In this paper, we theoretically propose and experimentally demonstrate a compact, optically-transparent metasurface radome with asymmetric electromagnetic absorption for making low-radar cross section (RCS) and gain-enhanced multifunctional antennas. The proposed unseeable metasurface has a bilayer structure consisting of periodically patterned and unpatterned transparent conductive films separated by a thin acrylic layer. Such a bilayer metasurface is highly reflective when illuminated by microwave from one side, while exhibit a high absorption when illuminated from the other side. Moreover, when the optically-transparent, weather-proofing bilayer metasurface acts as a radome, it can greatly enhance the gain and reduce RCS of the solar panel-integrated microstrip antenna without affecting the performance of optical devices (e.g., photovoltaic panels, flat panel displays, or light emitting devices). We provide the analytical formulation and design guidelines for the bilayer metasurface and the integrated cavity antenna. Our experimental results show that the realized gain of the microstrip antenna can be increased by 6.1 dBi and its RCS can be reduced by more than 20 dB around the operating frequency of 8.1 GHz. The proposed low-profile, flexible, hydrophobic, and optically-transparent bilayer metasurface may be beneficial for many applications, including the next-generation radomes, self-powered 5G/6G base stations, satellite communication (CubSat), and other compact, multifunctional RF and microwave modulus integrated with optical sensors, lidar, displays, and solar panels.

We propose here a fully-passive wireless liquid sensor using a harmonic transponder, which comprises a dual-band microstrip antenna reconfigured by different types of liquids injected in a fluidic cavity. Different from traditional radio-frequency (RF) backscatter sensors, the proposed harmonic-transponder sensor (or harmonic sensor) receives frequency-hopped RF monotones and backscatters their second harmonics, with the peak frequency shifted by dielectric properties of liquid mixtures. This microstrip antenna has a hybrid-feed structure, of which an outer split-ring patch exhibits a narrow-band TM ₃₁₀ mode at the fundamental frequency ( ${f}_{0}$ ) and an inner elliptical patch displays a wideband resonance centered at the second-harmonic frequency ( $2{f}_{0}$ ), achieved with hybridization of TM $_{e110}$ and TM $_{o110}$ modes. In particular, the outer split-ring patch is loaded with a fluidic channel system to tune the resonance frequency of the TM $_{3}10$ mode ( ${f}_{0}$ ). We demonstrate that the type of liquid mixture filling in the fluidic cavity can be clearly perceived by reading the peak received signal strength indicator (RSSI) in the spectrum of second harmonics. Our results show the potential for deploying this passive wireless sensor in noisy environments that include clutters, multiple reflections, jamming, and crosstalks.

Cough detection has aroused great interest because the assessment of cough frequency may improve diagnosis accuracy for dealing with several diseases, such as chronic obstructive pulmonary disease (COPD) and the recent COVID-19 global pandemic crisis. Here, we propose and experimentally demonstrate a wireless smart face mask based on a passive harmonic tag for real-time cough monitoring and alert. Our results show that the cough events can be successfully monitored through non-contact track of the received signal strength indicator (RSSI) at the harmonic frequency. Owing to the frequency orthogonality between the launched and backscattered radio-frequency (RF) signals, the harmonic tag-based smart mask can well suppress the electromagnetic interferences, such as clutters and crosstalks in noisy environments. We envision that this zero-power and lightweight wireless wearable device may be beneficial for cough monitoring and the public health condition in terms of tracking potential contagious person and virus-transmissive events.

In this Letter, we propose and experimentally validate a compact hybrid-fed microstrip antenna for the harmonics-based radar and sensor systems, which receive the fundamental tone and re-transmit the modulated second harmonic. The proposed microstrip antenna is based on a simple single-layered and dual-feed structure, consisting of an inner circular patch operating in the TM110 mode (second harmonic; 6 GHz) and an outer split-ring patch operating in the higher-order TM210 or TM310 mode (fundamental frequency; 3 GHz). By varying the feed position and the geometry of the split-ring patch, specific higher-order mode can be excited at the fundamental frequency. Moreover, analytical expressions were derived for calculating the antenna's resonant frequencies. Our measurement results show that the maximum realized gain is 1.17 dB at 3 GHz and is 3.33 dB at 6 GHz, with good impedance matching (|S11| < -15 dB) and high isolation (< -20 dB).

In this article, we design and experimentally demonstrate a flexible, low-profile harmonic transponder sensor, capable of performing multiplexed fluidic sensing. The harmonic transponder comprises multiple reconfigurable electrically small antennas (ESAs) integrated with microfluidic channels for receiving radio frequency (RF) signals at the fundamental frequencies (f₀), as well as a broadband microstrip monopole for retransmitting the second-harmonic signal (2 f₀) to a sniffer. Such a frequency orthogonality can help suppressing possible echoes, clutters, and crosstalks in the rich-scattering environment. We show that injection of different liquid samples into a microfluidic channel, which tunes the operating frequency of an ESA, can be precisely recognized by analyzing the shift of peak second-harmonic received signal strength indicator (RSSI) in the frequency-hopping spread spectrum (FHSS). We also demonstrate the possibility of performing rapid and multiplexed noncontact detection with the proposed harmonic transponder sensor. We envision that this battery-free and lightweight multiplexing wireless sensor may benefit various Internet-of-Things (IoTs) and healthcare applications, such as rapid contactless point-of-care (POC) and drive-through tests.

Harmonic transponder sensors, receiving radio-frequency (RF) signal and converting it to a modulated second harmonic, have been demonstrated to be effective for signal interrogation in environments with strong clutters. Here, we propose a compact dual-resonance microstrip antenna consisting of an elliptical patch loaded with shorting pins. Such an antenna utilizes the TMe110 resonant mode to intercept the perpendicularly-polarized fundamental tone (2.86 GHz) and uses the TMo110 resonant mode to retransmits the parallelly-polarized second harmonic (5.72 GHz). Moreover, we have developed a harmonic-based wireless liquid sensor comprising the proposed antenna and a micromachined liquid channel. Our measurement results show that even in the noisy environment, the second harmonic strength can precisely indicate the dielectric property of liquid filled in the microfluidic tank (e.g., acetone-water mixtures with various concentrations). However, a similar setup without frequency and polarization modulations (i.e., linear and passive backscatter tag) fails to provide a sensitive and quantitative measurement of liquid property. The proposed low-profile dual-resonance antenna with sensing capability will pave the way for the development of harmonic sensors and nonlinear radio-frequency identification (RFID) tags.