Microwave and Optical Technology Letters最新文献

A spoof surface plasmon polariton superstrate structure-based decoupling method is proposed in this paper to reduce mutual coupling between two tightly coupled quarter-mode substrate integrated waveguide antenna elements. This superstrate is a periodic structure suspended over the antenna elements with an inter-element spacing of 0.02 λ₀ (1 mm) at 6 GHz. It has been demonstrated that the isolation can be improved from 6.5 dB to more than 39 dB within the operation band in measurement. Further, it has been shown that the proposed method can improve both the total efficiency (by 25%) and gain (1.4 dBi) of the multiple-input multiple-output antenna.

In this paper, we present a tunable bandpass frequency-selective surface (FSS) with wide tuning range. The FSS consists of a metallic rectangular waveguide array integrated into the double-sided printed circuit board (DSPCB) and an array of embedded varactors with a bias network on the backside of the DSPCB. The waveguide array exhibits a frequency-selective transmission peak due to the tunneling effect, and the transmission frequency can be adjusted by modifying the capacitance of varactors using different bias voltages. One benefit of this setup is that the varactors, along with the bias network, are situated on the same layer, resulting in a significant reduction in overall thickness to just 0.006λ_c (where λ_c represents the free-space wavelength at the highest transmission frequency). Additionally, the insertion loss related to the parasitic resistance of varactors is notably reduced due to the innovative integrated design of biasing lines with varactors and the nearly full-metal waveguide array. A prototype has been manufactured and tested, demonstrating that by varying the varactor capacitance from 0.12 to 1 pF, the passband showcases a frequency tuning range of 2.32:1, spanning from 5.8 to 2.5 GHz with an insertion loss ranging from 0.2 to 2.6 dB.

A compact dual-band omnidirectional antenna with folded microstrip lines and slotlines is proposed. The antenna is composed of a substrate and a superstrate. The superstrate is employed to broaden the antenna bandwidth. The radiating patch is situated on the upper surface of the substrate and consists of folded microstrip lines. The performance of the high-frequency band can be optimized based on the analysis of the current distribution intensity. The bottom of the substrate is composed of a ground plane with slotlines. The introduction and improvement of the low-frequency band are achieved by modifying the ground plane structure with characteristic modal analysis. The antenna demonstrates omnidirectional radiation characteristics across both frequency bands. The dimensions of the antenna are π × 0.178 λ₀ × 0.178 λ₀ × 0.049 λ₀ (π × 22 × 22 × 6 mm). The antenna achieves a maximum gain of 3.37 dBi at 2.5 GHz and 2.74 dBi at 5.8 GHz. The measured impedance bandwidths for the two frequency bands are 17.3% and 21.7%, respectively.

In this work, a V₂AlC-based saturable absorber (SA) was applied to a 2 μm band passively mode-locked (PML) laser. The saturation intensity and modulation depth of the V₂AlC-based SA were detected by the I-scan method, which were 225.05 MW/cm² and 7.68%, respectively. In the PML mode, an average power of 2.12 W at 1939 nm and a pulse width of 1077 ps at 82.49 MHz were obtained from the Tm:YAP laser. In addition, the laser exhibited a good beam quality factor (M²) which was measured to be less than 1.2. These results will pave the way for the application of V₂AlC-based SA in ultrafast lasers.

A gas analyzer has been developed using cavity ring-down spectroscopy (CRDS) technology, designed to simultaneously measure the concentrations of CO₂ and CO. These gases are critical indicators of transformer insulation degradation. The analyzer covers a CO₂ concentration range from a few ppm to several thousand ppm and can measure CO/CO₂ concentration ratios from 0.076 to 0.8. The selected absorption lines for CO and CO₂ are centered at 6337.99 and 6338.59 cm⁻¹, respectively. The two candidate lines are proximate to each other and nonoverlapping. The experimental results indicate that the system accurately fits the measured and calculated signals, enabling concentration measurements at the ppm level. This confirms its ability to measure the gas concentrations and ratios critical for assessing the condition of transformer insulation. This CRDS-based system offers a reliable and sensitive method for early detection of potential transformer faults.

The integration of health monitoring and wearable antenna technologies has enhanced patient assessment and the provision of healthcare. Traditional health monitoring methods have evoked a surge in interest toward embroidered antennas as a viable solution for constant physiological monitoring in a noninvasive way. This study examines the performance of an embroidered wearable textile (EWT) antenna intended for breathing rate applications. The antenna is seamlessly integrated into a commercially available T-shirt, extending the confines of “smart” textiles. The compact (45 mm × 26 mm) EWT antenna operates in 2.4 and 5.8 GHz with gains of 2.07 and 5.85 dBi, respectively. Crumpling and stretching analysis have also been investigated to ensure antenna's mechanical stability and reliability. Subsequently a specific absorption rate analysis has also been performed to quantify the radiation within specified limits.

This study explores tightly coupling technology applied in the design of an ultra-wideband (UWB), compact, dual-polarized antenna. The proposed design utilizes tightly coupling effects to achieve UWB performance, with a notable feature being its double-dipole topology. This topology significantly reduces the difficulty of impedance matching across the ultra-wide band. To suppress the significant edge effect that occurs in tightly coupled dipoles, coupled grounded posts are added to the surrounding dipoles, effectively attenuating edge reflections and significantly expanding the bandwidth to lower frequencies. A periodic patch superstrate is loaded above the dipoles to improve the impedance matching and radiation performance. A prototype was fabricated and measured, showing a bandwidth is 1.8–12.5 GHz (149.7%) with VSWR $��\; ��<��$ 2.2, fully covering the S, C, and X bands. The proposed antenna is printed on a thin substrate and its size is very small, only $��\; ��33�\; �\times �\; �33�\; �\times �\; �18.2��$ mm ($��\; ��0.2�\; �\times �\; �0.2�\; �\times �\; �0.11�\; �{\lambda }_L��$), where the $��\; ��{\lambda }_L��$ is the wavelength of the lowest operating frequency.

This paper aims to study the effect of eliminating some elements from the original planar array (OPA) to generate a useful flat-top coverage pattern (FTCP) using a genetic algorithm. The study involves dividing the fully planar array (i.e., OPA) into two regions with an unequal number of elements. The first region includes the elements located in the core (center) of the array, and the remaining elements are the second region, and their location is within the boundary (side) of the array. Only the elements of the first region are included in the optimization process, while the elements of the second region are eliminated in the form of a specific geometric composition without affecting the FTCP generation. Comparing the OPA that N×M elements with the first region (active subarray) elements, it is noted that the number of elements has been reduced from N×M to (N − 2x) × (M − 2y) to reduce the complexity of the system as much as possible without affecting the performance in general. Different boundary subarray shapes are presented in this paper with different numbers of elements. The obtained results showed the effectiveness of the proposed method in generating FTCP close to its counterpart in the OPA.

A new decoupling method for microstrip antenna arrays has been proposed in this paper, which is based on characteristic mode analysis. This method works by suppressing the nonworking mode that contributes the most to mutual coupling. An eight-element microstrip antenna array is designed as the reference antenna, and for this antenna, mode suppression is achieved by etching gaps on the patch and adding metal columns between the patch and the ground. The antenna array after the mode suppression is fabricated and measured. The results show that the measured S₃₄ after the mode suppression is below −21 dB, whereas the simulated S₃₄ of the reference antenna array is about −15 dB. This indicated a 7–10.5 dB reduction in coupling across the band. Additionally, the measured ActiveS₃ after the mode suppression is still below −11 dB. Besides, the measured and simulated normalized array patterns after the mode suppression agree well at different frequencies.