An improved S-parameter extraction method, based on the forward and backward propagating waves under oblique incidence on metamaterials (MMs), is proposed to accurately extract electromagnetic parameters for asymmetric uniaxial MMs in a broad frequency range. The proposed approach equivalently models asymmetric MMs as two isotropic media (distinct from the 3 & times; 3 matrix-form anisotropic medium). To validate the effectiveness of the proposed method, a low-thickness asymmetric absorptive frequency-selective surface (AFSS) and a high-thickness 7-layer absorber are designed, simulated, and analyzed.

Megahertz wireless power transfer (MHz-WPT) enables compact resonant components; yet the matching, compensation, and filtering stages used in conventional systems can dominate loss and standby dissipation at MHz operation. To address this issue, this work proposes a compact 13.56 MHz WPT architecture in which impedance transformation is integrated into the resonant hardware. A self-resonant transmitting coil is co-designed with a Class-E power amplifier to shape the reflected load toward the optimum operating condition, thereby removing the external compensation network, additional matching stage, and lumped-element LC output filter. The analysis shows that, when the receiver is removed, the effective load becomes dominated by the transmitter resistance, inherently suppressing delivered power without sensing or closed-loop control. A prototype delivers 9 W over 30 mm with 81.5% end-to-end DC-DC efficiency, while under receiver absence, the DC input power decreases from 11 W to 1.15 W. These results demonstrate a simplified and robust MHz-WPT architecture with reduced component count and inherently low standby dissipation.

In this letter, a broadband high-power eight-way coaxial waveguide power divider is proposed. The structure consists of one large coaxial tapered waveguide and eight branched coaxial tapered waveguides. A coaxial stepped impedance matching structure is introduced to suppress reflection peaks and broaden the operating bandwidth, while rectangular slots are incorporated near the inner conductor plane to redistribute the electric field and thereby enhance the power capacity. A prototype is designed, fabricated, and experimentally tested. The results show that the proposed power divider operates from 4.8 to 14.95 GHz with a power capacity exceeding 100 kW. Within the 100.8% relative bandwidth, the measured input return loss is better than-10 dB, and the maximum efficiency reaches 99.44%.

A switchable integrated absorber and radiator are proposed in this letter based on a planar ultrawideband modular antenna (PUMA) structure integrated with p-i-n diodes. The switchable PUMA can serve as a resistor-free absorber by utilizing the semi-conductive mode of p-i-n diodes, or a radiator when the diodes are off. Additionally, a simplified-fed method for PUMA array is proposed, achieving excellent radiation performance comparable to a fully fed array with per-element excitation. A 9 x 9 PUMA array prototype is fabricated and measured. For linear polarization, the measured 10 dB radar cross-section reduction (RCSR) bandwidths are 4 GHz to 15.6 GHz (normal incidence), 4.3 GHz to 14.8 GHz (TM 70 degrees), and 5.8 GHz to 16.6 GHz (TE 30 degrees). For circular polarization, the 10 dB RCSR bandwidths are 4 GHz to 15.6 GHz (normal incidence) and 5.5 GHz to 14.9 GHz (CP 45 degrees). Meanwhile, the prototype exhibits a measured -10 dB radiation bandwidth of 9.17 GHz to 9.9 GHz and a peak gain of 14.9 dBi at 9.5 GHz. This work provides a new approach to designing integrated radiator and absorber structures, with potential applications in satellite or aircraft communications.

This letter proposes a compact low-profile 5G multiple-input-multiple-output (MIMO) antenna system. The system comprises four antenna elements: two main antennas and two diversity antennas. The two main antennas are symmetrical to each other and both cover the identical frequency bands of 700 MHz to 960 MHz and 1710 MHz to 6000 MHz, while the two diversity antennas are also symmetrical to each other and cover the 1710 MHz to 6000 MHz band. The proposed main antenna exhibits polarization orientation differences at different frequencies; thus, arranging four antennas enables adjacent antennas to have mutually orthogonal polarizations, thereby achieving low mutual coupling. Moreover, the overall size of the MIMO antenna system is only 74 mm x 74 mm x 11 mm, and the isolation between each antenna exceeds 10 dB. Experimental test results indicate that the MIMO antenna system performs excellently, meeting the requirements for practical engineering applications, and particularly demonstrates significant size advantages in MIMO antenna applications.

The Smith chart is a graphical approach for solving impedance matching problems in the microwave circuit design, which provides visualized traces, but it is not applicable to address impedance discontinuities graphically, especially for multiple-impedance transmission lines (TLs) within microwave networks. In this letter, a novel transformation of normalization standard (TNS) method is proposed, which is applied to the Smith chart to solve the aforementioned problem. Then, TNS traces are derived and applied to the classical Smith chart, obtaining an improved Smith chart. In addition, the TNS idea is applied to the microwave circuit design examples. The proposed TNS method provides a convenient visualized way for the design of microwave circuits with multi-impedance TLs.

This letter presents a compact, broadband, Global Navigation Satellite System (GNSS) antenna with wide axial ratio (AR) beamwidth. The proposed antenna consists of multiple coplanar metal patches, square dielectric substrate and compact transformed metallic cavity. Owing to the coupling feeding between those coplanar metal patches, the proposed antenna can realize a relatively broad bandwidth in the operating frequency band. Meanwhile, by utilization of single-layer coplanar coupling technology, the present antenna is only 0.30 lambda(0) x 0.30 lambda(0) x 0.07 lambda(0). In order to increase the half-power beamwidth and AR beamwidth, claw patches and a compact transformed metallic cavity are introduced. Simulation and measurement results show that the proposed antenna possesses a broadband frequency range from 1.05 GHz to 1.73 GHz that meets the operating frequency of all satellite navigation systems, and the maximum AR beamwidth range is achieved from -104 degrees to 98 degrees (1.575 GHz) by using the diffraction effect of the electromagnetic wave in the transformed metallic cavity.

This study first proposes a planar alternating winding coil (PAWC) integrating a magnetic backplate, adopting a dual-sided alternating winding architecture on dielectric substrates. The proposed PAWC system features three fundamental characteristics: load-independent constant-current output, broad-range high-efficiency power transfer, and inherent no-load protection. During no-load operation, the transmitter exhibits high-impedance characteristics, effectively entering a zero-current shutdown state that completely prevents the overcurrent risks typical of series-series (S-S) compensated systems. When the receiver approaches the charging position, the system autonomously shifts to a zero-reactance operating mode at the same working frequency, achieving efficient power transfer without supplemental compensation networks. This design enables state switching purely through relative coil positioning, eliminating the need for additional sensing or switching components. Experimental validation confirms 92.8% coil-to-coil transfer efficiency at 5 mm separation, with efficiency exceeding 90% across 50-300 Omega load variations. These advancements offer particular value for safety-critical applications, including smart home ecosystems and medical implants, where the system's self-regulating switching capability and robust efficiency significantly enhance operational reliability.

Self-resonant (SR) coils replace lumped with distributed compensation components, enabling efficient power transfer and high integration. However, current SR coils function merely as series (S) or parallel (P) resonant tanks, thereby supporting merely four low-order compensation networks: SS, SP, PS, and PP. High-order compensation networks have demonstrated superiority over low-order counterparts, providing enhanced safety, stability, and design flexibility. Unfortunately, no SR coil currently with high-order compensation network characteristics. This article proposes a novel SR coil with LCC compensation characteristics that integrates the required compensation components in the form of distributed structures with the main coil on a single substrate, eliminating lumped components. To verify feasibility, a pair of SR coils, incorporating a normal SR coil, are fabricated, which constitute a SR coil inductive power transfer (IPT) system with LCC-S compensation characteristics. Different with traditional LCC-S systems, the compensation inductor in the proposed SR coil participates in power transfer. Therefore, corresponding formulas for the resonant frequency, ac-ac power transfer efficiency (PTE), output voltages and power are provided. Favorably, the proposed IPT system exhibits three advantages: a constant voltage output independent of the ac load, high ac-ac PTE over a wide range of ac load values, independence of resonant frequency from coupling coefficients, and no damage to the front-end power source during no-load operation. Experimental results demonstrate a coil-to-coil PTE of 93.2% and dc-dc efficiency of 80.5% over a 35 mm spacing gap. In addition, a LED monitor is successfully powered, indicating its potential for application in electronic devices, such as mobile phones and laptops.

A compact dual-band, lightweight, circularly-polarized (CP) microstrip patch antenna with a wide axial-ratio beamwidth (ARBW) for global navigation satellite system (GNSS) is proposed in this paper. This antenna consists of two stacked patches, specifically by utilizing coupled feeding mechanism and air dielectric to provide dual-band broadband operation. In order to achieve miniaturization, the radiation patches are specially shaped and loaded with multiple slots, resulting in a compact antenna size of 0.376 lambda(0) x 0.376 lambda(0) x 0.069 lambda(0). Furthermore, eight T-shaped branches are symmetrically arranged around the radiation patch to broaden the 3 dB ARBW. To verify the design, a prototype is manufactured and measured. The experimental data closely matches the simulations, indicating that the proposed antenna possesses -10 dB impedance bandwidths (IBW) of 1.12 GHz-1.45 GHz (25.68%) in the lower band and 1.49 GHz-1.75 GHz (16.05%) in the upper band, adequately covering all GNSS bands. Moreover, its measured ARBW reaches maximum values of 170 degrees and 227 degrees at 1.227 GHz and 1.575 GHz, respectively, and the measured gains are 4.5 dBic and 5.09 dBic, respectively. With these excellent performance characteristics, the designed antenna is well-suited for high-precision positioning and navigation.

The MHz inductive power transfer (IPT) system has broad application prospects due to its high safety, longer transmission distance, and better tolerance for misalignment. However, the increase in frequency will lead to an increased loss of lumped components, such as inductors and capacitors, in peripheral circuits, thereby reducing efficiency. This is currently the bottleneck problem encountered by MHz IPT. To address this key challenge, researchers have tried to remove the lumped components in the peripheral circuit while ensuring the functions of the IPT system. One successful and beneficial attempt is the self-resonant coil, which replaces traditional lumped resonant capacitors with distributed capacitors formed by coil turn-to-turn capacitance, ensuring compensation for leakage inductance and eliminating reactive power. This innovative integration makes efficiency and integration greatly improved. In this article, the novel coil named Recticoil, on which the rectification function is integrated further, is proposed for efficient compact IPT. Recticoil can directly output dc power without additional compensation and rectification circuits. By analyzing a toroidal coil where two separate conductors generate a voltage differential, and incorporating two diodes, resulting in a dc output. Subsequently, the self-resonant coil replaces the toroidal coil to form Recticoil, and the equivalent circuits of Recticoil are given and analyzed. In addition, the analysis and test method of rectification efficiency is given. The experimental results show that the Recticoil achieves a rectification efficiency as high as 97.25% at 6.78 MHz, outperforming both the bridge rectifier and the Class E rectifier. Additionally, the compact construction of Recticoil eliminates the volume of traditional rectifiers. All of these advantages of Recticoil render it a promising candidate for applications in wearables and miniaturized devices.

In this paper, a palisade-type defected ground structure(DGS) is proposed for decoupling a miniaturized lowprofile dual-polarized antenna array. The palisade-type DGS loaded between the array elements can achieve both TE decoupling and TM decoupling simultaneously. The decoupling mechanism is elucidated via analyzing and calculating the equivalent capacitance and inductance values of the parallel resonant circuit of the palisade-type DGS when the antenna array is under TE coupling as well as TM coupling. For verification, a dual-polarized antenna array loaded with palisade-type DGS is designed, whose profile is 0.006λ0 and the center-to-center distance between antenna elements is 0.36λ0 at the working frequency of 8.1 GHz. The coupled current analysis of the array loaded with DGS reveals that the palisade-type DGS has the spatial band-stop characteristic, changing the direction of the partially coupled current and thereby reducing coupling. Consequently, the simulated and measured results indicate that TE coupling and TM coupling are reduced about 11 dB and 33 dB respectively.

For a phased-array antenna system, position mismatches in spacing distances between source's output and antenna arrays' input ports are common. At present, flexible radiofrequency cables are always utilized to electrically connect those two parts, which will result in the reduced integration. To tackle this problem, a low-profile aperture converter array (ACA) is first proposed, which is applied between a microwave source and antenna arrays. Each stripline-structured channel of the ACA is composed of the signal lines with different routing shapes to achieve nearly the same phase shift. In this study, a high-power and high-efficiency 16-channel ACA is designed, fabricated and measured at X-band. Experimental results show that it meets the requirement of 5-kW pulse wave input, and the power transfer efficiency of each channel is more than 93.8%. Furthermore, its profile is only 0.42 lambda 0 ${\lambda }_{0}$ at 9 GHz, which is favorable to transforming the integration of the whole system.

This letter proposes a N-way planar power divider/combiner using a combination of a Gysel power divider and a ring coupler. This innovative approach enables the realization of a fully planar multiway (>2) Gysel power divider. By replacing the common point of the conventional Gysel power divider with a ring coupler, the entire symmetrical divider can be implemented on a two-layer printed circuit board. To validate the design concept, an eight-way planar power divider is designed, fabricated, and tested. Experimental results obtained within the frequency range of 5.10-7.25 GHz (34.8% fractional bandwidth) demonstrate excellent performance. The insertion loss is below 1.24 dB, the input port return loss exceeds 12.65 dB, the output ports exhibit return loss greater than 14.83 dB, the isolation exceeds 22.41 dB, and the maximum measured phase imbalance is within +/- 2.7 degrees.

Self-resonant (SR) coils with high quality factor Q play a critical role for wireless energy transmission systems. In this article, a new single-layered spiral SR coil is introduced based on coplanar waveguide, which consists of two gradual-width spiral copper traces. Compared with traditional equal-width coils, the proposed coil increases the conductive area at specific location. This design significantly reduces loss during the current transmission, further improving Q of the coil. The simulation and experimental tests have confirmed that the proposed coil exhibits an outstanding Q of 436 at a resonant frequency of 13.58 MHz, which is the highest Q of the same frequency and size at present, and indicates a good application prospect for future inductive power transfer (IPT) transmission system.

With the rapid development of industrial and service robots, the cross-joint dynamic wireless power transfer (WPT) technology has attracted more and more attention. Unfortunately, the current range of motion angles that can achieve WPT coverage is limited, with the maximum coverage spanning from an axial angle theta ranging from 0 degrees to 75 degrees and a circumferential angle phi of 360 degrees. Currently, the realization of an omnidirectional WPT (omni-WPT) system remains elusive. This article pioneers in highlighting that maintaining constant coupling polarity (CCP) between the transmitting ( Tx ) and receiving ( Rx ) coils throughout the entire relative motion range is a fundamental requirement for achieving omni-WPT. Leveraging this insight, the cross-joint WPT with an omnidirectional operating angle is achieved for the first time. Subsequently, guidelines for designing a coupler based on CCP are provided, balancing the need for high transfer efficiency and low coupling fluctuation. To validate the effectiveness, a 13.56 MHz cross-joint omni-WPT system with theta epsilon [0 degrees, 180 degrees], phi epsilon [0 degrees, 360 degrees] is designed, constructed and tested. The experimental results show that high efficiencies of more than 90% are achieved with low efficiency fluctuation of 1.73%. Meanwhile the output power fluctuation on the load is only 4.89%.

This letter proposes an ultra-wideband, wide-angle absorber based on mushroom-type high impedance surface (HIS). The absorber is composed of six frequency selective surface (FSS) layers and six polymethyl acrylamide (PMI) foam substrates. Metal column is introduced into substrates to obtain a stable absorption within the broad oblique incidence angle theta range of 0 degrees similar to 70 degrees under TM polarization. Additionally, the absorber also has good absorption performance of the incident angle theta in the range of 0 degrees similar to 55 degrees under TE polarization. Good agreement can be obtained between the simulated and experimental results, which indicates that the absorber has an absorption of at least 90% within the frequency range of 1.18 GHz similar to 41.43 GHz, boasting fractional bandwidth (FBW) of 188.9%, and the thickness is only 0.116 lambda (max).

In this letter, a low insertion loss power divider/combiner with a power capacity exceeding 100 kW is presented. The discussion focuses on the simultaneous achievement of high efficiency and high-power capacity, based on the compensation analysis of discontinuous coaxial lines. An 8-way power divider/combiner, operating within the frequency range of 7.73-9.33 GHz, has been designed, fabricated, and subjected to experimental testing. The experimental results demonstrate that within a relative bandwidth of 18.8%, the highest efficiency surpasses 98.49% and the power capacity exceeds 100 kW. Furthermore, the return loss of the multiport is better than 22.5 dB, the single port exhibits a return loss better than 20.7 dB, and the minimum isolation among the multiport is better than 27.5 dB. By comparison with other recently reported power dividers/combiners, the proposed power divider/combiner possesses both high-power capacity and high efficiency simultaneously.

An antenna array based on multiple mechanisms, which are miniaturization techniques, scattering cancellation and absorptive frequency selective surface (AFSS) loading, is proposed for broadband radar cross section (RCS) reduction. Miniaturized antenna contributes to broadband low-RCS performance, while the average RCS reduction is limited. To address this, the miniaturized antennas are arranged to form a checkerboard-like array for scattering cancellation, effectively reducing the in-band RCS. Additionally, an AFSS unit with a transmission band is designed to absorb incident waves out of the operation band. Simulated and measured results are found in good agreement, indicating a broadband RCS reduction of 1 GHz to 14 GHz with a measured average RCS reduction of 10.9 dB under linearly polarized (LP) normal incidence. While under circularly polarized (CP) normal incidence, a notable RCS reduction within 1 GHz to 14 GHz is also obtained. The RCS performance under LP incidence maintains relatively stable as the incident angle increases up to 40 degrees. Moreover, the antenna exhibits good radiation performance, achieving a measured peak gain of 11.4 dBi with a radiation efficiency of 87.2%.