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A unidirectional planar electrically small antenna shape synthesis method is proposed through the use of characteristic mode concepts. Antenna feed does not have to be specified beforehand, and can be easily selected on the synthesized conductor suggested by the eigencurrents distribution. The afforded freedom leads to potentially successful high-directivity electrically small antenna designs. By properly choosing the starting shape and size, the synthesized antenna can well approach the maximum achievable directivity, the associated gain, and Q-factor predicted by the circular plate of the same size. [ABSTRACT FROM AUTHOR]

An advanced characteristic mode tracking method is investigated in this article. The tracking method is based on the eigenvector correlation, and a novel tracking error correction algorithm (TECA) is proposed to solve the challenging issues caused by the mode swapping and the mode degeneracy. As the core of the modal tracking method, the TECA can quickly find out the cross-mode pairs according to the eigenvalue differences among different modes, identify the wrongly mapped modes using the eigenvector correlation, and finally correct all the possible tracking errors. The detailed process of each step is described and discussed. In addition, the characteristic modes of two classical fractal antenna structures are investigated for validation. It has been shown that the modes including the swapped modes and degenerated modes are all tracked correctly, verifying the modal tracking method is reliable and efficient. Compared with the previous approaches, the proposed method is not subject to the number of tracked modes and the magnitude of eigenvalue. Also, it requires less computational time. Moreover, the sensitivity to the sizes of frequency step and mesh is reduced to some extent. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The theory of characteristic modes (TCM) based on the combined source integral operator (CSIO) is developed for perfect electric conductors and for impedance boundary condition. This formulation is similar to the one based on the combined field integral operator (CFIO), circumvents the interior resonance problem appearing in the electric field integral operator-based TCM. The eigenvalues obtained by the CSIO-based formulation agree well with the analytical and numerical CFIO-based solutions. However, the eigenvectors of the resonating modes computed with the CSIO-based approach are found to be corrupted at the internal resonance frequencies. [ABSTRACT FROM AUTHOR]

The characteristic mode theory is a promising tool for extracting the physical properties of scattering and radiating structures. The accuracy of these extracted data is mainly related to the tracking process. Several efficient tracking algorithms exist, nevertheless some shortcomings are reported in the literature. This paper treats two of the most challenging issues in modal tracking, modes swapping, and degenerated modes. The adopted approach was applied to two planar antennas, and the obtained results are discussed. [ABSTRACT FROM AUTHOR]

An interpolation algorithm of the eigenvalues and eigencurrents (eigenpairs) is proposed in this study to accelerate the broadband evaluation of characteristic modes. A bowtie antenna, an aircraft and a spherical shell are selected for numerical verification. To improve the stability and accuracy of the interpolation algorithm, a sampling scheme integrating non‐uniform frequency step (NUFS) into adaptive mode tracking is adopted. For comparison, the same problems are computed also by the method of moments (MoM) and a uniform frequency step (UFS) interpolation scheme. It is found that the interpolated eigenvalues and eigenpatterns by the NUFS scheme are very close to those by MoM while the ones by the UFS scheme yield larger deviations. The peak memory and computation time have been also studied. Compared with the computation by MoM, the memory consumed by the interpolation with NUFS is less and up to 80% of the computational time can be saved. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

This paper studies the design tradeoffs and modal excitation of closely spaced finite-sized ground plane backed phased arrays in the framework of characteristic mode theory, and proposes a method to suppress the higher order modes using a metasurface. The characteristic mode analysis also reveals the dominant modes and the excited modes for wideband operation. From the generalized method of moments impedance matrix and N-port network impedance matrix, key parameters are derived for the analysis, such as modal currents, modal radiation patterns, modal significance, modal weighting, modal radiated power, and modal near fields. The analysis includes the broadside and beamsteering radiation of closely spaced bowtie array antennas. The simulated and measured results show that it is essential for the beamsteering to have multiple modes excited simultaneously. By suppressing the higher order modes using the metasurface, the aperture efficiency and the realized gain can be improved for beamsteering cases. This analysis can be extended to a larger array and used to identify array impedance bandwidth performance in scan cases. [ABSTRACT FROM AUTHOR]

Five types of surface integral equation (IE) formulations are presented to determine the characteristic modes of dielectric and magnetic bodies. It should be noted that these five formulations are derived in a unified manner. Instead of directly using the IE coefficient matrices of scattering problems, this paper presents the formulations in a more natural and rigorous way. The IE formulations result in the generalized real symmetric eigenvalue equations for the characteristic modes. It is found that the eigenvalues of the first four formulations indicate the negative or positive imaginary part of the complex power of the characteristic modes, which are zero at resonance. The first two IE formulations result in two new generalized eigenvalue equations, while the last three IE formulations lead to the same generalized eigenvalue equations as in the references. Moreover, the first two formulations can be immune from spurious modes from which the third and fourth formulations suffer. [ABSTRACT FROM PUBLISHER]

We propose a method for modeling planar multiport antennas of arbitrary shape using characteristic mode theory (CMT) without physically including the feeds. The characteristic modes of the feed-free structure are expanded to form a basis for the eigenfields, and a virtual probe is introduced to excite the antenna. We develop a broadband multiport circuit model for the antenna impedance based on the excitation of each mode, where the feed locations only affect transformer ratios in the model, enabling design and analysis of arbitrary feed combinations over a wide frequency range. Because a CMT expansion can be computed for any planar geometry, the shape of the radiating element can also be arbitrary. While this approach is approximate, several examples are presented to demonstrate that its accuracy and flexibility make it suitable for various planar antenna design applications. With the rapid evaluation of input impedance at multiple excitation points, input parameters, such as the multiport S, Y, or Z parameters, can be plotted as a heat map on the antenna structure, facilitating planar multiport antenna optimization and feed selection. [ABSTRACT FROM AUTHOR]

The behavior of a mobile handset chassis-mode antenna with a metal bezel is analyzed in this paper by utilizing the theory of characteristic modes (TCM). Two eigenmodes, one from the chassis (\text0.5\uplambda\-\textdipole mode) and the other one from the bezel (\text1\uplambda\-\textperimeter mode), are merged by a monopole exciter to broaden the antenna bandwidth below 1 GHz. A new eigenmode from the bezel (\text1.5\uplambda\-\textperimeter mode) is also exploited to achieve multiple-input multiple-output (MIMO) performance. The characteristics of the antenna are verified with an experimental prototype and measurements. The measured input reflection bandwidths from the two ports are 263 MHz (742–1005 MHz) and 93 MHz (863–956 MHz), respectively. The port isolation is more than 7.4 dB across the whole band of interest. Finally, the monopole exciter, which excites two eigenmodes in the low-frequency band, is modified to provide more resonances in the upper frequency band, providing coverage in the range of 1715–2060 MHz and 2280–2710 MHz. [ABSTRACT FROM AUTHOR]

An antenna shape synthesis method is proposed that allows shaping of the antenna geometry prior to specification of the feed location and type. This reduces the constraints placed on the optimization process and can lead to potentially new designs due to the increased degree of freedom afforded. An appropriate feedpoint is easily chosen after shape optimization by selecting a location on the resulting structure for best impedance matching. The procedure is made possible through the use of characteristic mode concepts. Examples show that the antenna-Q values of the resulting shaped radiators closely approach the fundamental bounds. [ABSTRACT FROM PUBLISHER]

Reconfigurable multiple-input multiple-output (MIMO) antennas have the potential to improve the performance of a MIMO antenna system operating in a dynamic environment. This paper presents a reconfigurable MIMO antenna that utilizes two tunable parasitic elements that manipulate the characteristic modes of the structure to provide pattern diversity. Simulation studies and calculation of envelope correlation coefficients from this two-port antenna confirm that the radiation patterns are orthogonal to each other in each of the two states of the antenna, as well as between the two states. Measured two-port S-parameters of a prototype employing switched PIN diode loading correspond well with simulations. Radiation pattern measurements in the two principal planes indicate overall correlation with simulations. [ABSTRACT FROM PUBLISHER]

This paper presents a novel concept of antenna design for an electrically small unmanned aerial vehicle (UAV) using the theory of characteristic modes. With the knowledge of the characteristic modes of the UAV body, a multi-objective evolutionary algorithm is implemented to synthesize currents on the UAV body for achieving desired power patterns. Compact and low-profile feed structures are then designed to excite the synthesized currents. The feed structures are termed as probes, as they are used to excite the currents on the UAV body and cannot work as standalone antennas. The UAV body excited by these probes serves as the radiating aperture. Its reconfigurable radiation patterns are obtained by feeding each probe with proper magnitude and phase. In this method, the aperture of the UAV body is fully utilized. Practical issues associated with large antennas at low frequency band are eliminated. Knowledge for probe placement on such platform becomes more explicit. Simulated and measured results are presented to verify the design concept. [ABSTRACT FROM PUBLISHER]

A method is proposed by which a large number of characteristic modes of an arbitrary structure are tracked over a very wide frequency range; it is necessary for any serious wideband modal analysis of complex structures. The method can track a large number of modes using the modal eigenvectors from a generalized eigenvalue problem parameterized by frequency and has been successfully applied to over one hundred antenna geometries. The method is detailed and applied to two distinct geometries and the results discussed. [ABSTRACT FROM PUBLISHER]

We describe the physical phenomena that contribute to the behavior of an electrically small TM10 antenna using characteristic mode theory. The application of characteristic modes to antenna tuning and bandwidth enhancement serves as demonstration of the broad utility of the modal technique. A modal analysis of the TM10 antenna's impedance match yields several interesting observations as to the nature of resonances and antiresonances, which has implications for the impedance matching of small antennas in general. Furthermore, to overcome the bandwidth limitations inherent in small antennas, we determine that multiple resonances must be combined and use a conductance ratio as a figure of merit for design. We then investigate the TM10 antenna's potential for multiresonant operation by examining different candidate modes. Using the appropriate characteristic modes to form multiple resonances, we show how the bandwidth of the TM10 antenna can be designed to be nearly double that expected from the physical limit for a single resonance. [ABSTRACT FROM AUTHOR]