Search Results (2,311)

This paper presents a novel ultrathin frequency selective surface (FSS) bandpass filter with an extraordinary wideband tailored for operating within the S-C bands. The filter structure entails a double-layer FSS structure with mutually perpendicular unit cells etched on the top and bottom sides of a 0.003${\lambda }_L$ thick FR4 dielectric substrate, where ${\lambda }_L$ is the free space wavelength at the lowest operating frequency. Thus, both TE and TM polarizations can be covered, ensuring the polarization insensitivity of the structure. The two FSS layers are loaded with resistors to implement the harmonic suppression principle. The overall periodicity is extremely compact, measuring 0.16${\lambda }_L$× 0.16${\lambda }_L$. An equivalent circuit analysis was conducted to comprehensively evaluate the structure and provide design guidelines. Numerical simulations and experimental measurements demonstrated that the proposed filter achieved a −3 dB transmission band spanning from 2 to 6.76 GHz (fractional bandwidth equal to 108.7%) under normal incidence. Moreover, aside from excellent wideband performance, the filter showcased a flat bandpass and stable responses up to 40° of incidence angle. These remarkable capabilities position the proposed filter as a valuable asset in advancing the development of radomes and applications relevant to electromagnetic interference (EMI) shielding, promising significant contributions to the field. Full article

The early diagnosis of Alzheimer’s disease remains an unmet medical need due to the cost and invasiveness of current methods. Early detection would ensure a higher quality of life for patients, enabling timely and suitable treatment. We investigate microwave sensing for low-cost, non-intrusive early detection and assessment of Alzheimer’s disease. This study is based on the emerging evidence that the electromagnetic properties of cerebrospinal fluid are affected by abnormal concentrations of proteins recognized as early-stage biomarkers. We design a conformal six-element antenna array placed on the upper portion of the head, operating in the 500 MHz to 6.5 GHz band. It measures scattering response due to changes in the dielectric properties of intracranial cerebrospinal fluid. A multi-layer perceptron network extracts the diagnostic information. Data classification consists of two steps: binary classification to identify the disease presence and multi-class classification to evaluate its stage. The algorithm is trained and validated through controlled experiments mimicking various pathological severities with an anthropomorphic multi-tissue head phantom. Results support the feasibility of the proposed method using only amplitude data and lay the foundation for more extensive studies on microwave sensing for early Alzheimer’s detection. Full article

Objective: The air content within the lungs directly influences the dielectric properties of lung tissue; however, previous studies were conducted under ex vivo conditions and without quantitatively controlling air volume. This study aims to develop an improved model using in vivo measurements to accurately characterize the dielectric properties of rabbit lung tissue across various tidal volumes. Methods: In this study, six sets of different tidal volumes (30, 40, 50, 60, 70, 80 mL) were set in the frequency band of 100 MHz~1 GHz to analyze the trend of the dielectric properties, and the dielectric parameters were systematically constructed under the conditions of different tidal volumes. Results: It was found that the conductivity and permittivity of rabbit lung tissue showed a decreasing trend with increasing tidal volume in the measuring frequency band. The traditional Cole–Cole model has limitations in simulating the dielectric properties of in vivo lung tissues. Therefore, by refining and optimizing the model, this study successfully reduced the average error between the measured data and the model fitting to less than 5%. Conclusions: This study lays the groundwork for investigating the relationship between total air volume within the lungs and their dielectric properties in vivo. Full article

Amplitude equalizers play an important role in microwave transmission systems, and their performance can directly improve the signal transmission quality. Nowadays, the miniaturized equalizer is in good need of a highly integrated radio frequency (RF) front end. In this paper, a wafer-level amplitude equalizer based on an integrated passive device (IPD) process is proposed. Moreover, the equalizer circuit containing two resonance points within the transmission frequency band is also proposed. The amplitude equalizer operates at the center frequency of 3.88 GHz, with two resonance points of 2.9 GHz and 5.2 GHz; the minimum insertion loss is 1.17 dB, the maximum attenuation is 4 dB, and the in-band voltage standing wave ratio (VSWR) is less than 1.6:1. All the measured results are in good agreement with the designed results. The size of the proposed equalizer is 0.8 mm × 0.65 mm × 0.1 mm (0.011λ × 0.008λ × 0.001λ), which shows great potential in the miniaturization application of the RF microsystem. Furthermore, the new equalizer circuit with two resonance points is especially suitable for wavy in-band transmission. Full article

A low profile dual polarized transmitarray antenna, made of three identical layers, is proposed in this paper for Ku-band applications. The transmitarray comprises 22 × 22 symmetrical unit cells. A 3-bit phase compensation layer with less than α_T = 1.3 dB transmission loss and 2π transmission phase coverage for both linear polarized components at the central frequency f₀ = 12 GHz is designed. Moreover, for an incidence angle θ = 30°, the unit cell transmission loss is less than 2 dB; the transmission phase is close to the transmission phase at zero incidence angle θ = 0°. The fabricated transmitarray exhibits a measured peak gain of Gm₀ = 21 dB at the frequency f₀ = 12 GHz. The corresponding measured 1 dB gain bandwidth is BWg = 10.8% (11.1–12.4 GHz). The measured peak side lobe levels are SLL₀ = −20.8 dB at f₀ = 12 GHz. The transmitarray antenna can be used for beam steering up to an angle of γ_max = ±30° with a measured scan loss △G_MSL1 = 2.73 dB at f₁ = 12.4 GHz. Full article

The Finite Difference Time-Domain (FDTD) method is a powerful tool for electromagnetic field analysis. In this work, we develop a variation of the algorithm to accurately calculate antenna, microwave circuit, and target scattering problems. To improve efficiency, a single-field (SF) FDTD method is proposed as a numerical solution to the time-domain Helmholtz equations. New formulas incorporating resistors and voltage sources are derived for the SF-FDTD algorithm, including hybrid implicit–explicit and weakly conditionally stable SF-FDTD methods. The correctness of these formulas is verified through numerical simulations of a newly designed dual-band wearable antenna with an artificial magnetic conductor (AMC) structure. A novel antenna fed by a coplanar waveguide with a compact size of 15.6 × 20 mm² has been obtained after being optimized through an artificial intelligent method. A double-layer, dual-frequency AMC structure is designed to improve the isolation between the antenna and the human body. The simulation and experiment results with different bending degrees show that the antenna with the AMC structure can cover two frequency bands, 2.4 GHz–2.48 GHz and 5.725 GHz–5.875 GHz. The gain at 2.45 GHz and 5.8 GHz reaches 5.3 dBi and 8.9 dBi, respectively. The specific absorption rate has been reduced to the international standard range. In particular, this proposed SF-FDTD method can be extended to analyze other electromagnetic problems with fine details in one or two directions. Full article

Substrate Integrated Circuits (SICs) represent a significant advancement in microwave communication systems due to their high efficiency, performance, and integration capabilities. Empty Substrate-Integrated Waveguides (ESIWs) are a type of SIC that offers benefits such as cost-effectiveness while maintaining high performance. This paper presents the design and implementation of the first floating patch antenna integrated into an ESIW, fed by a metallic rod. The proposed antenna is designed to operate in the X-band (8–12 GHz), with a resonance peak at 10 GHz. The patch antenna is square, which provides interesting radiation characteristics. It is excited by a metallic rod that connects the patch to the ESIW line, resulting in excellent performance in terms of measured radiation efficiency (over 90%) and −10 dB impedance bandwidth (approximately 20%). The prototype demonstrates minimal differences between the simulated and manufactured versions. These results highlight the potential of ESIW-fed floating patch antennas for advanced satellite communication systems. This will enable the integration of complete communication systems within ESIWs and facilitate the straightforward development of 2D element arrays. Full article

In this article, a miniaturized frequency-selective surface (FSS)-based electromagnetic shield is investigated for EMI mitigation in the X-band. The FSS comprises a convoluted conducting loop designed over an FR-4 substrate. It operates at 10 GHz X-band frequency and offers an effective shielding of at least 33 dB. It reveals rejection bandwidths of 26% for the TE and TM wave modes at normal incidence. Moreover, it accomplishes polarization-insensitive and angularly stable spectral responses owing to its structural symmetry and compact size. In addition, an equivalent circuit model (ECM) and a finite prototype of the shield are developed to verify EM simulations. A comparison of the results indicates that the FSS offers wide angular stability and excellent shielding performance, which makes it a suitable candidate for applications requiring targeted EMI mitigation. Full article

Laser-induced graphene (LIG) has become a promising stealth material due to its excellent electromagnetic loss characteristics in the terahertz and microwave bands (2–18 Ghz) and the advantages of low-cost large-scale manufacturing. With the rapid advancement of electromagnetic detection technologies toward multispectral and high-dynamic-range capabilities, there is an increasing demand for LIG-based stealth materials with superior absorption performance. The synergistic design of functional material doping and structural configurations has been identified as a critical approach to achieve high electromagnetic shielding performance in LIG-based composites. This article briefly reviews the developmental progress of LIG-based electromagnetic stealth materials, with a particular emphasis on doping technologies and shielding mechanisms tailored for stealth applications. Furthermore, we propose potential future development pathways for LIG-based stealth materials to facilitate their transition toward broader practical applications. Full article

This paper proposes a tri-band wide-angle polarization-insensitive absorber operating in the C-band and Ku-band, based on the design concept of metal–dielectric–metal. The absorber achieves absorption efficiencies of 99.05%, 99.3%, and 97.9% at 4.23 GHz, 7.403 GHz, and 14.813 GHz, respectively. The first two absorption frequencies are in the C-band, while the third absorption frequency is in the Ku-band, both of which are commonly used in satellite communication. The designed absorber consists of three differently sized regular hexagonal rings. To analyze the interaction mechanism between the electromagnetic wave and the absorber, we applied the theory of impedance matching and equivalent media to analyze the metamaterial properties of the absorber. In addition, the equivalent circuit model of the absorber has been analyzed. We then determined the existence of coupled electromagnetic resonances between the top and bottom surfaces by analyzing the distribution of the electric field, magnetic field, and surface currents on the absorber. By varying the polarization angle and incident angle of the incoming wave, we found that the absorber exhibits polarization insensitivity and wide-angle absorption characteristics. The TE and TM waves maintain more than 90% absorption efficiency up to incident angles of 50° and 60°, respectively. The absorber’s thickness is 1.07 mm, which is 0.0154 times the wavelength corresponding to the lowest resonant frequency ${(\lambda }_0)$, and the edge length of the subunit’s regular hexagon is 7.5 mm (0.108${\lambda }_0$), making the absorber sub-wavelength in scale while maintaining its compactness. The proposed absorber operates in the C-band and Ku-band, and can be applied in the field of satellite communications, achieving functions such as electromagnetic shielding and stealth. Full article

A mechanical beam-scanning reflectarray (RA) antenna is presented for Ka band. The 1D steering of the beam is achieved through linear in-plane panel translations, which can be implemented at low cost using a rail-mounted moving RA panel. Compared to related works, a highly uniform beam level is achieved with a remarkably compact antenna profile. A new technique is also proposed to mitigate the high side lobes caused by the compact antenna optics, achieving an estimated 2.3 dB reduction in maximum SLL. The manufactured prototype has a panel size of 256.4 by 187.2 mm with 2898 elements, and an $F/D$ of only 0.47. A measured scan loss of 1.1 dB is achieved over a 45-degree scanning range. The measured gain is 31.6 dBi and the aperture efficiency is 24.7% at the design frequency of 29.5 GHz, with SLL between −9.4 and −17.5 dB. In-band measurements show a 1 dB bandwidth from 28 to over 32 GHz (11.9%). Full article

This research presents a novel approach to 28 $\mathrm{GHz}$ impulse radio ultra-wideband (IR-UWB) transmission using pulse position modulation (PPM) over an analog radio-over-fiber (ARoF) link, investigating the impact of fiber-based fronthaul on the overall performance of the communication system. In this setup, an arbitrary waveform generator (AWG) is employed for PPM signal generation, while demodulation is performed with a commercial time-to-digital converter (TDC) based on an event timer. To enhance the reliability of transmitted reference PPM (TR-PPM) signals, the transmission system integrates Gray coding and Consultative Committee for Space Data Systems (CCSDS)-standard-compliant Reed-Solomon (RS) error correcting code (ECC). System performance was evaluated by transmitting pseudorandom binary sequences (PRBSs) and measuring the bit error ratio (BER) across a 5-m wireless link between two 20 $\mathrm{d}\mathrm{B}\mathrm{i}$ gain horn (Ka-band) antennas, with and without a 20 $\mathrm{k}\mathrm{m}$ single-mode optical fiber (SMF) link in transmitter side and ECC at the receiver side. The system achieved a BER of less than 8.17 × 10⁻⁷, using a time bin duration of 200 $\mathrm{p}\mathrm{s}$ and a pulse duration of 100 $\mathrm{p}\mathrm{s}$, demonstrating robust performance and significant potential for space-to-ground telecommunication applications. Full article

This study presents an extensive analysis of electromagnetic field (EMF) exposure in Greek urban and suburban areas during the ongoing deployment of the 5G technology. A total of 400 in situ measurements of electric field strength were conducted at ground level across four distinct regions. The first part of the study provides a statistical analysis of the overall EMF exposure, highlighting the contributions of different cellular generations (2G–5G) to the total field. The results show that 4G networks are the dominant contributors, while the impact of 5G remains limited. The second part investigates the correlation between electric field levels in the 3.5 GHz band and the distance from 5G base stations (BS). Using geospatial analysis, we computed distances to the nearest BS and examined their relationship with measured EMF levels. The findings indicate a general inverse correlation between EMF levels and BS distance, although variations occur due to factors such as line-of-sight conditions and urban built environment characteristics. These results confirm that distance remains a key predictive feature for AI-based EMF exposure models, even in the case of 5G networks. Full article

This paper proposes a detailed design study of resonating high-frequency notch filters driven by RF MEMS switches and their optimization for dual-band operation in the X-Band. Microstrip configurations will be considered for single and dual-band applications. An SPDT (single-pole-double-thru) switch composed of double-clamped ohmic microswitches has been introduced to connect triangular resonators with Sierpinski geometry, symmetrically placed with respect to a microstrip line to obtain a dual notch response. Close frequencies or spans as wide as 2 GHz can be obtained depending on the internal complexity and the edge side. The internal complexity has been modified to introduce the possibility of using the same edge size for the frequency tuning of an elementary cell, maintaining a fixed footprint, and allowing coupled structures to implement high-frequency filters of the same size and variable operational frequencies. Preliminary experimental results have been obtained as a confirmation of the predicted device functionality. Full article

This paper presents the calibration process involved in a planar active phased array antenna operating in the K-band (17.7–20.2 GHz). The array consists of eight columns, each containing a 1 × 8 subarray of patch antennas. To enhance the antenna bandwidth, a double-stacked patch structure is employed. We analyze the challenges encountered when measuring active antennas. Additionally, we discuss the solutions and calibration techniques used to improve the array performance. Finally, we present the results of the optimal calibration approach, comparing simulated and measured data, both with and without calibration, to evaluate the improvements achieved. Full article