Theory Of Characteristic Modes Research Articles

Broadband low‐profile monopole antenna design with coplanar waveguide , substrate integrated waveguide, and slots based on characteristic mode theory

This article presented a broadband low-profile monopole antenna that was equipped with coplanar waveguide (CPW), substrate integrated waveguide (SIW), and slots based on characteristic mode theory (CMT). Compared with the traditional antenna design method, the antenna design method based on CMT can design the antenna more effectively. A prototype of the proposed antenna with an overall size of 0.98 λ0 × 0.98 λ0 × 0.027 λ0 (λ0 is the free-space wavelength at the X-band of 8.2 GHz center frequency) was fabricated and measured. The measurement results showed that the prototype had a impedance bandwidth of 3.2 GHz (6.8‑10 GHz), a relative bandwidth of 38.1%, the maximum gain of 4.1 dBi at 9.5 GHz, and the maximum radiation efficiency of 92%. Besides, it was found that the prototype almost achieved symmetrical radiation in E-plane and omnidirectional radiation in H-plane. Its operating frequency band covered that of broadcast band (7.1–7.3 GHz), the water mobile band (8.1–8.815 GHz), and the aviation mobile band (8.815 ‑9.04 GHz).

Analysis of Archimedean Spiral Slot Antennas in Infinite Ground Planes Using the Theory of Characteristic Modes

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.

Characteristic Mode-Based Efficient Broadband Electromagnetic Scattering Analysis Method for Array Structures

A fast and accurate numerical method based on the theory of characteristic mode (TCM) is proposed for analyzing electromagnetic scattering from repetitive multiscale array structures. In order to overcome the low frequency breakdown problem when interacting at short distances between observation and source basis functions, the CMs are extracted from the impedance matrix generated by the augmented electric field integral equation (AEFIE), which is discretized by the method of moments (MoM). Then the CMs of a single unit are used as the global basis functions to reduce the unknowns of the array structures. In addition, the broadband multilevel fast multipole algorithm (MLFMA) based on approximate diagonalization of the Green’s function is implemented to accelerate the matrix-vector multiplications between the impedance matrix and CM vectors. Several numerical examples are given to demonstrate the high efficiency and accuracy of the proposed scheme.

A Reconfigurable Metal Rim Antenna With Smallest Clearance for Smartphone Applications

In this paper, a reconfigurable metal rim antenna with a small clearance and compact structure for 4G/5G smartphones is proposed. It consists of the system ground plane and a metal rim with two slits, in which a U-shaped slot of 1 mm (the smallest) clearance is realized between the system ground and metal rim. The theory of characteristic modes is applied to analyze the eigenmodes of the antenna so as to aid the optimum design. The characteristic modes (CMs) in the band of interest are studied, and an inductive coupling exciter is identified as the best since three CMs can therefore be generated in the lower frequency band (0.69-0.96 GHz), and an SP4T switch can be utilized to increase the bandwidth of the whole lower band. A prototype antenna of the optimized design was fabricated. A good agreement was obtained between the measured and simulated results. The 6-dB frequency bands are the lower band (0.69-0.96 GHz), the middle band (1.7- 2.7 GHz), and the higher 5G band (3.4- 3.6 GHz), respectively, and the total efficiencies are over 50% in all these frequency bands. They indicate that the antenna is a very good candidate for smartphone applications.

Characteristic Modes Analysis of a Near-Field Polarization-Conversion Metasurface for the Design of a Wideband Circularly Polarized X-Band Antenna

A metasurface (MS) based on loop elements operating in the near field of a linearly-polarized microstrip antenna is employed to realize a circularly polarized radiated field. The properties of the loop unit cell are highlighted with the help of the Characteristic Mode Analysis that is employed for investigating the achievable linear to circular polarization conversion bandwidth and providing the guidelines for the design of the final antenna. A finite structure comprising 4×4 unit cells is tailored for achieving a circularly polarized far field within the whole X-band adopted for satellite communications (7.25 GHz-7.75 GHz, 7.9 GHz-8.4 GHz). A simple but effective single-port excitation scheme is adopted, and the overall performance are assessed by measurements on the fabricated prototype. The good agreement between simulated and measured results confirms the reliability of the proposed approach as well as the meaningful insight provided by Characteristic Modes Theory.

Near-Omnidirectional Broadband Metamaterial Absorber for TM-Polarized Wave Based on Radiation Pattern Synthesis

The satisfactory realization of a metamaterial (MM) absorber simultaneously occupying low-profile, wide-bandwidth, particularly and fully angular stability characteristics is a long-term challenge. In this article, we propose a new design method to accomplish above realization based on the radiation pattern synthesis. In the design, a mushroom-type MM absorber is simply covered with different resistive sheets, which empower it with the admirable low-profile, wide-bandwidth, and near-omnidirectional absorption for the TM-polarized wave. In order to guide our design, the characteristic mode theory is first employed to synthesize the omnidirectional radiation pattern and the dissipation tailoring is then utilized to achieve the impedance matching and near-omnidirectional absorption conversion. A proof-of-concept sample is fabricated and measured for verification. At quasi-normal incidence, the measured bandwidth characterized by absorption more than 90% covers 1.5–4.5 GHz with fractional bandwidth (FBW) 100% and the thickness about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.11\lambda _{0}$ </tex-math></inline-formula> at 1.5 GHz. In the frequency band of 1.9–4.2 GHz, the simulation results indicate that the absorption for TM and TE polarizations of approximately 90% can be, respectively, up to incident angles of 82° and 45°, and its effectiveness within 10°–70° is verified with measurements.

Application of Theory of Characteristics Modes for Bandwidth Enhancement of a Miniaturized Minkowski Fractal Antenna

This article presents a miniaturized modified Minkowski fractal antenna with enhanced linearly polarized impedance bandwidth (IBW) in the lower frequency region. Applying theory of characteristic modes (TCM) analysis on a second iterated modified Minkowski radiator without feeding structure, modal currents and their corresponding modal fields (radiation patterns) were determined to recognize symmetric dominant modes to be specifically excited to generate the required radiation pattern. It gave direction for choosing quarter-wave microwave line feeding as the feed of choice to excite the four desired modes at 2.7, 5.7, 5.9, and 8.7 GHz and providing excellent impedance matching to generate the very wide IBW. Measured results matched well with simulations done using Ansys HFSS 2020R1. Measured IBW is 9.7 GHz (160%), from 1.2 to 10.9 GHz, with center resonance frequency (frc) at 6.05 GHz, whereas simulated IBW is 9.4 GHz (162%), from 1.1 to 10.5 GHz with frc at 5.8 GHz. The antenna size is 36 × 34 × 1.6 mm3 (0.216λgL × 0.204λgL where at lower resonating frequency frL = 1.1 GHz, λgL is the guided wavelength) with 82.24% reduction in size. This antenna may be utilized for “L”, “S” band and UWB wireless communication.

Wideband metasurface absorber (metabsorber) using characteristic mode analysis.

Designing an ultra-wideband metasurface absorber is challenging because of its lossy characteristic that hinders the understanding of its resonance behavior. In this study, a framework was formulated to extend the application of the theory of characteristic modes to the analysis and design of metabsorbers. The metabsorber and its lossless counterpart exhibited similar modal behaviors, hence revealing the absorption mechanism of metabsorber. By introducing absorption modes, a dual-band metabsorber was converted into an ultra-wideband metabsorber. This proposed metabsorber with a thickness of 1.99 times the Rozanov's limit could measure a bandwidth of 5.51-36.56 GHz (or 6 octaves) with 90% absorptance.

Low-profile UWB antenna with unidirectional radiation pattern analyzed with the theory of characteristic modes

This paper presents a low-profile Ultra Wide Band (UWB) antenna with polarization diversity for new 5G base stations. The antenna consists of a metallic ring capacitively fed with four circular monopoles and reinforced with a cylindrical cavity to generate a unidirectional radiation pattern. The Theory of Characteristic Modes is used to analyze and explain the behaviour of the proposed antenna excited with differential feeding configurations. A prototype of the optimized design has been manufactured and measured, yielding good impedance matching (S11<-10 dB) from 3 GHz to 6 GHz with maximum directivity of 10.51 dBi. Measured and simulated results are in good agreement.

Characteristic Mode Formulation for Antennas With Waveguide Port Feeding Structures

In conventional characteristic mode (CM) theory, modal weighted coefficient (MWC) is often adopted to measure the interaction of CMs with external excitations. However, the MWC is only available in simple source models like plane wave incidence and the delta-gap source. In this letter, a CMs formulation is proposed for analyzing the radiation or scattering properties of antennas with practical waveguide port (WP) feedings. Owing to the energy-based derivation, the resultant CMs have a clear physical interpretation and forms an orthogonal set of current basis and far-field basis which can be used for modal decomposition. The following two benefits are obtained with the proposed method: first, it considers the influence of the feeding structures and can measuring the mode excitation level due to the more practical WP excitations; and second, the CMs solved from radiation and scattering problem are consistent, which provide the possibility to optimize the radiation and scattering performance of the antennas with the same set of CMs. Numerical examples are presented to illustrate the accuracy and effectiveness of the proposed CMs method in analysis of antennas with practical WPs.

Radiation Pattern Reconfigurable Antenna for IoT Devices

Based on the characteristic mode theory, a versatile radiation pattern reconfigurable antenna is proposed. The analysis starts from two parallel metallic plates with the same and different dimensions. By means of two PIN diodes, the size of one of the parallel metallic plates can be modified and consequently the behavior of the radiation pattern can be switched between bidirectional and unidirectional radiation patterns. Moreover, a SPDT switch is used to adjust the frequency and match the input impedance. The reconfigurable antenna prototype has been assembled and tested, and a good agreement between simulated and measured results is obtained at 2.5 GHz band which fits the IoT applications.

Design and experimental validation of a low-profile wideband metamaterial absorber by characteristic modes analysis

A wideband microwave metamaterial absorber (MMA) with a relative absorption bandwidth of 127.3% and the actual thickness of only 0.084λL at the lowest operating frequency is presented in this paper. Compared with other MMAs, the characteristic modes theory is proposed to guide our design and shows an excellent performance with wider bandwidth, lower profile, and fewer modes. Characteristic modes analysis (CMA) shows a wideband modal significance (MS) over 0.707 (MS > 0.707) by exciting one mode from 1.38 GHz to 6.44 GHz, suggesting that the MMA achieves a wideband absorption performance by exciting only one mode. According to the surface current distributions, the natural modes’ current under the normal direction has the same direction with modal current shows that the mode has been successfully excited. The input impedance also illustrates a wideband match characteristic with free space. A prototype has been designed, fabricated, and measured. Measurements and simulation results show a wideband absorption with a high absorption rate of over 90% range from 1.44 to 6.32 GHz. Our design demonstrates that CMA is an effective method to design absorbers.

Wideband Decoupled 8-Element MIMO Antenna for 5G Mobile Terminal Applications

A wideband decoupled dual-antenna pair is proposed and designed for 5G mobile terminal applications, with the help of characteristic mode theory. Parasitic strip and defective ground structure are used simultaneously to realize broadband and high isolation. Based on the proposed decoupled antenna pair, an eight-element multiple-input--multiple-output (MIMO) antenna is presented by placing four pairs at both side frames. By etching slits on the system ground, the mutual coupling between antenna pairs is effectively reduced. A prototype of the proposed antenna array is successfully fabricated and measured. Both simulated and experimental results indicate that the proposed eight-element MIMO antenna can completely cover n77/n78/n79 bands of the 5G New Radio and wireless local area network 5 GHz band. Furthermore, an isolation better than 15 dB is achieved between any two ports.

A pattern‐reconfigurable antenna design for body area networks communications

This article presents a novel pattern-reconfigurable antenna based on theory of characteristic mode for body area networks (BAN) communication. The radiation pattern of the antenna is able to switch between the omnidirectional pattern and broadside pattern, which can satisfy on-body and off-body communication links simultaneously. An array of 4 × 2 rectangular plates with the off-body communication mode at BAN frequency band is used as the main radiator, and four copper posts between the radiator and the ground are introduced to generate the on-body mode. A simple feeding network is designed to selectively excite the two modes. The footprint of the proposed antenna is 0.43 λ0 × 0.43 λ0 × 0.05 λ0. For demonstration, the proposed antenna has been fabricated and measured both in the free space and on the phantom. The measured results agree well with the simulated ones. Furthermore, the issue of the specific absorption rate (SAR) is studied. The peak SAR values of the proposed antenna in two communication modes are far below the Federal Communication Commission standard. In addition, for practical application, the link budget of the on-body link is investigated. The stable impedance and radiation properties, the low SAR values and the reliable communication ability make the proposed antenna be a suitable candidate for BAN communications.

Ground radiation antenna utilizing characteristic mode theory

This article presents two ground radiation antennas with simple feeding structures for mobile applications. Characteristic mode theory is exploited to design the proposed antennas. For antenna 1, a small loop without any lumped elements is used as the feeding structure. And the total length of the feeding loop is only 20.2 mm (0.06 λ0.9GHz). The operating band of the proposed antenna 1 is 834-1020 MHz. Across the whole operating band, the measured efficiencies of antenna 1 are higher than 72%. In addition, a new method to generate an extra resonant frequency is proposed, which has little effect on the existed resonant modes at the lower band. The dual-mode antenna 2 without loading any lumped elements is obtained by implementing this method, which covers 812-940 MHz and 1543-1576 MHz bands. Furthermore, the new resonant frequency can be controlled independently. Namely, the higher frequency band of antenna 2 can be adjusted freely without changing the lower band. To validate the design strategy, two ground radiation antennas are fabricated and measured. The measured results indicate that the proposed antennas are suitable for mobile terminals.

Bandwidth enhancement of low-profile slot antennas using theory of characteristic modes

In this article, we have presented a novel design of wideband slot antennas using theory of characteristic modes and theory of mode coupling. First, the narrow slot antenna, etched in a finite ground, is investigated with combined characteristic modes. While most of resonant slot antennas show narrowband operation, the modification of modal properties using structural manipulations is studied in detail and this analysis helps in the design of low-profile wideband slot antennas. Characteristic mode analysis explains the physical insight of resonance and radiation mechanism of proposed wideband slot antennas. Bandwidth enhancement techniques of these slot antennas are presented and validated with measured results of wideband slot antennas. This approach gives a large fractional bandwidth of >35% and high gain of >3 dB.

Analytical study of Fano phenomenon in plasmonic hexamer and heptamer using characteristic mode theory

In this paper, the optical properties of several Plasmonic structures including single nanodisk, Hexamer and Heptamer, using the Characteristic Mode Theory (CMT) are investigated. Single nanodisk and Hexamer have shown a bright mode with Lorentzian response. In Heptamer, a strong interaction is created between the nanoparticles and thus the Fano resonance occurs with a bright and a dark mode. Based on the CMT analysis and the definition of the Fano response, it has been observed that the collision of the modes in the modal significance diagrams is where the Fano response occurs. Due to the characteristic current distributions, bright and dark modes have been observed that are necessary to create a Fano response. So, using CMT, it can be determined whether the structure is capable of generating a Fano response or not. Also, the frequency of bright and dark modes and the Fano minimum can be obtained using CMT. Moreover, according to the characteristic currents, the structure can be modified to achieve a sharper Fano response. We have selected such structures to investigate this method in these types of Heptamers and it has been shown that the proposed method is applicable for such structures.

Analysis of double‐Xi‐shaped millimetre‐wave patch antenna backed by a high‐order‐mode cavity using characteristic mode design

The authors propose an innovative double-Xi-shaped Chinese character artistic antenna backed by a high-order-mode cavity for 5G millimetre-wave (mm-wave) applications. The initial double-Xi-shaped microstrip antenna element is designed by using characteristic mode theory, and stable gain can be obtained across the whole 5G mm-wave frequency band. By employing a slotted cavity with TE230 high-order-mode to excite 2×2 antenna subarray, a high gain of 13.2 ± 0.8 dBi and a high isolation better than 45 dB without additional feeding networks can be simultaneously achieved. To further improve the gain for reliable transmission, a 4×4 dual linearly polarised (LP) antenna array is also designed and fabricated. Measured results show that a wide frequency bandwidth of 11.45% (24.7–27.7 GHz) with a peak gain of 19.2 dBi and a high isolation of 33 dB is realised for the dual LP antenna array. The proposed method provides a simple and efficient prediction-theory way to design artistic antenna for the first time, which solves the problem of designing specific-shaped antenna without theoretical support.

A broadband metamaterial absorber design using characteristic modes analysis

In this paper, a broadband metamaterial absorber with a fractional bandwidth of 126.88% was presented. The characteristic mode theory was used to guide the design of the absorber. According to the analysis of characteristic mode and characteristic current, the resistance value of resistive films can be determined. The different modal information obtained through parameter changes can also better guide the design of the absorber. To study its operation mechanism, the equivalent impedance and surface current distribution of the proposed absorber have been analyzed. The final simulation and measurement results show that the proposed absorber has a wide absorbing bandwidth which is from 3.21 to 14.35 GHz, and the absorptivity is greater than 90%, covering the S, C, X, and Ku bands. In addition, for TE and TM polarization, it can achieve an absorptivity of more than 85% at 45° oblique incident and has good angular stability. Hence, the absorber has great potential applications in the field of electromagnetic stealth technology and Radar Cross Section reduction.

Oriented beamformig of a multi-mode antenna based on characteristic mode analysis

In this paper, a method is presented for antenna beamforming based on characteristic mode theory. A multi-mode antenna with two elements and four significant orthogonal modes is presented in 5 GHz. Feed positions of the proposed antenna to excite its four characteristic modes are determined; Then with the help of characteristic mode analysis and the reciprocity theorem, amplitude and phase of the excitation coefficients of the proposed antenna are found to change the antenna beam direction in whole azimuth plane. Moreover, changing the weights of the antenna feeds enables elevation beamforming of the proposed antenna.