Theory Of Characteristic Modes Research Articles

Circularly polarized edge‐placed printed monopole antenna using theory of characteristic modes

AbstractThis 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.

Design of Broadband Low-RCS Array Antennas Based on Characteristic Mode Cancellation

In this letter, a design method for low radar cross section (RCS) array antennas based on characteristic mode cancellation (CMC) is presented. Based on the characteristic mode theory (CMT), two novel microstrip elements are designed by introducing rectangular slots and cross slots, which produce 180° scattering phase difference by adjusting the size of slots. The dominant characteristic modes of the two elements achieve broadband dual-linear polarization CMC and similar radiation performances. The 4 × 4 array antenna consisting of these two antenna elements is designed. The operating band is from 4.55 GHz to 5.49 GHz (relative bandwidth 18.7%). The gain loss of the proposed array is about 0.1 dB compared to the reference array. The monostatic RCS is reduced for dual−linear polarized waves, and the 6 dB radar cross section reduction (RCSR) bandwidths are 62.3% and 35.7%, respectively. The prototype is fabricated and measured. The measured results of radiation pattern and RCS are in good agreement with the simulated results.

Characteristic-Mode-Analysis-Aided Design of Filtering Patch Antennas

A characteristic-mode-analysis (CMA)-aided design method for filtering patch antennas is proposed in this article. The method mainly includes three parts: first, use the theory of characteristic mode (TCM) to analyze the filtering antenna without feed structure (i.e., the radiator) to find the resonant modes in the passband and predict the positions of the inherent radiation nulls, which are then utilized to obtain the suitable size of the radiator and filtering elements; second, use modal analysis to find the best feed position to excite the resonant modes and simultaneously generate the inherent radiation nulls; and third, optimize the filtering antenna to obtain the optimal overall performance. For demonstration, two different types of filtering patch antennas are investigated using the proposed method. It has been shown that the method can truly realize the independent analysis and design of the radiator structure and the feed with the help of characteristic modal analysis, making the design process of both filtering antennas clearer and more efficient. In addition, the proposed method can find out and fully utilize the inherent radiation nulls, including the modal nulls and the nonmodal nulls of the radiator itself.

A flexible transparent absorber bandwidth expansion design based on characteristic modes

In this paper, a flexible transparent absorber with broadband absorption is presented theoretically and demonstrated experimentally, which is composed of five layers. Based on the characteristic mode theory, the absorption bandwidth and modal significance of a narrow band absorber were analyzed with the modal radiation patterns and surface current, respectively. To expand the bandwidth, the effective modes and the positions where the resistors should be loaded were determined with the help of characteristic mode analysis. It has a broadband absorption in the frequency range of 14.3–28.3 GHz with a relative absorption bandwidth of about 65.72 %, and the corresponding absorptance is more than 90 %. Meanwhile, the optimized absorber also achieves a satisfactory absorption rate up to 45° incident angle for both TE and TM polarizations. The experimental results show excellent absorption rates, which indicates a good agreement with the simulated results.

Design of a Transmission-Type Polarization-Insensitive and Angularly Stable Polarization Rotator by Using Characteristic Modes Theory

A novel design strategy for realizing a transmission-type polarization rotator of linearly polarized (LP) plane waves by exploiting the characteristic modes (CMs) theory is described. Design guidelines for the excitation of two current modes on a frequency selective surface (FSS), both exhibiting a circularly polarized (CP) radiated field, are provided to obtain the polarization rotation. The proposed converter exhibits remarkable performance also in the case of oblique incidence and a polarization-insensitive response thanks to the FSS unit cell compactness along with its fourfold rotational symmetry. Specifically, it provides a 3 dB cross-polar transmission percentage bandwidth up to 16.5% with a minimum insertion loss (IL) of 0.1 dB for a normally impinging plane wave whereas in case of an incidence angle of 60° the 3 dB cross-polar transmission percentage bandwidth turns out to be around 14% with a minimum IL of 0.7 dB. Measurements on a realized prototype are in good agreement with simulations, confirming the reliability of the proposed theoretical study.

Design of Low-SAR and High On-Body Efficiency Tri-Band Smartwatch Antenna Utilizing the Theory of Characteristic Modes of Composite PEC-Lossy Dielectric Structures

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.

Design of a circularly polarized printed open‐slot antenna using theory of characteristic modes

AbstractIn 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.

Design of Dual-Port Pattern-Reconfigurable MIMO Antenna With Four Operating States Using the TCM

Based on the theory of characteristic modes (TCM), this letter presents a novel pattern-reconfigurable dual-port multiple-input–multiple-output (MIMO) antenna with four operating states at the 2.4 GHz ISM band. The proposed antenna consists of the rectangular platform (RP), excitation units, and reconfigurable feeding network. Four elaborated characteristic modes (CMs) of the RP can be stimulated effectively by six groups of balanced inductive exciters. By using a reconfigurable feeding network controlled by four switches, two orthogonal CMs associated with the two ports are switched, and four possible operating states for the reconfigurable MIMO antenna are generated. In order to demonstrate the proposed TCM-based conceptual design, a prototype is fabricated and the measured results agree well with the simulated ones. Furthermore, the port isolations of the proposed MIMO antenna are higher than 36 dB.

DESIGN OF A COMPACT BROADBAND ANTENNA USING CHARACTERISTIC MODE ANALYSIS FOR MICROWAVE APPLICATIONS

A compact broadband antenna of dimensions 27 mm x 28 mm x 1.6 mm and with good impedance matching is designed for high-bandwidth radio systems with a short range. To improve the impedance matching, two rectangular slots are created on the radiating element, and the ground plane size is reduced to extend the ultra-wideband frequency band. The antenna bandwidth and radiation performance are analysed using characteristic mode theory (TCM). The performance is compared to the desired specifications, and the shape and size are modified to produce efficient radiation and dominant radiation patterns. The findings clearly demonstrate that the six modes are resonant with (λn = 0). This implies that the eigenvalues of the six modes contribute strongly to dominant electromagnetic radiation and have high modal significance values around 1 at their respective frequencies. Furthermore, the characteristic angle indicates that the antenna resonates at 180°, since the six modes intersect the axis line at 180° at their respective frequencies. Experimental results show a bandwidth of 109.7% between 5.64 and 19.34 GHz, a maximum gain of 6.3 dB, and a maximum efficiency of approximately 86.5%. These results make this antenna a versatile and effective choice for a wide variety of communications and electronics applications and easy to install in narrow spaces due to its easy design characteristics, small size, and light weight.

Research on Electromagnetic Scattering Characteristics of Transmission Tower With Different Tower Types in Short Wave Band Based on the Characteristic Mode Theory

Research on Electromagnetic Scattering Characteristics of Transmission Tower With Different Tower Types in Short Wave Band Based on the Characteristic Mode Theory

Investigation of Conformal MIMO Antenna for Implantable Devices Based on Theory of Characteristic Modes

To meet an implantable device for human disease detection, a conformal multiple-input–multiple-output (MIMO) antenna is investigated in this article. The layout and decoupling of the antenna are designed according to the inherent electromagnetic properties of the implanted device based on the theory of characteristic modes (TCM). Compared with traditional methods, the TCM provides a method and theory to design a conformal implantable antenna with MIMO properties. The designed antenna has the advantage of simple structure, low space occupation, and little multipath reflection. The newly designed structure adopts two planar helical radiators to achieve size miniaturization. Some I-type patches are located between two radiators and slotting structures on the ground plane to reduce coupling. The biocompatible flexible polyimide is used as a dielectric substrate, enabling the conformal property and compatibility with the human tissue. Measurements of the fabricated prototype have been finished in the minced pork and skin-mimicking gel. Results show very good agreement with the simulations, which validate the suitability of the proposed antenna for the implantable devices. To the best of our knowledge, this is the first time that the TCM has been applied to guiding the design of implantable MIMO antennas.

Evaluation of Scattering by Spacecrafts Using Characteristic Mode Theory Considering Ionosphere Faraday Effect

This article is concerned with developing a viable procedure to evaluate the scattering property of a satellite object. First, to deal with the uncertainty of attitudes of the object with respect to the direction of incidence, the characteristic mode theory is exploited, which is independent of the incident waves. The total scattered field is obtained by summing up the individual characteristic scattered fields with modal weighting coefficients. Second, the ionosphere Faraday effect that may significantly change the polarization behaviors is considered when the frequency is at L-band or below for a ground-based identifying radar. The ionosphere is taken as a sparse and weakly magnetized plasma, and the approximate Wentzel–Kramers–Brillouin method is adopted to calculate the propagations of two intrinsic modes through the ionosphere. The Stokes vector is assumed to be the measured quantities, and the Mueller matrix is employed to fully describe the polarized scattering process.

Thin Dielectric Sheet Approximation-Based Characteristic Mode Formulations for Lossless Dielectric Coated Conducting Bodies

The thin dielectric sheet (TDS) approximation-based theory of characteristic modes (TCM) is formulated for lossless thin dielectric-coated conducting bodies. The proposed formulations are derived from the TDS-electric, magnetic, and combined field integral operators (TDS-EFIO, TDS-MFIO, and TDS-CFIO). Similar to the cases of perfect electric conductor (PEC) and impedance surface, the TDS-CFIO-based formulation can avoid modes corruption by spurious solutions at the internal resonant frequencies of the PEC cavity filled with the material of the background. Due to the utilization of TDS approximation and PEC boundary conditions, the eigencurrents are expressed only by electric currents on the surface of PEC. Therefore, the proposed formulations are more efficient than the conventional formulations based on the surface integral operator (SIO) or volume-surface integral operator (VSIO). The coating thickness of TDS approximation is recommended to be less than one tenth of a wavelength in the dielectric for accurate analysis of characteristic mode (CM). Several numerical examples are given to validate the proposed formulations.

Unified Theory of Characteristic Modes—Part II: Tracking, Losses, and FEM Evaluation

This is the second component of a two-part article dealing with a unification of characteristic mode decomposition. This second part addresses modal tracking, interpolation, and the role of ohmic losses, and presents several numerical examples for surface-based method-of-moment (MoM) formulations. A new tracking algorithm based on the algebraic properties of the transition matrix is developed, achieving excellent precision and requiring a very low number of frequency samples when compared with procedures previously reported in the literature. The transition matrix is further used to show that characteristic mode decomposition of lossy objects fails to deliver orthogonal far-fields and to demonstrate how characteristic modes can be evaluated using the finite element method (FEM).

Unified Theory of Characteristic Modes—Part I: Fundamentals

A unification of characteristic mode decomposition for all method-of-moment (MoM) formulations of the field integral equations describing free-space scattering is derived. The work is based on an algebraic link between the impedance and transition matrices, the latter of which was used in early definitions of characteristic modes and is uniquely defined for all the scattering scenarios. This also makes it possible to extend the known application domain of characteristic mode decomposition to any other frequency-domain solver capable of generating the transition matrices, such as finite difference or finite element methods. The formulation of characteristic modes using a transition matrix allows for the decomposition of induced currents and scattered fields from arbitrarily shaped objects, providing high numerical dynamics and increased stability, removing the issue of spurious modes, and offering good control of convergence. This first part of a two-part article introduces the entire theory, extensively discusses its properties, and offers its basic numerical validation.

Design of Wideband Decoupling Antenna Pairs for 5G Portable Devices at N77/N78/N79 Bands.

We proposed an antenna pair that is applicable for mobile terminals: a type of coupling feed planar antenna that has a size of 28×6 mm2, a wide band, coupling suppression and compact size. Different from the conventional antenna design, the process of band expansion comes from a dual-band antenna pair and is based on characteristic modes theory (CMA). By observing characteristic current distributions, the optimization emphasis is placed on current sensitive parts in antenna structures, which is an effective method to create or modify resonant points for exciting band potential. Meanwhile, the multiple defective ground structure (DGS) is introduced for isolation enhancement. The -10 dB bandwidth of 3.23-5.24 GHz can be realized, and the isolation of two antenna pairs with only 2 mm spacing is lower (-17.34 dB). The eight-port MIMO system constructed by four proposed antenna pairs has been fabricated. The simulated and measured results of MIMO indicate that its -10 dB operating bandwidth can work at N77/N78/N79/WLAN 5 GHz bands. Moreover, a lower ECC of less than 0.04 and a high efficiency of more than 60% can be obtained, which confirms that it is capable of excellent data transmission as a terminal MIMO antenna system.

Efficiency Improvement of Smartphone Antennas Using Higher-Order Mode Suppression Under Characteristic Mode Analysis

Radiation efficiency is one of the key parameters for smartphone antennas. However, the actual mechanism governing radiation efficiency remains unclear, and a few works have been published. This work is an attempt to extend the theory of characteristic modes (TCMs) to the lossy smartphone antenna for gaining insight into what causes efficiency variation and realizing efficiency improvement. Compared with the classical TCM, the CM analysis (CMA) adopted in this work has the capacity to handle lossy smartphone antenna. By using such an analysis approach, we find, quite interestingly, that suppressing the higher-order modes is an effective optimization strategy to enhance efficiency. Then, an optimized antenna with considerable improvement in efficiency is proposed. Finally, to meet the requirement of practical application, the additional LC circuit and through holes are introduced into this proposed optimized antenna. GSM 900 of 890–960MHz can be covered by this finally proposed antenna, and the measured peak total efficiency is improved from 26% to 43%. Due to the attractive features of small heights (4 mm) and tiny clearances (0.25 mm), the proposed designs are promising for application in a smartphone with a full-view display and thin profile.

Characteristic Mode Inspired Single-Plate Unidirectional Antenna Using Complementary Characteristic Radiation

Based on characteristic mode theory (CMT), a novel single-plate complementary antenna with a unidirectional radiation pattern is proposed in this communication. The proposed antenna is composed of an isosceles right triangular conductive slab (IRTCS), three inductive coupling elements (ICEs), and a feeding network. By exciting two complementary current modes of IRTCS simultaneously, the radiation of IRTCS can be tailored. More specifically, ICEs can stimulate the electric dipole and magnetic dipole current modes of IRTCS concurrently through appropriate excitation, thereby realizing an endfire radiation pattern by combining the two complementary characteristic fields with suitable modal weighting coefficients (MWC). For proof of concept, a prototype is fabricated, and the measured results demonstrate that a good unidirectional radiation pattern is achieved with a front-to-back ratio (FBR) larger than 20 dB, which verifies the feasibility and reliability of the proposed method. Compared with the conventional complementary antennas or Huygens source antennas which need two different radiating sources to realize the electric and magnetic dipole radiations individually, the proposed method reuses the radiating aperture and achieves the unidirectional radiation by a single radiating aperture.

Design of a Broadband High-Gain Omnidirectional Antenna With Low Cross Polarization Based on Characteristic Mode Theory

A low-profile, high-gain, and low cross-polarization omnidirectional antenna is proposed. The antenna structure is pretty concise with convenient probe feeding and sandwich-structure design. The characteristic mode analysis shows that the antenna can excite quasi-TM01 and quasi-TM02 modes in a broadband and, thus, results in monopole radiation mode and low cross polarization in a broad operating band. The full-wave simulation and experiments were performed to verify the antenna design. The fabricated prototype antenna showed <2 dB unroundness and <−25 dB cross polarization in (4.8–11.1) GHz, with a relative bandwidth of 79.2% and a peak gain of 8.4 dBi at 9 GHz.

A Broadband Metasurface Antenna Array With Ultrawideband RCS Reduction

In this communication, a metasurface-based antenna array with wideband radiation and broadband radar cross section (RCS) reduction is constructed. Without introducing additional radiators, the broadband radiation performance of the antenna array is achieved by inspiring the metasurface itself. In addition, the characteristic mode theory is introduced to analyze the radiation mechanism of the radiator elements and the proposed antenna array. Both simulations and measurements are carried out to verify the performance of the proposed antenna array. The results show that the impedance bandwidth and 3 dB gain bandwidth of the proposed antenna array are 5.1–9.0 and 5.1–8.0 GHz, respectively, with a peak gain of 12.1 dBi. Monostatic RCS reduction more than 10 dB is achieved within the frequency band of 6.0–26.2 GHz.