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In this paper, a new approach to analyzing graphene-based antennas in the terahertz frequency range using the characteristic mode analysis method is presented. By the proposed method, graphene antenna parameters are obtained with simpler analysis and better physical visibility compared to numerical methods. In this regard, two structures of a graphene-based rectangular microstrip patch antenna for resonance in the terahertz frequency range are considered. The first structure consists of a metal ground plane and a substrate on which a layer of graphene is placed instead of a 10 μm × 7 μm metal patch. The second structure includes a metal ground plane with dimensions 19 μm × 16 μm, a substrate, a graphene layer on the substrate and a 10 μm × 7 μm metal patch placed on a graphene layer. Characteristic mode analysis method has been applied to the mentioned structure with different substrates, and the effect of graphene chemical potential on the modal behavior of the structure has been analyzed. For the first structure, there are two modes in the frequency range of 1–3 THz, but they do not have good resonance and they can be considered as non-resonant mode. Due to the conductivity of graphene in low frequency range of terahertz, which causes graphene to be a dielectric in this area. For the second structure, the 7.6 THz resonant frequency corresponds to the first mode of the structure and the 10 THz resonant frequency corresponds to the second mode of the structure, respectively, in the vertically and horizontally feeding stimulated. [ABSTRACT FROM AUTHOR]

Fifth generation (5G) wireless communication systems demand compact and low-profile high gain wideband antenna. Planar log-periodic array antenna (pLPA) has such potential qualities for 5G base-station and user-devices. However, many important features and RF analysis are rarely elaborated for pLPA which include equivalent circuit modeling. In this article, authors present the systematic RF analysis, fabrication, and equivalent circuit modeling for sub-6GHz 5G application using theory of characteristic modes (TCM). The fabricated pLPA is designed to cover the 5G bands with a high gain of >5 dBi and a fractional bandwidth of >1. Using simulation tool based on method of moments, radiating and non-radiating modes are tracked and studied with detailed physics. These modes are modeled using analytically derived equivalent circuit and assembled in an RF/microwave circuit simulator. The characteristic mode analysis and RF circuit modeling of pLPA help in the advancement of future 5G RF front-end modules.

This paper presents a novel method for assessing misalignment scenarios in Magnetically Coupled Resonant Wireless Power Transfer (MCR-WPT) system through measurements using Characteristic Modes Analysis (CMA). Based on the metric parameters of Characteristic Modes (CMs), including resonant frequencies, magnetic field plots, and current distributions, the dominant CM is identified. Its impact on the system’s total power is evaluated and compared with the system’s behavior in proximity to resonance. To establish a comprehensive and concise reference delineating the applicability of CMA in the field of Wireless Power Transmission, this contribution conducts a detailed investigation about the various types of unexpected misalignments between the Transmitting (Tx) and the Receiving (Rx) coils due to typical movement of the device or the user from both analytical and experimental perspectives. Besides, the various misalignment scenarios were systematically and progressively explored, accounting for both minor and significant degrees of variation. The proposed MCR-WPT system consists of two identical loops characterized by a singular turn, a radius R of $0.0625\lambda $ and a width w of $0.025R$ . Notably, results based on CMA guided by The Coupled Mode Theory (CMT) demonstrate a great affinity with the measurements of the fabricated prototypes. Consequently, this marks the inaugural confirmation of CMA’s applicability in the field of Wireless Power Transmission, which opens avenues for improving PTE using previous research findings deploying principles of selective excitation through CMA concepts.

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. [JJCIT 2023; 9(2.000): 107-117]

In this paper, a dual-band WiFi antenna and its Multiple Input Multiple Output (MIMO) system application is designed, fabricated, and measured based on the Chinese characters "Men" and "Wei". The antenna uses a 40 × 40 × 1.6 mm3 FR4 substrate to analyze the combinatorial structure of Chinese characters using the theory of characteristic modes, to optimize the antenna dimensions by analyzing the mode current distribution, and to broaden the antenna bandwidth by etching rectangular slots on the ground. The measured and simulated results show that the four-element MIMO antenna covers 5.68–8.01 GHz, the isolation between the antennas is higher than 20 dB in the working band, the envelope correlation coefficient and the channel capacity losses of the simulation are lower than 0.001 and 0.18 bits/s/HZ, respectively. The efficiency of the antenna is higher than 90%, and it can be used for WiFi communication bands (5.8 GHz and 6 GHz). [ABSTRACT FROM AUTHOR]

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A printed ultra-wideband (UWB) multiple input multiple output antenna with isolation enhancement and pattern diversity is presented, which is based on characteristic modes (CM) for upcoming wireless mobile platforms. The presented antenna is composed of dual UWB antenna elements (AEs) – an antipodal Vivaldi and a circular-shaped monopole positioned on the two opposite edges of the antenna's rectangular-shaped ground surface. The two AEs are capable to stimulate distinct modes in the ground surface and therefore obtaining the required distinct radiation (far-field) patterns and highly isolated AEs without introducing any decoupling structure (DS). The measured results demonstrate that both AEs are UWB with an operating frequency from 2.6 to 11 GHz. The measured isolation is more than 30.1 dB with a maximum value of 55 dB in UWB. The peak realized gain is over 0.83 dBi. The radiation efficiency is greater than 75%. The Envelope Correlation Coefficient is <0.08. [ABSTRACT FROM AUTHOR]

This paper is concerned with the extractions of electromagnetic characteristic modes (CMs) for lossless dielectric bodies, for which spurious modes are prone to generate using the traditional definition of CMs based on the Poggio–Miller–Chang–Harrington–Wu–Tsai (PMCHWT) equations. It is found that the impedance matrix of PMCHWT equations cannot distinguish (i) which domain is the dielectric body and which domain is the background and (ii) from which domain the excitation source was applied. If the system is taken as a scattering problem, the spurious modes are solutions to a reverse media problem, i.e., exchanging the media of the dielectric body and the background space. However, if the system is taken as a radiation problem, no appropriate CMs that meet the specified boundary conditions are obtained. These phenomena indicate that CMs developed from scattering systems are not suitable for radiation systems. To clarify the issue, four cases with reverse media and with excitation sources in either domain are examined. The four cases are distinct in essence, but the PMCHWT equations cannot distinguish them. As a result, definitions of CMs for the four cases should be given along with their specific boundary conditions. Especially, the CMs for the radiation problems we consider here show that the excitation source inside the material object should be properly defined in order to be distinguished from scattering problems. [ABSTRACT FROM AUTHOR]

We show theoretically that the characteristic modes of dielectric resonator antennas (DRAs) must be capacitive in the low frequency limit and that as a consequence of this constraint and the Poincaré Separation Theorem, the modes of any DRA consisting of partial elements of an encompassing super-structure with the same spatial material properties cannot resonate at a lower frequency than the encompassing structure. Thus, design techniques relying on complex sub-structures to miniaturize the antenna, including topology optimization and meandered windings, cannot apply to DRAs. Due to the capacitive nature of the DRA modes, it is also shown that the Q factor of any DRA sub-structure will be bounded from below by that of the super-structure at frequencies below the first self-resonance of the super-structure. We demonstrate these bounding relations with numerical examples.

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.

A novel Green’s function-free characteristic modes formulation is introduced in this work. The desired impedance or admittance matrix is obtained utilizing and appropriately modifying the versatile finite element method. For this purpose, the generalized eigenvalue problem of the electric or magnetic field vector wave equation is formulated. In the case of the electric field wave equation, using the Schur complement, the system is reformulated and expressed only in terms of the tangential electric field over the radiating apertures, retaining the equivalent magnetic currents. Similarly, in the case of the magnetic field wave equation, the electric current density on radiating metallic surfaces is isolated using the Schur complement. In both cases, the obtained matrix is split into its real and imaginary part to yield the characteristic modes eigenvalue problem. Key advantage of the proposed formulation is that it does not require the evaluation of Green’s function, thereby the study of any arbitrarily shaped, multilayered geometry loaded with anisotropic and inhomogeneous materials is feasible. To prove the validity of the proposed methodology various classical structures, with both homogeneous, and inhomogeneous and anisotropic materials, published in the bibliography are studied. Both the eigenvalues and eigenvectors compared with the published results show good agreement.

This paper presents the systematic design of an elliptical ring monopole wideband antenna using characteristic mode analysis. The modal analysis is used to analyze the bandwidth and the radiation performance of the radiating patch. The resonance frequencies of three characteristic modes are close to each other with similar modal current distributions and characteristic fields. These three characteristic modes are simultaneously excited by an effective feeding technique. The proposed model achieves wideband characteristics. The proposed model is printed on an inexpensive FR4 substrate with a size of 20 mm × 18 mm × 1.6 mm and has a wide impedance bandwidth of 124.4% in the range of 3.6–15.46 GHz. The prototype has been fabricated and the measured results show good agreement with simulated results. The antenna will cover WLAN, WiMAX, Wi-Fi, and X-band applications.

Natural gas pipeline leaks can cause serious hazards to natural gas transportation and pose considerable risks to the environment and the safety of residents. Therefore, feature extraction of pipeline signals is crucial in natural gas pipeline leak detection. However, the quality of feature extraction directly affects the effectiveness of pipeline leak detection. Therefore, this paper proposes a pipeline leakage feature extraction method based on variable mode decomposition (VMD) and local linear embedding (LLE). First, the pipeline signal is decomposed into several modal components by VMD; then, the dispersion entropy is used to select the feature modes. Secondly, the time-frequency domain features of different components are extracted to construct a high-dimensional feature matrix, which LLE reduces to obtain the classified low-dimensional feature vectors. Finally, the extracted feature vectors are used to train and test the support vector machine (SVM). By analyzing the experimental results, it can be seen that the proposed method can classify pipeline signals with an accuracy of up to 95%, which effectively solves the problem of false alarms and missed alarms in pipeline leakage detection. [ABSTRACT FROM AUTHOR]

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]

This communication proposes a compact 4-shaped monopole annular ring UWB antenna design. The proposed structure contains multiple radiating strips inside the annular ring, in the form of a 4-shaped and a 50Ω microstrip feed line. A tapered structure with a feed point is chosen to achieve wideband characteristics. The proposed model is printed on a low-priced FR4 substrate with a size of 0.180λ 0 × 0.225λ 0 (20 × 25mm 2). The proposed model achieves a fractional bandwidth of 133.74% in the 2.7 to 13.6 GHz range with S11

This paper presents a novel planar filtering antenna (filtenna) with simple layout based on tightly-coupled different-length monopoles. Its design methodology is based on an even-mode/odd-mode decomposition of the monopole currents, and the combination of both modes in a multimodal circuit model that facilitates the design and tuning of the antenna and the accompanying feeding network, since most of the process is performed using fast circuit simulations. The circuit model casts light into the interaction between conducting and radiating modes in the bulk of the antenna, and on how this interaction can be used to propose feedback loops that increase and define the radiation band. Using these modal tools, a filtenna was designed and fabricated to fully cover the new 5G frequency bands (n77, n78 and n79) from 3 to 5 GHz that are currently being adopted by many countries for high-capacity data transmission. It features a 62.9% FWB, dipole-like radiation patterns, good out-of-band rejection up to 10 GHz, and a very good trade-off between design/structural-simplicity and bandwidth. [ABSTRACT FROM AUTHOR]

This communication proposes a compact 4-shaped monopole annular ring UWB antenna design. The proposed structure contains multiple radiating strips inside the annular ring, in the form of a 4-shaped and a 50Ω microstrip feed line. A tapered structure with a feed point is chosen to achieve wideband characteristics. The proposed model is printed on a low-priced FR4 substrate with a size of 0.180λ0 × 0.225λ0 (20 × 25mm²). The proposed model achieves a fractional bandwidth of 133.74% in the 2.7 to 13.6 GHz range with S11<-10dB and covers the 3.1-10.6 GHz unlicensed band approved by FCC in 2002 and X-band applications. The antenna exhibits stable and Omni-directional radiation patterns in the operating frequency range. The analysis of the proposed monopole antenna using characteristic modes is performed to obtain a physical understanding of the radiation process occurring on the radiating antenna. The modal significance curves and the modal current distributions are used to analyze the radiating antenna using the first six characteristic modes. The measurement and simulation results show a good agreement. [ABSTRACT FROM AUTHOR]

A low-specific absorption rate (SAR) four-antenna multiple-input–multiple-output (MIMO) antenna array is designed for fifth-generation (5G) mobile phones based on the theory of characteristic modes (TCM) of the composite perfect electric conductor (PEC) and lossy dielectric structures. First, the TCM of composite PEC-lossy dielectric structures is reviewed briefly. Then, the application of the theory for low-SAR antenna design is demonstrated through a four-antenna MIMO antenna array. Finally, the proposed MIMO array is fabricated, and its performance is measured. Its −10 dB impedance bandwidth can cover the desired frequency band from 3.4 to 3.6 GHz. The isolations between different antenna elements are higher than 15.4 dB. Their total efficiencies range from 81.3% to 89.9%. The envelope correlation coefficient (ECC) is lower than 0.07. The 10 g spatial-average body SAR corresponding to the situation that four antennas simultaneously radiate is 0.47 W/kg. Compared with the state-of-the-art 5G MIMO antenna arrays, the proposed MIMO antenna array has the lowest peak SAR value, satisfactory efficiency, isolation, and ECC. This article provides a general methodology for the design of low-SAR mobile phone antennas. [ABSTRACT FROM AUTHOR]

A four-port wideband cavity-backed antenna is presented for indoor base stations applications. The antenna is composed of a square open cavity with an X-shaped isolating block and 4 feeding monopoles symmetrically and orthogonally arranged in the aperture of the cavity. A novel methodology based on Characteristic Modes Analysis (CMA) is used for identifying the modes which are contributing to the coupling. As a result of this analysis, an X-shaped isolating block placed at the center of the cavity is proposed for increasing the isolation between ports. A wide-band four-port antenna with unidirectional radiation patterns is obtained with a measured impedance bandwidth ( $\text{S}_{11} < -10$ dB) ranging from 1.55 to 6 GHz (118%), covering most of the sub-6 GHz 5G bands. The proposed antenna provides four independent radiation patterns, with 16 dB of measured minimum isolation between ports and an efficiency higher than 84%. Multiple-input multiple-output (MIMO) compatibility is confirmed with a $4\times 4$ MIMO simulated system with an envelope correlation coefficient (ECC)< 0.5 in different propagation conditions. The antenna is easy to fabricate and presents a compact size of $129.5\times 129.5 \times 28.2$ mm3 ( $0.68\lambda \times 0.68\lambda \times 0.15\lambda $ , at a frequency f $=\,\,\text{f}_{min} =1.55$ GHz). Moreover, the antenna has the advantage of avoiding complex feeding structures with baluns or directional couplers.

A co-designed (3+3)-port antenna for dual-band operation that requires no feeding or decoupling network is proposed and verified. The antenna consists of six ports which are divided into two groups that resonate in two different frequency ranges. The basic radiating element is a tri-modal patch in a folded snowflake-shaped structure with the largest antenna dimension of $0.48\mathit {\lambda _{1}}$ or $0.68\mathit {\lambda _{2}}$ , where $\mathit {\lambda _{1}}$ and $\mathit {\lambda _{2}}$ are the wavelengths in air at the center frequencies of the lower and upper operating bands, respectively. Measurement results show that the 10 dB impedance bandwidths of the two respective bands are 19.7% and 14.4%. The proposed antenna exhibits compact, multiport, multiband and broadside radiation characteristics which are not only suitable for dual-band MIMO applications, but also for energy harvesting systems with spatial and frequency diversities, or dual-function wireless systems with simultaneous information and power transfer.

Characteristic modes can be used to solve many radiation and scattering problems involving fully conducting structures. The basic concept is to use eigenvalues and eigenvectors of the characteristic mode equations of a certain topology, and to use these eigenmode currents to control the radiating structure’s behavior. This is accomplished by understanding the overall reflection and radiation characteristics across the operating frequency band. More specifically, characteristic modes are effective for example in determining the best location for the excitation of a radiating structure. In this paper, a comprehensive overview is provided to the different studies and their results obtained for different antennas that are based on the characteristic mode analysis method. A future perspective is given regarding the potential of this method in antenna analysis and design.

This paper is concerned with the electromagnetic theory of characteristic modes for general homogeneous material bodies using surface integral equations (SIEs), which are computationally more efficient than using volume integral equations (VIEs). However, the generalized eigenvalue equations (GEEs) based on SIEs are prone to generate spurious or extra modes, unlike their counterparts based on VIEs. In this work, several variant SIE-based GEEs are proposed to eliminate the spurious/extra modes and improve the numerical performances. These variant GEEs share a common point that the reactive power operator related to the material domain should be removed from the left-hand sides of GEEs by explicitly enforcing the boundary condition, because it seems sensitive to incur unwanted modes. These variant GEEs are diverse in choosing the active power operator related to the material domain on the right-hand sides to suit different medium parameters, which even can be dropped directly for lossless cases. Numerical results show that these variant GEEs do not generate spurious/extra resonant modes in observed frequency range, and at least one of the variants would have acceptable numerical accuracy.

A systematic procedure for constructing orthogonal antenna ports on arbitrary symmetric multimode antennas is proposed. The procedure is based on the projection operator method from group theory. With this method, optimal orthogonal port configurations are generated analytically. Electromagnetic simulations, modal analyses, or optimization techniques are not required. The optimal port configurations are solely governed by the symmetry of an antenna and can thus be considered as a priori knowledge. Accordingly, this knowledge is available right from the beginning of the design process and can be purposefully used to compare different suitable antenna geometries with respect to the practical implementation effort. The procedure is illustrated by means of an application example resulting in a complete prototype of a six-port multimode antenna. [ABSTRACT FROM AUTHOR]

In this paper, a new method based on characteristic mode analysis for designing wide-beam antennas is proposed. According to theory of characteristic modes, the total radiation pattern is the linear combination of modal currents. The phase of the excitation coefficient is demonstrated to be the characteristic angle. To get a wide-beam pattern, a fundamental mode and an even mode are selected to be excited and combined. The method is verified by the proposed antenna. First, two modes are selected based on characteristic mode analysis. Then the modes are excited separately using different feeding networks. Lastly, a 1-to-3 power divider is proposed to combine the feeding networks. The proposed antenna has a 3-dB beamwidth of over 200° in both 45° and 135° planes. The results have shown that the proposed antenna is suitable for a wide range of detection applications. [ABSTRACT FROM AUTHOR]

A simple pattern analysis method is presented for conformal antenna arrays, considering mutual coupling effects based on the characteristic modes of isolated elements. There are many methods to analyze the performance of conformal antenna arrays, but they seldom provide a clear insight into the coupling mechanism. Thus, the overall characteristic modes of the conformal array are calculated from the characteristic modes of the isolated elements and coordinate transformation, which are different from the traditional modal analysis for the entire array. And the radiation field of the conformal array is given. The modal coupling matrix which depends on the structural parameters and relative pose of conformal elements is used to characterize the mutual coupling effect between elements and explain the coupling mechanism from the perspective of the characteristic currents. Finally, the effectiveness and efficiency of the proposed method is verified by some numerical examples. [ABSTRACT FROM AUTHOR]

Characteristic mode analysis (CMA) is based on time-harmonic electromagnetics and applied mostly in frequency domain. In this paper, CMA is extended to time domain (TD) for transmission problems. Based on the modal self- and mutual-admittance of two-port systems, the modal TD response is obtained using inverse fast Fourier transform, extrapolation and Hilbert transform. TD-CMA represents modal impulse response on specified ports, and impulse response of the system is the linear superposition of the modal ones. Numerical examples are provided to explain this new concept. For a pair of parallel wires, the calculated TD-CMA is alike the common and differential modes. For a pair of dipoles, TD-CMA can be divided into strong and weak coupling modes according to their magnitudes. The strong coupling modes are due to the pulse radiation and oscillations at dipole tips and feedings. Time delay paths are clearly observed in the near field but are not discernable in the intermediate field. This shows different coupling mechanisms for the two cases. TD-CMA expands research field of CMA and may find applications for analyzing transient behaviors of microwave systems.

Multiple-input multiple-output (MIMO) is a key enabler for high data rates in mobile communications. However, it is challenging to design MIMO terminal antennas for LTE bands below 1 GHz, due to the conventional chassis offering only one resonant characteristic mode (CM). Recently, it was shown that minor structural changes can yield up to two additional resonant modes for designing two-port MIMO antennas. Nonetheless, the resulting bandwidth for the second port is relatively small. To simultaneously meet bandwidth and other practical requirements (including low profile and no off-ground clearance), a step-by-step approach for structural changes and feed design is applied in this work to exemplify the use of physical insights from CM analysis to achieve a competitive wideband two-port solution. The main novelty is that an entirely new mode is identified and appropriately tuned by structural modification for creating an additional resonance below 1 GHz. Moreover, two simple probe-feed ports are designed to jointly excite different subsets of four modes over frequency. In addition, far-field pattern orthogonality is guaranteed by the different phase shifts of the characteristic electric fields at the port locations. Furthermore, bulkier self-resonant antenna elements are avoided. To show design flexibility, a three-port version is also demonstrated.

A technique for enhancing the bandwidth of a broadside tri-modal patch antenna is described. The key idea of the technique is to incorporate a dual-resonance structure into the broadside tri-modal patch geometry. By increasing one edge of the tri-modal patch while decreasing its size at the opposite edge, the resulting structure can be viewed as two superimposed Y-shaped structures of different resonant frequencies. This intuition is confirmed using characteristic mode analysis (CMA). Furthermore, guided by CMA, further modifications enable two sets of resonant modes to be tuned for increasing the bandwidth of the tri-modal patch antenna. Importantly, the proposed bandwidth enhancement technique does not affect the desired broadside radiation patterns significantly. Therefore, it can be utilized to modify the tri-modal patch antenna without degrading its potential for massive MIMO array application. Measurement results show that the technique enhances the 10 dB impedance bandwidth from 4.3% to 19.7% with the largest antenna dimension of $0.48{\lambda _{c}}$ , where ${\lambda _{c}}$ is the wavelength in air at the center frequency. The design example of the proposed technique is able to cover widely used 3 GHz bands in 5G communication systems and its potential usage in massive MIMO arrays is demonstrated.

Theory of characteristic modes (TCM) can provide physical insight into the radiation mechanism of arbitrarily-shaped electromagnetic objects. However, how to compute the characteristic modes (CMs) of different structures is still an open problem. Even for the calculation of CMs of an isolated dielectric body, there are eleven integral equation (IE)-based formulations which result in different modal solutions. Such kind of non-uniqueness of solutions makes CMs community confused. One of the objectives of this paper is to outline the differences among all existing IE-based formulations for the CMs of dielectric bodies. The existing formulations are briefly reviewed and carefully compared. We present a procedure to implement the different formulations in a unified manner. Then, we make a complete comparison of the numerical results of existing methods for a dielectric cylinder, which also serves as cross-validation for these approaches. We hope that this paper will help researchers understand the calculation of CMs of dielectric bodies and explore the computation methods of CMs for more complex objects.

This work addresses the use of the Characteristic Modes Analysis (CMA) for tailoring a Metasurface (MTS) in view of exploiting it in compact, low-profile, Circularly Polarized (CP) antennas. The investigated non-uniform MTS is exploited as a superstrate in the design of a novel compact and low-profile antenna for nanosatellite applications. The MTS consists of a 3 × 4 array of unequal patches arranged in a rectangular lattice. The antenna parameters are carefully tailored by using the CMA to achieve a significant performance enhancement with respect to a uniform MTS in terms of both Axial Ratio (AR) bandwidth and aperture efficiency. The reliability of the CMA approach is verified by assessing the overall performance of the whole radiating structure comprising the stripline feeding excitation. The proposed MTS antenna is compact (0.068 λ0) provides a remarkable aperture efficiency going from 86 % up to 96 % and an AR coverage in the upper hemisphere greater than 84 % within the desired S-band (2.025 - 2.29 GHz).

It is generally regarded that some properties of the characteristic modes of multiconductor transmission lines, such as per-unit-length impedance, admittance, or characteristic impedance, cannot be meaningfully computed analytically. This inability arises from the nature of the definition of the modes – sets of relative voltages and/or currents – which in the prevailing understanding may be arbitrarily scaled. Several methods with which to compute scaling factors have been proposed, but often – and as demonstrated in this work – the results of these processes do not agree with values determined directly from field quantities or experiments. This work begins by examining several facets of multiconductor-transmission-line modes under the approximation of ideal TEM propagation: firstly, that transmission-line modes are normal, and secondly, that as a result of possessing this property, it is postulated that total currents in the terminal domain may be directly equated to those in the modal domain. It is then shown that these relations allow the scaling factors to be determined to within a sign, and as a result, modal properties may be directly computed. This technique allows for the extraction of the modal transmission-line properties for any arbitrary system of conductors. Multiple examples are studied numerically, in which it is shown that the proposed process results in much stronger agreement with field solution than other proposed processes, and further validation of the proposed process is provided though experimentally obtained data.

This paper presents the systematic design of an elliptical ring monopole wideband antenna using characteristic mode analysis. The modal analysis is used to analyze the bandwidth and the radiation performance of the radiating patch. The resonance frequencies of three characteristic modes are close to each other with similar modal current distributions and characteristic fields. These three characteristic modes are simultaneously excited by an effective feeding technique. The proposed model achieves wideband characteristics. The proposed model is printed on an inexpensive FR4 substrate with a size of 20 mm X 18 mm X 1.6 mm and has a wide impedance bandwidth of 124.4% in the range of 3.6-15.46 GHz. The prototype has been fabricated and the measured results show good agreement with simulated results. The antenna will cover WLAN, WiMAX, Wi-Fi, and X-band applications. [ABSTRACT FROM AUTHOR]

This communication presents the design of an asymmetrical hexagonal-shaped wideband antenna based on Characteristic Mode Analysis (CMA). An L-shaped slot is used to change the distribution of mode surface currents. Current modes and characteristic fields of the desired modes are studied to improve bandwidth. A simple 50 Ω microstrip line is used to excite the selected modes and obtain wideband impedance matching. The antenna is printed on a low-cost FR4 substrate with a size of 30 mm × 29 mm × 1.6 mm. The design approach is experimentally validated through fabricated prototype antenna. It exhibits impedance bandwidth of 110.22% from 2.7 to 9.33 GHz, and thus covers most wireless systems, such as WPAN, WiMax, WLAN, RFID and C-band. [ABSTRACT FROM AUTHOR]

In this article, characteristic modes for metallic‐dielectric composite structures (MDCS) are explored for fast and accurate analysis of scattering from large antenna arrays. Specifically, a generalized eigenvalue equation is formulated from a method of moments (MoM) impedance matrix of the electric field integral equation and Poggio, Miller, Chang, Harrington, Wu, and Tsai formulation. Modal currents for an antenna element with MDCS are then used as entire domain basis functions in the MoM for expanding equivalent currents over a large antenna array with MDCS. Only a small number of the modal currents are required to approximate the induced current over the antenna array with sufficient accuracy. Thus, the number of the unknowns and the corresponding CPU time in the MoM are significantly reduced in the proposed method. Furthermore, as the periodicity and intrinsic relationship of the matrix elements are considered when using the modal currents as entire domain basis functions, the computational efficiency of the proposed method is further improved. Numerical results of typical antenna arrays with MDCS are presented to illustrate the efficiency and accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

A circularly polarized loop-type ground radiation antenna using a ground mode tuning (GMT) structure is proposed for Internet of Things (IoT) devices. The antenna is designed to excite two orthogonal modes of equal magnitude on the ground plane. The GMT structure consists of an inductor and a metallic strip that has been installed at the edge of the ground plane to obtain a 90° phase shift between the two modes. The proposed antenna generates left-hand circularly polarized waves in the +z-direction and right-hand circularly polarized waves in the −z-direction. The antenna was fabricated to validate the simulation results. The measured −6 dB bandwidth of the antenna was 150 MHz and the axial ratio bandwidth with reference to 3 dB was 130 MHz, completely covering the 2.4–2.48 GHz band.

An 8-port multiple-input multiple-output (MIMO) array operating in the long-term evolution (LTE) band 42 (3400-3600 MHz) based on the characteristic mode theory (CMT) is described. As designing a multi-antenna system with multiple significant modes is difficult for the CMT, a new method is, therefore, proposed to effectively provide useful physical insights and improve the envelope correlation coefficient (ECC). Although many of the same modes are excited by two antennas, the proposed method is still effective to improve the ECC of the two antennas. A detailed and reasonable framework adopting the CMT to analyze geometries with multi-significant modes is presented. The consideration of the mode number can be avoided by using this method. For an arbitrary structure, it is verified that the modal weighting coefficient (MWC) phases of the same modes excited by two ports are in phase or out of phase. Therefore, for the design of a multi-port MIMO array, the same modes excited by different ports can be divided into two types: equiphase modes and antiphase modes in the proposed method, and the tradeoff of the equiphase modes and the antiphase modes is first proposed and analyzed as an efficient way to improve the ECC. Furthermore, the proposed method can also be applied to asymmetric structures. In order to prove the effectiveness of the proposed method in the design of MIMO antennas with desirable ECC, an 8-port MIMO array for 5G mobile applications is implemented and measured.

In previously published papers about the metal-frame smartphone antennas, the screen is usually not included in the antenna model. As the big-screen, or even full-screen (also called all-screen or bezel-less) smartphones become the trend in recent and foreseeable future years, the effect of the very close screen to metal-frame antennas becomes prominent, e.g., the antenna's bandwidth is significantly narrowed. In this paper, a metal-frame antenna for the big-screen smartphone covering 824-960 MHz (GSM850/900), 1452-1496 MHz (LTE1500), 1710-2690 MHz (DCS, PCS, UMTS2100, LTE2300/2500), and 3400-3600 MHz (3.5 GHz 5G) is introduced. For the first time, the screen is fully considered, even designed as part of the antenna. The theory of characteristic modes (TCM) is applied in the design method. A prototype is made and measured for verifying the proposed methodology. Some key parameters of this antenna design are also discussed. This methodology helps to improve the degraded performance of smartphone antennas due to the big-screen industrial design.

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]

This letter proposes a compact 435 MHz telemetry, tracking, and command (TT&C) CubeSat antenna, which does not require an in-orbit deployment mechanism for proper operation. It uses characteristic modes on spacecraft's body to increase the radiating aperture, resulting in a λ/4 sized antenna (as calculated along diagonal of the cube). The antenna aims to provide reliable communication, operating independently from spacecraft's orientation and does not rely on proper deployment of vulnerable mechanical components. The limited space in the launcher limits any protrusions to no more than 6 mm (1/115th of the wavelength) from the spacecraft's wall, which results in low input impedance of 0.4 – 3.2j. To overcome this issue, a differential feeding structure is proposed that incorporates a power divider with impedance matching circuit. It also incorporates a lumped capacitor that can adjust the antenna's operating frequency. The antenna offers a dipolelike radiation pattern with performances comparable to larger deployable UHF antennas. It is intended for reliable TT&C. [ABSTRACT FROM AUTHOR]

In this article, a novel metamaterial inspired UWB/multiple-input-multiple-output (MIMO) antenna is presented. The proposed antenna consists of a circular metallic part which formed the patch and a partial ground plane. Metamaterial structure is loaded at the top side of the patches for bandwidth improvement and mutual coupling reduction. The proposed antenna provides UWB mode of operation from 2.6–12 GHz. The characteristic mode theory is applied to examine each physical mode of the antenna aperture and access its many physical parameters without exciting the antenna. Mode 2 was the dominant mode among the three modes used. Considering the almost inevitable presence of mutual coupling effects within compact multiport antennas, we developed an additional decoupling technique in the form of perturbed stubs, which leads to a mutual coupling reduction of less than 20 dB. Finally, different performance parameters of the system, such as envelope correlation coefficient (ECC), channel capacity loss (CCL), diversity gain, total active reflection coefficient (TARC), mean effective gain (MEG), surface current, and radiation pattern, are presented. A prototype antenna is fabricated and measured for validation. [ABSTRACT FROM AUTHOR]

Recently, researchers noticed that there are non-physical modes in characteristic modes of material bodies. In this paper, those non-physical modes are investigated. The reason for the non-physical modes is discussed. The influence of integral equations’ internal resonance on the non-physical modes are studied. It is concluded that the internal resonance is not responsible for the non-physical modes. Meanwhile, based on the extinction theorem, the dependent relationship between the equivalent surface electric and magnetic currents are proposed, and it is observed that the non-physical modes do not obey the dependent relationship, whereas the physical modes are governed by the dependent relationship. Therefore, it can be concluded that the lack of the dependent relationship causes the non-physical modes. Besides, based on the dependent relationship, a novel post-processing method to remove the non-physical modes is proposed. Numerical results of a dielectric cylinder are presented to validate the discussions and new post-processing method.

A novel pattern-reconfigurable two-port multiple-input multiple-output (MIMO) antenna system at Wi-Fi band has been designed by resorting to characteristic modes analysis. The design method of the proposed MIMO antenna array is based on the selective excitation of three orthogonal characteristic modes. The selective excitation is achieved by integrating two pairs of capacitive coupling elements (CCEs) on the same plane with the ground. Two feeding networks related to two ports have been designed for achieving a good impedance matching and providing the feeding to the CCEs. By switching the p-i-n diodes in each feeding network, the selective excitation mode of the two ports can be varied, thus leading to the reconfigurable patterns. Three operating states can be obtained for the proposed antenna design. Simulation results demonstrate that the envelope correlation coefficients of the proposed MIMO antenna array are far less than 0.5 due to the orthogonality of radiation patterns between the characteristic modes in each of the three states. Measured S-parameters and the radiation patterns of a realized prototype are in good agreement with simulation ones confirming the reliability of the design method.

In this paper, the application of the Theory of Characteristic Modes for the design of narrow-in-width open slot antennas embedded in finite platforms is investigated. Purely magnetic and electric characteristic modes (CM) of a slot etched in both an infinite and a finite ground plane are analyzed, with the aim to provide a physical understanding of the slot behavior and its interaction with the metallic ground plane. Instead of traditional CM analysis, an alternative approach is proposed, which consists in dividing the design procedure into the separate and complementary analysis of purely magnetic and electric CM. Based on this analysis, some guidelines for the design of open slot antennas are provided, and a simple and compact narrow-in-width wideband open slot antenna is designed. Simulations and measurements are presented for the optimized slot antenna, showing an impedance bandwidth of 47.48% with a very stable radiation pattern.

We comprehensively study the characteristic-mode analysis (CMA) of arbitrarily shaped 3-D conducting objects embedded in a lossless planarly layered medium. The electric field integral equation (EFIE) with the layered medium Green's function is applied as the core equation for numerical analysis. The modal behavior of an object in a layered medium is first comparatively studied with that in free space. The low-frequency breakdown problem of the EFIE-based CMA and its remedy is then elaborated. The asymptotic frequency dependence of the characteristic eigenvalues for the low-order modes is numerically investigated. A theoretical analysis is conducted by constructing a frequency-independent generalized eigenvalue problem and applying Taylor expansions. Finally, the modal expansion for scattering problems and parameter extraction for circuit problems are discussed, both in the context of a layered background. Since the electromagnetic property of an electrically small object is mainly governed by a few of low-order modes, this paper may provide an effective way of model order reduction for complicated structures residing in a layered medium.

It is pointed out how the (sub-structure) characteristic modes of a perfect electrically conducting (PEC) object, in the presence of an electrically very large second PEC object, can be computed using a field-based hybrid method that combines the method of moments (MM) and the geometrical theory of diffraction (GTD). An example is provided to demonstrate this. This possibility has not yet been explicitly stated elsewhere (and so perhaps not realised), nor example computations provided. We believe this realisation might widen the scope of application of characteristic mode analysis. [ABSTRACT FROM AUTHOR]

In this letter, a slot‐loaded low‐profile wideband microstrip patch antenna is presented and analyzed. The theory of characteristic modes (TCM) is applied to study the bandwidth and the radiation performance of the proposed antenna. We find that the resonant frequencies of two characteristic modes, with similar characteristic current distributions and far field radiation patterns, are close to each other. By exciting these two modes simultaneously, the proposed microstrip patch antenna attains wide band properties. In addition, the radiation patterns are stable over the whole passband, which is consistent with the prediction by the TCM. The measurements reveal that the antenna's operating frequency ranges from 2.275 to 2.705 GHz with standing wave ratio less than 2, which exhibits a wide impedance bandwidth of 17.3%. The thickness of the proposed antenna is only 0.04λ0. The realized gain is stable and varies between 8.2 and 9.7 dBi within the whole passband. Consequently, the TCM shows potential applications in the area of the design of a high‐performance slot‐loaded microstrip patch antenna. [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel design approach for multi-mode antenna radiation patterns using characteristic mode (CM) theory. Specifically, we observe that the response of the multi-mode radiation pattern is a superposition of modal current responses, which can be controlled through port excitation. By combining the port excitations, we achieve the desired multi-mode radiation pattern. The feed structure can also be designed using CM theory. The validity of this intuition is confirmed through CM analysis. Subsequently, we use the proposed theory to design an antenna structure with a profile height of 0.07 λ 0 (λ 0 is the wavelength of the electromagnetic wave at 2.8 GHz in vacuum). This antenna is capable of achieving four orthogonal radiation patterns. Simulation and measurement of the antenna demonstrate that the four radiation patterns can operate simultaneously within a bandwidth of 17% (| S 11 | < −10 dB), with worst-case port isolation of −24 dB. [ABSTRACT FROM AUTHOR]

Basing on the characteristic mode theory, this paper develops the concepts of source-mode (SM) couplings and mode-mode (MM) couplings, which characterized by the modal weighting coefficients and the inter-modal coupling coefficients, respectively. It is found that SM couplings could be adjusted by MM couplings when feeding configuration remains unchanged. Therefore, just by properly adjusting mode-mode couplings, we can reduce Q factor of an antenna at its center frequency and hence its bandwidth could be extended. A new time-saving optimization method based on controlling mode-mode couplings is also proposed. Two examples (one is lossless and the other one is lossy) are given and analyzed in detail to show how to control mode-mode couplings in order to reduce Q factor at the center frequency and hence enhance -10 dB bandwidth.