We have proved in theory and simulation that the circularly polarized (CP) radiation from a resonant quadrifilar helix antenna (QHA) is independent of the phase between two composed bifilar helices, while the CP radiation pattern can be reconfigured by controlling this phase. It is demonstrated that the QHA is capable of generating a wide range of CP radiation patterns, including two cardioid patterns toward both ends, two vertical toroid patterns in two different orientations, and the mixed patterns of cardioid and toroid. A wideband and multilayer feeding network was developed to excite the four basic modes of the proposed QHA through phase control in the experiment. The simulated and measured right-handed circular polarization (RHCP) radiation patterns agree quite well. The proposed QHA was also designed to cover several global navigation satellite system (GNSS) bands. The measured impedance bandwidth is 1.26–1.65 GHz (26.8%) and the 3 dB axial ratio bandwidth is 1.1–1.8 GHz (41.4%) in the endfire mode. It is shown that the proposed QHA can be easily operated in two endfire modes to have an equivalent isotropic pattern for flying objects. When the antenna is operating in the endfire mode, the gain is in the range of 2.1–3.5 dBi, and the radiation efficiency is 70%–85% in the operating band.

This article presents our study on a wideband miniaturized 4 $\times$ 8 Butler matrix (BM) beamforming network based on a stacked metallic waveguide layout in millimeter-wave band. The size and complexity of the 4 $\times$ 8 BM have been substantially reduced by introducing a double-layer structure to avoid eight cross-overs and by using reverse feeding to eliminate four band limiting 180 $^{\circ}$ phase shifters. Additionally, by introducing two kinds of wideband hybrid couplers in $H$ -plane and $E$ -plane and a broadband 45 $^{\circ}$ phase shifter realized by loading inductive and capacitive irises in both paths, the operating bandwidth of the proposed 4 $\times$ 8 BM has been substantially increased. Furthermore, a Chebyshev amplitude distribution among the eight outputs has been realized through the unequal power dividers to suppress the sidelobes. The beamforming network and magnetoelectric (ME)-dipole array have been verified in the experiment, showing an operating bandwidth of 60–80 GHz with $S_{\mathrm{11}}< -15$ dB. The sidelobe of the radiation pattern is below $-$ 14 dB at 70 GHz and the peak gain is 18 dBi in the operating band. The beam switching angles are $\pm$ 11 $^{\circ}$ and $\pm$ 35 $^{\circ}$ for each port, respectively. Compared with the reported similar work on 4 $\times$ 8 BM, the bandwidth is increased by threefold and the size is reduced by more than 50%.

Cell dielectric property measurement holds significant potential for application in cell detection and diagnosis due to its label-free and noninvasive nature. In this study, we developed a biosensor designed to measure the permittivity of liquid samples, particularly cell suspensions at the nanoliter scale, utilizing microwave and millimeter wave coplanar waveguides in conjunction with a microchannel. This biosensor facilitates the measurement of scattering parameters within a frequency domain ranging from 1 GHz to 110 GHz. The obtained scattering parameters are then converted into dielectric constants using specific algorithms. A cell capture structure within the microchannel ensures that cell suspensions remain stable within the measurement zone. The feasibility of this biosensor was confirmed by comparison with a commercial Keysight probe. We measured the dielectric constants of three different cell suspensions (HepG2, A549, MCF-7) using our biosensor. We also counted the number of cells captured in multiple measurements for each cell type and compared the corresponding changes in permittivity. The results indicated that the real part of the permittivity of HepG2 cells is 0.2–0.8 lower than that of the other two cell types. The difference between A549 and MCF-7 was relatively minor, only 0.2–0.4. The fluctuations in the dielectric spectrum caused by changes in cell numbers during measurements were smaller than the differences observed between different cell types. Thus, the sensor is suitable for measuring cell suspensions and can be utilized for label-free, noninvasive studies in identifying biological cell suspensions.

Multi-band millimeter-wave (mm-wave) circuits are crucial for achieving high speed, high efficiency, flexible networking, and reducing hardware costs in mm-wave communication. This tutorial overviews three main types for multi-band mm-wave circuits, including wideband, concurrent multi-band, and reconfigurable multi-band. The implementation methods of multi-band circuits are analyzed and compared in detail from the perspective of device and circuit topology. The tutorial systematically reviews typical multi-band mm-wave circuits, including low noise amplifiers (LNAs), power amplifiers (PAs), variable gain amplifiers (VGAs) and attenuators (ATTs), phase shifters (PSs), mixers, signal sources, and transceivers (TRXs). Finally, the future trends are also discussed.

This paper presents a novel single-cell biosensor with pico-liter scale sensitivity. The high-sensitivity microwave biosensor integrates a microchannel and a coplanar waveguide (CPW) to facilitate the capture and release of single cells. This biosensor can identify and differentiate solution concentrations, various particles, and tumor cells by measuring the amplitude and phase of the transmission coefficient. The CPW structure enhances the electric field intensity in the sensing area by increasing capacitance, which reduces the sensing area size and improves sensor sensitivity. By measuring the amplitude and phase of the wide-band transmission coefficients of tumor cells, we achieved identification of subtle changes in the electrical properties of different cell types. The sensor is capable of detecting individual particles and cells. To validate its performance, we measured different concentrations of bovine serum albumin (BSA) solution, achieving a 0.5° phase difference, confirming the sensor’s capability to recognize minor changes in pico-liter samples. The measurement of three types of 10 μm particles verified the sensor’s ability to capture, release, and distinguish between different particle types. Measurements of three cell types showed that despite their small differences, the sensor could still distinguish them. Statistical results from 20 repeated measurements demonstrate the sensor’s feasibility in distinguishing single cells. This non-invasive, label-free sensor holds significant potential for tumor cell recognition, differentiation, and diagnosis.

This article presents a high-sensitivity biosensor based on a resonant slot structure with an embedded electrode. The electrodes are positioned at the center of the slot to concentrate the electric field, thereby significantly enhancing sensitivity. Through comparative analysis of various sensing structures and electrode dimensions, it is demonstrated that the proposed sensor exhibits substantially larger frequency shifts. The results underscore the sensor’s high sensitivity and its advantages for detecting trace liquid samples. The slot resonator structures are fabricated, and microfluidic channels are monolithically integrated using micro-electro-mechanical system (MEMS) technology. Experiments are conducted using low-volume bovine serum albumin (BSA) solutions at different concentrations, micro-particle suspensions, and clustered tumor cell suspensions. The slot sensor exhibits frequency shifts at the MHz level in response to individual micro-particles and cells, confirming its capability for high-sensitivity microwave detection. To reduce environmental interference, a low-temperature co-fired ceramic (LTCC) multilayer-based slot resonator with nonradiative characteristics is further developed. Integrated with a 3-D-printed microfluidic channel, the resulting millimeter-wave sensor achieves ultratrace sample detection. Measurements of BSA solutions show that a 2.5% change in concentration at the single-cell level induces a frequency shift of 10MHz, validating the high sensitivity of the sensor.