Selection of Dominant Characteristic Modes

The Theory of Characteristic Modes (TCM) had its humble beginnings in the early 1970's. The beauty of TCM lies in its ability to fully characterize the radiation and scattering properties of an arbitrary object based only on the object's geometry and material properties. This ability provides valuable insights into an antenna's behavior independent of the feeding arrangement as well as providing information about how desirable radiation modes can be excited. This feature has led to its use to design integrated antennas in the High Frequency (HF) band for land vehicles, ships and aircraft. However, TCM had largely remained a specialist field until it was rediscovered for aiding the design of mobile handset antennas about a decade ago. In particular, TCM provides a powerful tool to understand and exploit excitation of the terminal chassis to enhance antenna performance. Another powerful feature of TCM is that multiple characteristic modes at a given frequency facilitate orthogonal radiation patterns, which provide effective Multiple-Input Multiple-Output (MIMO) antennas.

The analysis of Archimedean spiral slots using the theory of characteristic modes is presented in this article. Purely magnetic characteristic modes are used to study the behavior of a spiral slot etched in an infinite ground plane. The effect of the spiral geometry parameters (slot width, separation between spiral slot arms, number of spiral turns, and inner spiral spacing) on the characteristic modes are investigated using the theory of characteristic modes. Based on the results of the characteristic mode analysis, guidelines are presented for selecting the appropriate slot dimensions for realizing a compact spiral slot assembly (miniaturization), multimode behavior and for improving the bandwidth of the purely magnetic characteristic modes. Additionally, analytical equations are presented for accurately predicting the resonant frequencies of the purely magnetic characteristic mode of the spiral slot antenna.

When analyzing object scattering, characteristic mode (CM) theory only focuses on the inherent properties of the target object and has nothing to do with any external factors. Different from traditional analytical eigenmode, characteristic mode method not only retains the advantages of clear theoretical concept of analytical eigenmode, but also combines the advantages of numerical method of moment (MoM) in dealing with irregular structures. Therefore the universality of the method is guaranteed. An aircraft example's analysis of radar cross section (RCS) using characteristic mode is given in this paper. The solution of characteristic equation can use implicit Arnoldi iterative algorithm. Characteristic mode theory provides theoretical support for the original antenna design so as to design an antenna with excellent performance.

In this paper, the theory of characteristic modes (TCMs) is used for the first time to analyze the behavior of defected ground structures (DGS), when added to antenna designs. A properly designed DGS introduces currents opposite in direction to the original characteristic modes (CM) currents thus reducing mutual coupling. TCM is also applied to multiple-input-multiple-output (MIMO) antenna systems to develop a systematic approach that can predict whether the isolation can be enhanced further or not. For this purpose two 4-element and one 2-element MIMO designs, i.e. monopole and planar inverted-F antennas are studied. The addition of different antenna elements affects the CM significantly as well as differently. Some of the CM excited on the antenna surface contributes to the coupling between the antenna ports that is why they can be classified as coupling modes. To improve the isolation, the DGS should be introduced at certain locations that blocks the coupling modes but at the same time does not affect the non-coupling modes. If there is no such location or the current on the surface of the chassis for coupling and non-coupling modes is approximately same, the isolation cannot be enhanced further. Using this approach, isolation was improved on an average by 11 dB in all the designs considered, giving the most isolation enhancement following a systematic way compared to other works.

A methodology to design an RFID tag antenna is presented. The tag is explicitly designed to account for changes in the dielectric constant of a thin substrate supporting the antenna. The theory of Characteristic Modes (CM), a modal-based approach, is used to study the effect of the dielectric variation on the dominant modes. The outcome of the CM analysis provides valuable guidelines to design a tag antenna that not only exhibits a large frequency bandwidth but also insensitivity to changes in the dielectric constant. Measurements and simulations show fairly good agreement. The proposed tag antenna is shown to have superior performance compared to commercially available antennas as well as antennas reported in the literature.

In this communication, the design procedure of a wideband antenna with omnidirectional radiation pattern is demonstrated based on the theory of characteristic modes. Consisting of a dipole and a loop antenna, the antenna has a very simple structure. A wide impedance bandwidth is obtained because of the simultaneous excitation of the antenna’s first two modes. Meanwhile, due to the fact that these two modes share a similar omnidirectional radiation pattern, a stable radiation pattern is also achieved across the operating frequency band. In order to identify the antenna’s different modes, a characteristic mode analysis of the antenna is carried out first. Then, a feed configuration is specifically designed to excite the desired modes. To validate the antenna design, a prototype was fabricated and tested. Measured results agree well with the simulated ones. Measurement shows that a wide impedance bandwidth of 44.2% with $\vert {\text{S}_{11}}\vert dB (1.85–2.9 GHz) and stable radiation patterns at both E-plane and H-plane were achieved over the operating frequency band.

In this paper, the rectangular microstrip antenna is analyzed by using the theory of characteristic modes (TCM). Characteristic modes of microstrip antennas with different shapes of ground are presented to provide physical understanding of the radiating behavior. The microstrip antennas operated at different modes are verified by both HFSS simulated and measured results. Resonance frequency, near field, eigencurrent, radiation pattern are analyzed and compared. The results show that some modes which similar to that obtained by cavity model could be carried out. The error of resonance frequencies of these modes predicted by TCM and cavity model is less than 2%. It is demonstrated that they are conventional TM modes.

In this article, an efficient and systematic method for fast and accurate computation of characteristic modes (CMs) and eigenvalues of the entire array antenna as a whole is proposed. This technique benefits from the symmetry and Toeplitz property of the method of moments (MOM) impedance matrix of the array structure. Using this technique, we are able to directly obtain CMs of a large-scale array structure with minimal computational resources. First, the self-impedance matrix of a radiating element and the mutual-impedance matrices between that element and the remaining ones are calculated numerically using MOM. Then, the theory of characteristic modes (TCM) is applied to the entire array. Using the proposed method, we decompose the coefficient matrix of the CM equation into smaller matrices. In addition, interesting and meaningful relations between currents flowing on elements associated with each CM of an array are derived and used to significantly reduce the computational complexity in the CM extraction. Moreover, numerical results and computation time are reported to validate the proposed method.

In this article, a novel design method is proposed based on characteristic mode theory (CMT). Based on the proposed method, a linearly polarised dual‐band wearable antenna with double‐layered substrates sandwiched by three layers of copper circle patches is designed and optimised step by step. The structure and excitation position of the antenna are obtained by progressively tailoring a classical circle patch antenna. Firstly, the characteristic mode analysis is performed on the middle‐layer planar circle patch to find the modes that can be excited in the designed frequency band. Secondly, the configuration of the middle layer is adjusted step by step based on the modal current distributions to realise the mode shift. Then, the top circle patch is changed to a metasurface to increase the number of modes and expand the mode bandwidth. Finally, the location of the excitation port is selected based on the modal electric fields to successfully obtain the desired performance of the antenna. The prototype of the proposed antenna with a radius of 2.6 cm fed by a single probe is fabricated and experimentally verified. The antenna operates in the 2.45 and 5 GHz bands, and the measured 10 dB impedance bandwidths on the two bands are 1.2% and 39.7%, respectively, which agrees well with the expectation predicted by the CMT. It should be noted that the desired antenna performance is achieved by step‐by‐step derivation using the CMT, rather than applying the theory to illustrate the working principle of antennas. The proposed design method can be broadly employed to various antenna designs.

A low specific absorption rate (SAR) and high on-body efficiency tri-band smartwatch antenna is designed utilizing the theory of characteristic modes (TCM) of composite perfect electric conductors and lossy dielectric structures. The antenna works in the GPS frequency band, WLAN 2.4 GHz/Bluetooth frequency band, and 5G frequency band. After analyzing the characteristic modes of the antenna and human body in the above three frequency bands, the characteristic modes with small modal coupling coefficients (MCCs) are selected and excited. Due to the small mutual coupling between the antenna and the human body, the proposed antenna has good performance on both SAR and on-body efficiency. Its measured 10 g average body SAR is lower than 1.3 W/kg. When it is placed 0 mm above the body, its on-body efficiency is 9.3%–9.88% in the GPS frequency band, 12%–14.2% in the WLAN 2.4 GHz/Bluetooth frequency band, and 17.3%–20.1% in the 5G frequency band.

This letter presents a pioneering magnetoelectric dipole (MED) resonator designed to enhance the directional sensitivity. Unlike traditional omnidirectional Rydberg atomic sensors (RASs) that struggle to selectively amplify signals from specific directions, this innovative resonator is directional and foregoes the need for a feed port, which is common in antenna designs. Instead, it boosts RAS sensitivity by intensifying local electric fields, thereby imparting the sensor with the directional properties of the MED and significantly reducing backlobe levels. Guided by the theory of characteristic mode, the resonator features an open cavity with a rectangular ring and two horizontal metal plates, creating magnetic and electric dipole resonator structures. It integrates a local oscillator port to support atomic heterodyne techniques, further improving sensitivity. The design impressively attains a peak electric field gain of 22 dB within a compact size while maintaining stable and directional patterns. These attributes render the resonator ideal for directional detection in RAS applications.

This paper presents a miniaturized Quad-Port MIMO antenna for UWB application. Employing Characteristic Mode Analysis, the single element UWB antenna is analysed to achieve a compact design. Using the Theory of Characteristic Mode, the significant modes and their current distribution are analysed, which helps to modify the radiator and ground plane to generate more significant modes to yield a wider frequency band. Four circular-shaped radiators with tapered feeds are placed orthogonally on the FR4 substrate to improve isolation, and partial ground planes are located on the opposite surface. Furthermore, a modified Pattee-cross-shaped parasitic element is introduced between the antenna elements and shorted with the ground planes via metallic shorting pins. This novel technique improves isolation compared to the conventional integration method of the partial ground planes through strip lines. The proposed antenna achieves less than −20 dB mutual coupling over most of the frequency band ranging from 2.51–18 GHz.

The theory of characteristic modes (TCM) provides a set of eigenvalues and eigenvectors that are determined entirely by a structure's geometry at a particular frequency. When examining a design over a range of frequencies, consistent interpretation of the modal data is crucial. Here, we present a way of interpreting crossing avoidances observed when tracking characteristic-mode eigenvalue traces over frequency. Using the framework of coupled-mode theory, a method for decomposing a pair of characteristic modes into auxiliary modes is developed. The auxiliary modes have a degeneracy (eigenvalue crossing) and maintain continuity of modal current distributions through the coupling region of the original characteristic modes. We demonstrate several examples using our method on simple objects. Furthermore, we explore the effect of multiplicative noise and lumped reactive loading on induced crossing avoidance.

A novel method is proposed for the synthesis of platform integrated antennas. The approach is based on the characteristic modes of the platform. Unlike traditional method that applying stochastic algorithms with full-wave simulators to find the optimal antenna placement, characteristic modes combined with the state of the art optimizer present physical insight into the currents induced on the platform and guide the engineer to quickly come up with the optimized antenna. The developed method for optimizing platform integrated antennas will find wide spread use in many applications. We demonstrate this method by a pattern reconfigurable aircraft integrated antenna design. Compared with traditional methods, the present method possesses advantages such as low computation cost, high radiation efficiency, easy to integrate antennas with platforms.

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.