This article provides a comprehensive review of recent applications of characteristic mode analysis (CMA) to innovative antenna element designs, including multiport, circularly polarized, wideband, reconfigurable, and dielectric resonator antennas (DRAs). Emphasis is placed on the interpretation of the characteristic modes (CMs) for those unfamiliar with the method and physical insights gained from the characteristic eigenvalues and eigenvectors of an antenna. In addition, we review CMA-based design strategies and specific design examples that highlight the application of CMA to various types of antennas. Ultimately, this article seeks to demonstrate the value of CMA-based design insights for antenna engineering and look toward promising new research directions for CMA and antenna research.

Antenna Element Design Using Characteristic Mode Analysis: Insights and research directions

In this letter, a wideband circular polarization (CP) metasurface (MS) antenna is proposed, which is composed of a centrosymmetric structure with a mode suppressor and an asymmetric aperture-coupled feed structure separated by dielectric substrates. Based on the characteristic mode theory, the aperture-coupled feed structure consisted of an L-shaped slot and a microstrip line is employed to excite two orthogonal modes, which have a 90° phase difference to obtain CP radiation. Meanwhile, two methods have been proposed to expand CP bandwidth: one is the direct suppression of characteristic mode by decreasing the overlap frequency of operating and nonoperating mode. The other one is indirect suppression using a mode suppressor. Finally, the MS antenna with a low profile of λ 0 × λ 0 × 0.075 λ 0 at 5 GHz is fabricated and measured. The measured results show that the proposed antenna has a 3 dB axial ratio bandwidth of 45.2% from 4.1 to 6.5 GHz and an impedance bandwidth of 41.8% from 4.1 to 6.31 GHz, and a peak gain of 7.94 dBic at 5.9 GHz.

Low-Profile Wideband Circular Polarization Metasurface Antenna With Characteristic Mode Analysis and Mode Suppression

Entire-structure-oriented work-energy theorem (WET) framework is used to build the characteristic mode theory (CMT) for material scatterers. Orthogonalizing driving power operator (DPO) method is proposed to construct characteristic modes (CMs). Solution domain compression (SDC) scheme is introduced to suppress spurious modes. Electromagnetic energy characteristic equation similar to the energy eigen-equation in quantum mechanics is derived for unifying the modal analysis theories of classical electrodynamics and quantum mechanics. Employing the concept of driving power in WET framework, the physical meaning of characteristic values is revealed; the reason leading to the non-orthogonal characteristic far fields of lossy scatterers is provided; it is explained why the conventional CMT fails to analyze some classical transmitting antennas; the physical interpretation for normalizing modal real power to 1 is given; the Parseval’s identity in CMT is derived; the classical concept of quality factor (Q-factor) is generalized to a novel concept of field-current phase-mismatching factor (Θ-factor). Employing the orthogonalizing DPO method with SDC scheme, the spurious modes outputted from the conventional orthogonalizing impedance matrix operator method are suppressed. Both the novel SDC and conventional dependent variable elimination schemes confirm the same conclusion that the spurious modes originate from overlooking the dependence relationships among the currents in DPO.

Entire-Structure-Oriented Work-Energy Theorem Based Characteristic Mode Theory for Material Scattering Objects

Entire-structure-oriented work-energy theorem (WET) framework is used to establish the characteristic mode theory (CMT) for material scatterers. Diagonalizing driving power operator (DPO) method is proposed to calculate characteristic modes (CMs). Solution domain compression (SDC) scheme is introduced to suppress spurious modes. An electromagnetic energy characteristic equation similar to the energy eigen-equation in quantum mechanics is derived for unifying the modal analysis theories of classical electrodynamics and quantum mechanics. Using the concept of driving power in the WET framework, the physical meaning of characteristic values is revealed; the reason leading to the nonorthogonal characteristic far fields of lossy scatterers is provided; it is explained why the conventional CMT fails to analyze some classical transmitting antennas; the physical interpretation for normalizing modal real power to 1 is given; the Parseval’s identity in CMT is derived; the classical concept of quality factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> -factor) is generalized to a novel concept of field-current phase-mismatching factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Theta $ </tex-math></inline-formula> -factor). Using the diagonalizing DPO method with the SDC scheme, the spurious modes outputted from the conventional diagonalizing impedance matrix operator method are suppressed. Both the novel SDC and conventional dependent variable elimination schemes confirm the same conclusion that the spurious modes originate from overlooking the dependence relationships among the currents in CM-generating operator.

Entire-Structure-Oriented Work-Energy Theorem (ES-WET)-Based Characteristic Mode Theory for Material Scattering Objects

In this letter, a wideband circular polarization (CP) metasurface (MS) antenna is proposed, which is composed of a centrosymmetric structure with a mode suppressor and an asymmetric aperture-coupled feed structure separated by dielectric substrates. Based on the characteristic mode theory, the aperture-coupled feed structure consisted of an L-shaped slot and a microstrip line is employed to excite two orthogonal modes, which have a 90° phase difference to obtain CP radiation. Meanwhile, two methods have been proposed to expand CP bandwidth: one is the direct suppression of characteristic mode by decreasing the overlap frequency of operating and nonoperating mode. The other one is indirect suppression using a mode suppressor. Finally, the MS antenna with a low profile of λ0 × λ0 × 0.075 λ0 at 5 GHz is fabricated and measured. The measured results show that the proposed antenna has a 3 dB axial ratio bandwidth of 45.2% from 4.1 to 6.5 GHz and an impedance bandwidth of 41.8% from 4.1 to 6.31 GHz, and a peak gain of 7.94 dBic at 5.9 GHz.

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 $0.07{\lambda _0}$ ( ${\lambda _0}$ 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.

A Low-Profile Broadband Circularly Polarized Patch Antenna Based on Characteristic Mode Analysis

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A Low-Profile Broadband Circularly Polarized Patch Antenna Based on Characteristic Mode Analysis