that flows on the metallic parts, eliminating all the other currents from the final system (Maximidis R.T., Zekios C.L., Kaifas T.N., Vafiadis E.E. and Kvriacou, G.A., EuCAP 2014, 2822666–2826, 2014).

This work analyses a low-profile, broadband monopolar patch antenna (which we call the Annular Coupled Patch) using Characteristic Mode Analysis. The full structure is divided into two resonators: a cylindrical cavity and shorted annular patch, which are electromagnetically coupled by the fringing fields about the annular ring of vias separating the two resonators. The analysis shows that Coupled Mode Theory accurately models the resonances of the full, coupled structure. The modal behavior is compared and contrasted with several other antennas that are also well-modeled by Coupled Mode Theory.

A new broadband circularly polarized (CP) metasurface (MS) antenna based on characteristic mode analysis (CMA) is proposed. The antenna consists of 4×4 truncated Γ-shaped unit cells and is fed by an improved slot coupling structure. The characteristic modes required to achieve CP are analyzed by CMA, and the CP performance is improved by changing the characteristic currents distribution. By introducing a stepped slot structure on the slot floor, the bandwidth of the antenna has been further broadened. By loading the microstrip line to feed the MS, left-handed circular polarization (LHCP) can be realized. In addition, the equivalent circuit method is used to explain how the designed MS antenna achieves CP. The overall size of the designed and fabricated MS antenna is 0.72 λ 0 ×0.72 λ 0 ×0.08 λ 0 at 6.2 GHz. The measured −10 dB impedance bandwidth of the antenna is 25.17% (5.21 GHz-6.71 GHz) and the measured 3 dB axial ratio bandwidth is 17.42% (5.87 GHz-6.99 GHz). Moreover, the highest gain in the working frequency band is 7.52 dBic. The proposed CP MS antenna has good application prospects in C-band satellite communications.

In this study a generic aircraft structure is numerically investigated to gain insight whether a HIRF excitation is equivalent to a DCI excitation. The investigation is based on Characteristic Mode Analysis where the different types of excitation are seen as equivalent if they are capable of exciting the same characteristic modes of a given structure. As main result it is observed that HIRF excitations and DCI excitations will lead to different characteristic modes since they are subject to different boundary conditions.

In this article, an approach combining the electric field integral equation (EFIE) and impedance boundary condition (IBC) is presented for the characteristic mode analysis (CMA) of composite metallic-dielectric structures. Different from published works, only the electric currents are used in this approach and the computed eigencurrents are in a similar form as those of purely metallic structures. To ensure completeness and orthogonality of the eigencurrents, symmetry of the IBC-EFIE operator has been carefully examined. For lossy structures, a new formulation is deduced for CMA. The presented IBC-EFIE formulation is able to handle a variety of problems, e.g., lossy metals, thin dielectrics, thin dielectric-coated conductors, and metasurfaces. Several numerical examples are provided for those problems, and full-wave CMA solutions are used to verify the IBC-EFIE formulation. It is shown that this method is fairly accurate while computationally efficient, which makes CMA a promising tool for large scale or complex composite structures.

Mobile communications technology is constantly changing with the increasing demands of the consumers and Long Term Evolution (LTE) is widely accepted as the technology that can meet these requirements. This paper presents an electrically small antenna designed for European LTE frequency band of 1800MHz. A systematic design procedure is developed using characteristic mode analysis (CMA). The CMA design procedure provides needed insights for the antenna designers to quickly come up with an antenna design using an arbitrarily shared structure.

Characteristic mode analysis has traditionally been constrained to problems which utilize only perfect electric conductors (PEC). Through forced symmetry of a method of moments surface integral equation and newly proposed post-processing, characteristic modes can be solved for any material in a computationally efficient manner. As an example, the characteristic modes are solved for a mobile terminal consisting of both PEC and dielectric materials.

Characteristic mode analysis provides unparalleled insights into designing high performance multiple-input multiple-output (MIMO) terminal antennas at frequencies where the antenna elements are constrained to be electrically small. Conventionally, an electrically small single-antenna utilizes the fundamental characteristic mode of the terminal chassis to obtain sufficient bandwidth while maintaining high radiation efficiency. However, modern MIMO terminals require two or more antennas per frequency band, and they tend to excite the same fundamental chassis mode, resulting in severe coupling, correlation, and poor overall system performance. Recently, characteristic mode analysis of the chassis is proposed to design highly efficient multimode multiband MIMO antennas with significant bandwidth using electrically small feed elements. Two distinct and excitable characteristic modes were created at frequencies above and below 1 GHz, for a typical smartphone's form factor. This paper provides an overview of the method and highlights its versatility for practical implementation in standard plastic cased smartphones as well as in the popular metal-bezeled smartphones, with only minor modifications to the chassis.

This paper presents computation and observations of the characteristic modes for radiating components in digital systems. Considerations collected during the analysis of different components are summarized for reference. Some observations on the computed results of multiple daughter cards and heatsink with fins are presented and discussed. It is shown that characteristic mode analysis is a useful tool to analyze metallic radiators and to reduce their radiated emission in digital systems.

Interacting chemical systems can be analyzed using the structures and lifetimes of characteristic modes of variation about a quasi‐steady state. For the atmospheric CH4‐CO‐OH system, characteristic modes of concentration variation involve predominantly a single species, but their lifetimes differ from the dominant species turnover times. Here a species turnover time is the species amount divided by its removal rate, whereas the characteristic mode lifetime is the amplitude of a perturbation pattern divided by its rate of change and includes feedback effects. Characteristic mode analysis is used here to consider variations in both concentrations and isotopic ratios for such systems. New ‘isotopic modes’ are identified which are only weakly affected by feedbacks and have lifetimes very close to species turnover times. This raises the possibility that analysis of variations in both concentration and isotopic ratios may provide a unique tool for resolving turnover times and chemical feedbacks in atmospheric chemistry.

A low-profile and broadband metasurface antenna is proposed for circular polarization radiation. The metasurface is an array of subwavelength square patches. The modal behaviors of the proposed metasurface are investigated by using the characteristic mode theory. Two characteristic modes with the same resonant frequencies and orthogonal current distributions are chosen as operation modes. Furthermore, a hybrid feed system consisting of a cross-slot and a microstrip line is employed to excite the two orthogonal modes having a 90° phase difference to obtain circular polarization radiation. Based on these concepts, an antenna with low profile of 0.07λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the free-space wavelength at an operation frequency of 5.5GHz) is designed. The measurement results show that the proposed antenna has -10 dB impedance bandwidth of 4.8-6.35 GHz and 3 dB axial-ratio bandwidth of 4.85-6 GHz. Moreover, the antenna gain is 6.8-9.7 dBic in the whole axial-ratio bandwidth.