To effectively receive circularly polarized (CP) waves at large angles, it is necessary for antennas to have large axial ratio beamwidth (ARBW). For instance, geostationary satellite receiver antennas employed in regions with high latitudes necessitate a broad ARBW. In this paper, the mechanism to broaden ARBW is analyzed, and a CP antenna with low profile is designed and fabricated. With L-shaped branches and vertical extension segments (VESs), ARBW of the proposed antenna has been effectively broadened. Moreover, compared with recently reported wide ARBW artworks, it features a simple structure and feeding mechanism. The proposed antenna within the working bandwidth (from 4.79 to 4.95 GHz, 3.3%) achieves a 3 dB ARBW of 243° in the φ= 0° plane while the 3 dB ARBW reaches 199° in the φ= 90° plane. The measured results agree well with the simulated ones.

In this paper, an enhanced FDTD algorithm is proposed for efficient characterization of HTS (high temperature superconductors) microwave planar filters. The developed algorithm, which can be generalized to any microwave planar circuit, is based on the two-fluid phenomenological model. Further, an irregular mesh discretization allowed improving the CPU time. Also, the thermal effects and the normal conductivity have been rigorously taken into account for better performance. The impact of the operating temperature as well as the choice of the superconductor thickness was investigated. Computed results are in good agreement with simulated data using commercial software.

The Double Stator Magnetic Field Modulation Motor (DSMFMM) realizes the magnetic field modulation effect and optimizes the torque ripple effect by accurately optimizing the flux line. In order to further explore the influence of permanent magnets (PMs) magnetization model on the performance of DSMFMM, this paper conducts a comparative study on the performance of three motors with radial magnetization, Halbach magnetization, and Spoke magnetization. Firstly, three motor models are designed based on the same outer radius and axial length, and the flux lines of the three motors are analyzed in detail. Secondly, the static and dynamic performances of the three models are compared by finite element analysis (FEA) method. Compared with the conventional radial magnetization structure, the DSMFMM structure with Halbach magnetization and Spoke magnetization improves the output torque and torque density of the motor.

This study proposes a dual-band microwave sensor based on a split-ring resonator (SRR) coupled with a pair of L-shaped structures. The proposed sensor has dual functionalities, including the detection of liquid permittivity and solid displacement. An ethanol-water mixture is selected as a sample to measure the permittivity of the liquid. Moreover, FR4 is chosen as the test sample to measure the displacement of the solid. As a result, the maximum frequency detection resolution (FDR) is 1.64, and the average FDR is 1.40. The maximum and average normalized sensitivity (NS) values are 0.073% and 0.06%, respectively. The maximum displacement sensitivity is 10.0 MHz/mm for f_DS2 and 10.5 MHz/mm for f_DS1, while the average displacement sensitivity values are 4.98 MHz/mm and 8.02 MHz/mm for f_DS2 and f_DS1, respectively. These values confirm the sensor's reliable performance and sensitivity across different measurements. In general, the proposed sensor offers several advantages: 1) it operates independently by isolating the electric fields generated by each sensor; 2) it demonstrates dual functionalities, including the detection of liquid permittivity and solid displacement; and 3) it is capable of handling both liquid and solid samples.

This paper introduces a decoupled dual-element array antenna designed to address the challenges of mutual coupling between elements. To tackle this issue, a neutralization line is strategically incorporated to suppress leaky surface currents, while ensuring the antenna's central frequency and radiation pattern remain intact. The dimensions of the neutralization line are carefully optimized using the Whale Optimization Algorithm (WOA) to achieve the best possible performance, focusing on minimizing mutual coupling and enhancing gain. By placing the neutralization line nearby between the two elements, surface currents are efficiently redirected back to the radiating element, preventing leakage to neighboring elements. This approach also results in a more compact structure. The proposed antenna, with overall dimensions of 50 mm x 30 mm x 1.6 mm, is simulated using analytic software. It achieves an impressive 27 dB reduction in mutual coupling and delivers an ultra-wide bandwidth of 1.2 GHz within the ISM band at an operating frequency of 2.4 GHz, with a measured maximum gain of -5 dB. The structure was fabricated, and experimental results closely matched the simulations, confirming the design's effectiveness. By leveraging the WOA optimization method, the geometry of the neutralization line was fine-tuned to maximize performance, significantly improving inter-element decoupling. This design approach is simple yet effective and can be readily extended to other antenna array configurations, demonstrating strong potential for compact and efficient Industrial Scientific and Medical (ISM) band applications.

Millimeter wave synthetic aperture interferometric radiometer (SAIR) can achieve high-resolution imaging without a large physical aperture antenna and has strong application advantages in the fields of earth remote sensing, astronomical observation, and meteorological monitoring. In order to adapt to various payload platforms and detection needs, the existing SAIR array structures are diverse, but the existing imaging methods are difficult to effectively deal with various arrays and achieve stable high-precision imaging inversion. Thus, this paper proposes a general multi-channel fusion imaging network to achieve SAIR imaging inversion of any array structure. First, with the help of the sensor matrix deduction subnet, a high-precision imaging sensor matrix is deduced according to the position of each array element of the SAIR system, and then high-precision image reconstruction is achieved with the help of the multi-channel fusion imaging subnet. The simulation results show that the network has good adaptability and can achieve high-precision imaging inversion of different SAIR array structures.

Neural networks have become a focal point for their ability to effectively capture the complex nonlinear characteristics of power amplifiers (PAs) and facilitate the design of digital predistortion (DPD) circuits. This is accomplished through the utilization of nonlinear activation functions (AFs) that are the cornerstone in a neural network architecture. In this paper, we delve into the influence of eight carefully selected AFs on the performance of the neural network–based DPD. We particularly explore their interaction with both the depth and width of the neural network. In addition, we provide an extensive performance analysis using two crucial metrics: the normalized mean square error (NMSE) and the adjacent channel power ratio (ACPR). Our findings highlight the superiority of the exponential linear unit activation function (ELU AF), particularly within deep neural network (DNN) frameworks, among the AFs under consideration.

The miniaturization of implantable antenna is one of the significant requirements, especially for those devices implanted under the skin, as it reduces prominent appearance and invasiveness. In this paper, we design, simulate, and implement a spiral resonator-based microstrip antenna utilizing the ISM band (2.4-2.48 GHz). A small size, light weight, and flat type are required for under-skin implantation. The proposed antenna dimensions were optimized for a miniaturized volume of (3 × 2.5 × 0.12) mm³, representing the smallest size for under-skin biomedical applications. This miniaturization is achieved using a spiral-shaped radiator and creating slots in the ground layer. In-vivo measurement parameters, including reflection coefficient, are measured on the suggested antenna, showing a gain of -19.9 dBi and a bandwidth of 90 MHz. Specific Absorption Rate (SAR) is evaluated at 316 W/kg, confirming that the proposed antenna meets the necessary human-use safety criteria.

This article presents a dual-band switchable and tunable band-pass filter for Bluetooth and 5G NR applications. The filter functions at 2.41 GHz for Bluetooth and 3.55 GHz for 5G, utilizing independent switching and tuning methods facilitated by PIN and varactor diodes. The suggested design exhibits compact dimensions of 0.177λ_g x 0.096λ_g, a minimal insertion loss of 0.35 dB, and a substantial return loss of 30 dB. Advanced design methodologies, including defective ground structures (DGS) and eigenmode analysis, were utilized to attain precise selectivity and exceptional out-of-band rejection. The engineered filter demonstrates superior performance, with outcomes closely aligning with models, and guarantees little interference with suppression up to 10 GHz. The tuning mechanism provides versatility by independently modifying the operating frequencies of the second band, rendering the design very flexible for dynamic wireless communication settings. This study emphasizes a robust and effective answer for contemporary mobile communication systems.

In this paper, a novel wideband reflectionless filtering patch antenna is proposed. The antenna consists of a filtering patch and an absorption network. The filtering patch includes an E-shaped radiator and two T-shaped radiators. The E-shaped radiator introduces a radiation null, which greatly improves lower-band edge selectivity. The T-shaped radiators introduce an additional radiation null, effectively increasing the filtering performance in the upper stopband. For the absorption network, a quarter-wavelength coupled-line section with two 200-ohm resistors and four short-circuited three-quarter-wavelength transmission lines are used to achieve reflectionless characteristics. To demonstrate the design, an antenna prototype with a center frequency of 3.5 GHz is fabricated and measured. Measurement results manifest that the input reflectionless bandwidth is 63.5% from 2.56 to 4.94 GHz with an antenna gain of 5.8 dBi. At 3.02 and 3.91 GHz, two radiation nulls are also obtained. The lower and upper stopband suppression levels are 18.1 and 14.5 dB, respectively.

The paper introduces a corrugated antenna structure suitable for 5G WLAN application and operates at a frequency of 5.52 GHz. Further, a periodic structure made up of square unit cells is combined with the antenna design, and improvement in gain and impedance bandwidth is observed. The antenna gain without periodic structure is 3.48 dB whereas with periodic structure it is noted as 4.09 dB. The antenna dimensions are 16 mm × 16 mm × 3 mm. Also, the measured bandwidth of the antenna structure without periodic structure is observed to be 210 MHz, and that with periodic structure is 310 MHz.

This paper investigates the low-frequency excitation of a non-magnetic stratified conducting sphere by external sources, using a classical quasi-static approach. We focus on point-impressed sources represented by charges or electric dipoles, which predominantly generate electric fields. The findings have implications for low-frequency scattering theory and can potentially support the assessment of localized human exposure to low-frequency electric fields, such as those from Wireless Power Transfer using capacitive coupling technology. For a sphere with an arbitrary number of homogeneous layers, we develop a numerical-analytical solution inspired by the Exact Difference Scheme. This approach yields a tridiagonal discretized representation of the continuous problem, ensuring uniqueness and computational stability, and allowing for efficient solution via the Thomas algorithm. For a sphere with general radial inhomogeneity, we apply the Finite Difference method. Computational experiments show a strong agreement between these two approaches. We also examine the physical aspects of electric field interaction with a four-layer model of the human head, using the concept of coupling coefficients for the electric field and the generated heat. Our results show that these coupling coefficients increase with the separation between the point sources and the sphere, converging in certain cases to those for a uniform incident electric field. A comparison with the relevant ICNIRP reference levels for the incident electric field is also provided. The comprehensive Wolfram Mathematica code, consisting of multiple modules and including theoretical definitions and explanations of the computed quantities, is available as a supplement to the paper.

A compact spiral cavity backed substrate integrated waveguide (SIW) multiple input multiple output (MIMO) antenna is presented in this paper. The edge-shaped spiral on top of the SIW cavity acts as a dipole antenna. The dual spiral arms are excited from their symmetrical connecting center. The single antenna element in MIMO is rotated such that unit cells are orthogonal to each other forming a compact 2 × 2 and 4 × 4 MIMO SIW antenna. The proposed design shows a wide bandwidth of 930 MHz (13.74 GHz to 14.67 GHz) and 67.68% impedance bandwidth. The overall size of proposed MIMO SIW antenna is 0.9λ_o × 0.9λ_o × 0.024λ_o, where λ_o is the operating wavelength. A return loss of 18.4 dB at 14.17 GHz is achieved. The series of metal pins (in plus shape) at the center of 4 × 4 MIMO improves the isolation to 19.6 dB at resonant frequency. A pattern diversity in broadside direction is achieved by the top spiral arms and its complementary spiral arms at the bottom. The beamwidth of the proposed antenna is 90˚ varying from -45 deg to +45 deg, useful for reliable signal transmission and reception. Thus, the proposed antenna is a symmetrical compact design working at Ku band suitable for radio telescope application.

This study proposes designing and developing a metamaterial absorber that improves the efficiency of solar cells. The design includes circular forms with rectangle gaps etched on the upper surface of an FR-4 substrate, with a copper sheet serving as an isolating substrate for the ground beneath. The structure operates in the tera frequency ranges to accommodate all infrared wavelengths of the sun's spectrum. Furthermore, the constructed metamaterial unit cell is used to build a metamaterial array absorber, which increases the rate of energy harvest from the sun spectrum. The two designs showed absorption rates of approximately 96.75% and 99.85% at 94.85 THz and 109.08 THz resonant frequencies respectively. In addition, a top surface of microwave cross-polarization conversion (CPC) is also generated and simulated. The structure of the proposed microwave unit cell consists of the same metamaterial absorber design. Efficient cross-conversion is achieved across a wide frequency band (9 GHz to 15 GHz), with polarization conversion effectiveness exceeding 99%. The suggested CPC design has three resonance bands with 50% fractional bandwidth (FBW) and achieves a stable polarization response at oblique incidence angles up to 35˚.

This paper proposes a CPW-fed wideband slot antenna with a modified Minkowski fractal island geometry. The antenna comprises a CPW-fed monopole placed within a modified Minkowski fractal island slot. The resonance introduced by the fractal slot combines with the monopole's resonance, resulting in an expanded operational frequency range. The interaction between the monopole and the fractal slot significantly broadens the bandwidth. Return loss measurements confirm a wide bandwidth extending from 2.27 GHz to 7.91 GHz, achieving a fractional bandwidth of 111% covering WLAN, WiMAX, Wi-Fi, and 5G sub-6 GHz bands.

A prototype filter design exhibiting Negative Group Delay (NGD) is presented, based on the ratio of two low-pass classical Bessel filter transfer functions of the same order, but with different 3dB-bandwidths. The resulting design is a reciprocal-Bessel filter transfer function, capped at a finite out-of-band gain. The proposed capped reciprocal-Bessel design is based on a similar concept applied to previously reported capped reciprocal-Butterworth and reciprocal-Chebyshev NGD designs, which use ratios of corresponding classical low-pass filter transfer functions. It is shown that within the in-band frequency range, the synthesized NGD transfer function exhibits a maximally flat group delay characteristic (Bessel-like property). Due to its near-flat in-band group delay characteristic, the design is suitable for constant phase shifter applications. For high design orders, it is shown that the achieved NGD-bandwidth product has an upper asymptotic limit, given by the square root of the out-of-band gain in decibels. When the prototype baseband transfer function is translated to a non-zero center frequency, it is demonstrated that resonator-based implementations are feasible via Sallen-Key, as well as all-passive ladder topologies. A combined in-band magnitude/phase distortion metric is evaluated for selected design examples and applied Gaussian and sinc input waveforms, and it is shown to be proportional to the design order and out-of-band gain. The proposed design's distortion metric is also shown to be generally lower than the previously reported capped reciprocal-Butterworth and reciprocal-Chebyshev designs.

To deal with the problems of low gain and low data transmission rate of millimeter wave antenna during long-distance transmission, a high-gain millimeter wave array multiple-input-multiple-output (MIMO) antenna with series-parallel hybrid feed is proposed. The radiating structure consists of a combination of multiple rectangular patches, to make the proposed design resonate within the desired frequency band of 39 GHz. The antenna line array consists of eight radiating patches connected in series via transmission lines, providing an operating bandwidth of 1.02 GHz and a peak gain of 15.9 dB, and utilizing the Chebyshev synthesis method to control the side lobe level below -20 dB. In order to obtain higher gain, two antenna line arrays are connected through a Y-shaped feeding network, which utilizes the mutual coupling between the antennas to increase the bandwidth of the antenna to 1.25 GHz and provide a simulated gain of 17.6 dBi. Furthermore, the proposed array antennas are placed side-by-side to form a four-port MIMO antenna, which does not require any decoupling structure and has the isolation of more than 25 dB. The radiation efficiency is as high as 99%, the Envelope Correlation Coefficient (ECC) less than 0.003, and the Diversity Gain (DG) greater than 9.98. The measured results show that the operating frequency band of the antenna is 38.0∼39.6 GHz, and the operating bandwidth is 1.6 GHz. In the operating frequency band, the peak gain of the antenna is 17.45 dBi, Finally, the frequency characteristics and radiation characteristics of the antenna when bending are analyzed. The results show that the bending of the antenna leads to a slight shift in the resonant frequency, but the relative bandwidth remains unchanged. The gain has decreased, indicating that the antenna is able to work normally after bending and has a wider range of application scenarios.

This article presents an approach to expanding the impedance bandwidth of a bilateral slotted antenna backed by a substrate-integrated waveguide (SIW) cavity using high-order radiation modes. By trimming a section of the conductive ground plane and connecting one side of the bottom long slot with a via, the three hybrid modes of the cavity are perturbed and merged to achieve a broad bandwidth. The optimized antenna is fed by a microstrip transmission line for Ku-band applications, demonstrating an impressive impedance bandwidth of 6.68 GHz (a fractional bandwidth of 41%) ranging from 12.82 GHz to 19.5 GHz, with a peak gain of 6.6 dBi. Compared to previous studies, the proposed antenna offers not only a wide bandwidth but also a compact size, with dimensions of 29.7 × 22 mm², and its electrical dimensions are 1.6λ₀ × 1.19λ₀, where λ₀ is the free space wavelength at the center frequency f_center = 16.16 GHz. Additionally, it has low production costs due to fabrication on an inexpensive FR4 substrate. The antenna was initially simulated using HFSS software, and to validate the accuracy of the results, it was also analyzed with CST Microwave Studio. Moreover, a prototype was constructed for experimental testing, with measured results showing strong agreement with the simulations.

This study presents the design of a MIMO (multiple inputs, multiple outputs) antenna for the 5G application. This is an inexpensive, low-profile antenna with a dimension of 9 x 18 x 1 mm³. The highest gain of the antenna in the operating frequency range is 7.79 dBi. This antenna structure provides a minimum isolation of less than -20 dB for the working bandwidth. The antenna's operational bandwidth covers the 26 GHz band mm-wave (millimeter-wave) spectrum, from 26.86 to 31.11 GHz. Its salient features make it appropriate for 5G applications.

To explore higher-performance satellite communication antennas, a dual-frequency dual-circularly polarized antenna based on a Fabry-Perot (F-P) resonant cavity is proposed in this letter. An artificial magnetic conductor (AMC) is loaded onto the resonant cavity as a partial reflection surface (PRS) to reduce the profile. The electromagnetic (EM) waves from the feeder are reflected multiple times within the cavity and subsequently superimposed in phase, thereby enabling dual-frequency operation and high gain. Right-handed circularly polarized (RHCP) and left-handed circularly polarized (LHCP) waves are respectively generated in the lower and higher frequency bands by incorporating a dual-frequency polarization conversion surface (PCS). Two rectangular microstrip patch antennas with a simple feeding network are employed as the feeder for RHCP and LHCP, respectively. The measurement results show that the operating bandwidth is 4.77% (12.47-13.08 GHz) for the low-frequency band and 5.36% (16.51-17.42 GHz) for the high-frequency band. The maximum gains of 14.91 dBi and 14.33 dBi are achieved for the lower and higher frequency bands, respectively. The proposed antenna fulfills the requirements of the frequency division duplex satellite communication system, providing a promising candidate for ground equipment in high-speed satellite Internet applications.