The replacement of lumped resistors with absorbing materials for absorptive frequency-selective transmission (AFST) structures enables a wider range of applications in low radar cross section (RCS) radomes. However, the transmission band within the absorption band for most absorbers is naturally nonexistent. It is shown that such a limitation can be overcome with the engineered frequency-selective surfaces (FSSs) and microwave absorbers to obtain a frequency response of absorption–transmission–absorption. In particular, a transmission window of low insertion loss within the absorption band is introduced by the integration of bandpass FSS and wideband absorber, while a bandstop FSS in the front minimizes the insertion loss in the passband. This design methodology is successfully demonstrated by two examples of AFST structures with single and dual polarizations.

Microwave transduction is a novel research field in gas sensing owing to its simplicity, low cost, passive, and non-contact operations that indicate a huge potential in applications for gas sensing. At present, the study of microwave gas sensors is limited to testing the macroscopic performance of materials. Although the permittivity change of materials is the reason for microwave transduction, the knowledge of the fundamental mechanism is an urgent requirement for boosting the development of microwave gas sensors. In this review, we summarized the presented progresses of microwave gas sensors, including the performance of the different materials and propagative structures, as well as their sensitive signal. We have attempted to explain the sensitive mechanism of these materials, discuss the function of the propagative structure, and analyze the sensitive signal extracted from the parameters of electromagnetic waves. Finally, the challenges in the study of sensitive materials and propagative structures used in microwave gas sensors are analyzed. It is clear from the published literatures that microwave transduction provide a new route for gas detection, and it is expected that commercial manifestation will occur. The future research on microwave gas sensors will continue to exploit their sensitive materials and propagative structure for different applications. The understanding of the microscopic polarization interactions with gases will assist in and guide the fabrication of high-performance microwave gas sensors in the future.

This paper presents a design of dual-polarized antenna with embedded filtering circuits by using a printed Jerusalem-cross radiator and a ring slot-coupled feed structure. A square ring slot has been adopted to implement the coupling mechanism. It is excited by two orthogonal microstrip feedlines to achieve dual polarizations. Meanwhile, the slot-coupled feed network and printed Jerusalem-cross radiator form a coupled-resonator circuit that realizes a second-order bandpass filtering performance. Consequently, a dual-polarized antenna with a filtering performance is obtained without using extra filtering circuits. To demonstrate the idea, an antenna prototype operating at LTE bands 42 and 43 (3.4–3.8 GHz) is designed, fabricated, and measured. Measured results show that the proposed antenna provides good filtering and dual-polarized radiation performances. Furthermore, a 4-element dual-polarized linear array is designed for high-gain applications.

In this letter, a behavior model, namely CP- LSTM, composed of Chebyshev polynomials (CP) and a long short-term memory (LSTM) network is built to linearize the wideband millimeter-wave power amplifier (mmW PA) in the fifth-generation (5G) mobile communication system. In order to verify the linearization performance of the CP- LSTM predistorter, a 100-MHz bandwidth 5G new radio (5G NR) signal is employed to test the 28-GHz mmW PA under the text. Experimental results show that the adjacent channel power ratio (ACPR) of the mmW PA with the CP- LSTM can be improved by 20 dB which is 5-dB better than with the LSTM and 3-dB better than with the generalized memory polynomial (GMP). Therefore, the proposed CP- LSTM model is very effective to linearize 5G mmW PAs.

This paper presents a compact circularly polarized UHF RFID tag antenna based on complementary split ring resonator (CSRR). Its operating frequency varies with the injected liquid inside the proposed antenna, which is fabricated by 3D printing technology. Based on this feature, the complex relative permittivity of the liquid under test can be extracted wirelessly at a distance from the reader antenna by measuring the read range. The real and imaginary parts of the complex permittivity of the liquid are then extracted from the frequency with the maximum read range and two neighboring frequency points, at which a read range related parameter is 90% of its maximum value. The circular polarization of the proposed antenna is realized by a pair of CSRRs for achieving a relatively large read range and miniaturizing the antenna size. According to the measurement results, the complex relative permittivity of liquids with real part between 1 and 81 can be extracted from a reader with the distance up to 1.97 m.

A planar low-profile log-periodic antenna based on cavity-backed slot is proposed in this letter. The slot element is made of a microstrip patch on a large ground plane with three edges shorted and one edge open. It is excited by a probe protruding into the cavity from the back of the ground. Through choosing different length and width, each slot element of the array can resonant at different frequency with a same height. A meandered microstrip line on the bottom is used to feed the array, achieving end-fire radiation with wide bandwidth. The proposed antenna has been fabricated using printing technology. Measured results show that an operating frequency band from 6.9 GHz to 17.4 GHz (2.52:1) for VSWR < 2.2 and a low profile of 0.047 $\lambda_L$ at the lowest operating frequency are obtained. The proposed array has a good overall performance of wide bandwidth, low profile, and planar structure with easy printing fabrication process, which may be useful for the demand of end-fire radiation with low profile.