Nonlinear radars can be utilized to detect electronic devices, which are difficult to detect with conventional radars due to their small radar cross sections (RCS). Since the receiver in a nonlinear radar is designed to only receive harmonic or intermodulated echoes from electronic devices, it is able to separate electronic devices from non-electronic scatters (clutter) by rejecting their echoes at fundamental frequencies. This paper presents a harmonic-based nonlinear radar scheme utilizing frequency-modulated continuous-wave (FMCW) signals for the detection of various electronic devices at short range. Using a laboratory experiment setup for FMCW radar at S-band for Tx (C-band for Rx), measurements are carried out to detect electronic devices of various sizes. The results show that the detection of small electronic devices is possible with nonlinear FMCW radar when appropriate system parameters are selected. Furthermore, we also discuss the maximum detectable range estimation for electronic targets using the radar range equation for FMCW nonlinear radar.

Nonlinear radars detect nonlinear (harmonic or intermodulation) responses excited from semiconductor junctions within electronic targets, allowing for the detection of small targets that are otherwise difficult to detect. To properly detect the nonlinear responses from targets, which are typically much lower than the responses at the fundamental frequency, the transmit, and receive antennas in a nonlinear radar, must ensure high isolation between the transmit and receive ports to suppress unwanted harmonics from the transmit side interfering with target responses. In this letter, a circularly polarized dual-band shared-aperture antenna, applicable to nonlinear radar, is presented. The S -band part of the antenna has a center frequency of 3 GHz for transmit, while the C -band part is designed to operate at the second-harmonic band of 6 GHz for receive. The proposed antenna is designed to exhibit high isolation between the transmit and receive ports in both bands. The characteristics of the proposed antenna are demonstrated via simulation and measurement, where it shows good circular polarization (CP) characteristics at both S -and C -bands with high isolation between ports. Furthermore, the performance of the proposed antenna for nonlinear detection is verified through an experiment where a nonlinear target is detected.

With the recent surge in popularity of the Internet of Things (IoT), there also has been a notable increase in research on wirelessly charging IoT enabled electronic devices. Millimeter-wave retrodirective power beaming has recently emerged as a suitable candidate for wirelessly charging small electronic devices. In this paper, we propose and demonstrate a millimeter-wave power beaming concept applicable to IoT ecosystem. The proposed system incorporates a Rotman lens retrodirective beamformer as a Tx unit. To demonstrate the retrodirective power beaming, a Rotman lens beamformer on a waveguide platform is designed to operate at Ka-band. The proposed Rotman lens operates with seven beam ports capable of radiating seven beams within $\pm$ 30° steering range. On the receiving end, a rectenna consisting of a patch array and rectifier is designed. We then conducted an end-to-end power beaming experiment as proof of concept, where the results demonstrate the capability of the proposed system as a retrodirective millimeter-wave power beaming system.

There has been growing interest in the field of wireless power transfer (WPT), especially for a high-power (Watt-level) applications. Rectifiers are essential components for WPT, as they convert incoming RF into DC power. However, conventional Schottky diodes used in WPT exhibit their highest efficiency at low power levels typically less than few Watts. To address this limitation, we introduce a power dividing rectenna system that can handle beyond several Watts of input RF power. The proposed rectenna consists of a single patch antenna integrated with a radial power divider, whose output ports are connected to rectifiers. In this way, incident RF power can be distributed into lower levels at which Schottky diode-based-rectifier can accommodate. Via simulation and measurement, we demonstrate that the input power (about 10 W) received through the proposed structure can be uniformly distributed to each output port of the divider to be rectified at lower power levels, which are then combined at DC to provide several Watts of DC power. The proposed rectenna can be utilized in WPT applications requiring beyond several Watts. Furthermore, by configuring the proposed structure into an array, even higher power handling can be achieved for applications that require hundreds of Watts.

This paper proposes a system for detecting and classifying realistic electronic devices in the presence of linear clutter objects based on a frequency-modulated con-tinuous-wave (FMCW) nonlinear radar. For classification, the proposed system utilizes statistical features of the time-segmented baseband signal envelope of the second har-monic responses from targets, which contain nonlinear co-efficients of targets, and a support vector machine (SVM) is used as the classifier. The system is validated using an ex-perimental apparatus in an anechoic chamber, and a mini-mum allowable SNR is set to ensure accurate target classi-fication in the presence of a linear clutter object. The trans-mit frequency band of the FMCW nonlinear radar used for the experiment is 3.0-3.2 GHz, and the receive frequency band is 6-6.4 GHz, which is the second harmonic of the transmit band. The experimental results show that the proposed system can detect and classify targets with a detection rate of 85.5 % and classification accuracy of 89.2%. The pro-posed system has the potential to provide an effective solution for detecting and classifying unauthorized electronic devices in various scenarios.

As the applications utilizing harmonic backscattering continue to expand, the works on analyzing the radar cross section (RCS) through harmonic radar equation have also gained attention. Although various approaches exist for deriving harmonic RCS, there remains a gap in the theoretical derivation and experimental validation of the radar equation that accurately accommodates the specific harmonic responses of a given nonlinear target. This paper introduces a comprehensive radar modeling approach designed for practical harmonic radar system development. A mathematical model of reference harmonic radar system is proposed, which precisely takes into account the harmonic responses from a reference target engineered with a circuit simulation. To validate the effectiveness of this approach, two experimental scenarios are conducted, demonstrating that the proposed approach can be applied to harmonic radar models. Furthermore, this method enables the radar designers to systematically adjust radar parameters based on specific target characteristics and operational requirements.