A low-profile double U-slot microstrip patch antenna with a wide operating bandwidth has been developed in this article. This design produces the dual radiative resonant modes adjacent to each even at the thickness of the antenna of $0.02 {\lambda _{0}}$ . An extremely stable radiation gain that varied from 6.2 to 6.8 dBi is achieved within the whole operating band. In addition, detailed parametric studies in this article provide antenna engineers with simple approximate rules that result in an excellent first-pass design with prescribed characteristics requiring only minimal tuning. Finally, the proposed antenna with the thickness of $0.04 {\lambda _{0}}$ is fabricated and measured. Simulated and measured results are found in good agreement with each other. They have evidently illustrated that the bandwidth is increased by 50% compared to the traditional U-slot antenna. With the help of these simple design procedures and examples in this article, antenna designers could easily develop a low-profile broadband antenna.

Gesture air-writing is an advanced nontouching interaction method that replaces traditional typewriter keyboards, touch screens and other input devices. Due to its low power consumption, noncontact detection and independence from light conditions, millimeter wave radar has become a valuable gesture air-writing solution. In this paper, we proposed a prototype of a gesture air-writing tracking system that is based on the 24-GHz frequency-modulated continuous-wave (FMCW) radar system-on-chip (SoC). The transmitted chirp signal of this radar chip covers up to a 4-GHz bandwidth, which provides sufficient range resolution to track hand gestures. With the development of single input, multiple output (SIMO) antennas, the air-writing symbols can be reconstructed in an observation plane. A system design and signal processing algorithm for gesture air-writing applications is proposed for the prototype system in this paper. To test the performance of the proposed method, experiments with cases of five simple gestures, air-writing numbers and letter tracking are carried out. The experimental results verify the efficiency and accuracy of the proposed method.

Automatic fall detection plays a significant role in monitoring the health of senior citizens. In particular, millimeter-wave radar sensors are relevant for human pose recognition in an indoor environment due to their advantages of privacy protection, low hardware cost, and wide range of working conditions. However, low-quality point clouds from 4D radar diminish the reliability of fall detection. To improve the detection accuracy, conventional methods utilize more costly hardware. In this study, we propose a model that can provide high-quality three-dimensional point cloud images of the human body at a low cost. To improve the accuracy and effectiveness of fall detection, a system that extracts distribution features through small radar antenna arrays is developed. The proposed system achieved 99.1% and 98.9% accuracy on test datasets pertaining to new subjects and new environments, respectively.

A wideband hybrid phase shifter (PS) with low phase and amplitude error is presented, consisting of a 180 $^{\circ}$ reflection-type PS (RTPS) and a phase invertible variable gain amplifier (PIVGA). The effects of a nonideal coupler and reflective loads on the limitation of RTPS’s bandwidth are analyzed. To achieve a wide operation bandwidth and a compact chip area, the 180 $^{\circ}$ RTPS is realized by cascading three stages of 60 $^{\circ}$ RTPS with capacitive load (CL). A PIVGA follows the RTPS to achieve a full 360 $^\circ$ phase-shift range (PSR) and compensate for the loss of RTPS. The proposed wideband 6-bit PS is implemented in a 40-nm CMOS process with a core area of 0.11 mm $^{2}$ . It achieves 39.3% (22.5–33.5 GHz) fractional bandwidth (FBW) with low rms phase and an amplitude error of less than 2.8 $^{\circ}$ and 0.45 dB, respectively. Moreover, the average gain of the proposed PS is 2.5 dB at 28.5 GHz with $S11$ and $S22$ both lower than $-$ 8 dB from 22.5 to 33.5 GHz.

This letter presents a fractional-N frequency synthesizer with an ultrawideband continuously tuning range of 32.4% centered at 26.85- and 8.7-GHz chirp for frequency-modulated continuous-wave (FMCW) radars. The synthesizer uses a fractional-N 9-GHz charge-pump phase-locked loop (CPPLL) cascaded by an injection-locked frequency tripler (ILFT). A voltage-controlled oscillator (VCO) with transformer (TFM) coupling for an enhanced linear control voltage range, a better quality factor and good isolation from the load seen by the resonator, and a wide output voltage range charge-pump (CP) are used to achieve the ultrawideband CPPLL. The ILFT is implemented using a weakly coupled TFM for broadband locking range and 3-dB output power. The synthesizer is fabricated in the 40-nm CMOS technology and consumes 28.5 mW. The phase noise (PN) of the synthesizer at 27 GHz is -97.87 dBc/Hz at 1 MHz and the prototype is capable of generating a triangle or sawtooth chirp of 8.7-GHz modulation bandwidth with an rms FM error of 0.04% of BWchirp.

This article presents the design methodology of millimeter-wave wireless power transfer (WPT) system and the implementation of a 24-GHz beam-steerable multinode WPT system. We first propose a new figure of merit (FoM) to evaluate the cost efficiency and spatial coverage of WPT system. Considering loss in a practical system, we establish a mathematical model to optimize FoM and draw design curves to get some insights into tradeoff among frequency, area cost, and power efficiency at the target distance. To demonstrate the utility of the design methodology, we implement a 24-GHz WPT system consisting of an 8 $\times$ 8 phased-array transmitter with 56-dBm effective isotropic radiated power (EIRP) and 5 $\times$ 5 high-gain angle-deflection planar rectennas with LED loads. The spacing between the transmitter (TX) and receivers (RXs) is 1 m and the aperture size of TX antenna array and high-gain receiving antenna is 6 cm $\times$ 6 cm. Due to the beam-steering capability of TX, no calibration is required for the precise placement of individual rectennas. The measured results show that all LEDs can be lightened with maximum 5.7-dBm dc power and custom patterns can be displayed using such LED array by sequential beamforming of the TX.