In this communication, a microfluidic antenna is proposed for chemical sensor applications. An eighth-mode substrate-integrated-waveguide (EMSIW) antenna with a microfluidic channel is introduced in order to devise a compact and nondestructive chemical sensor. The frequency response of the proposed antenna is controlled by changing the nanoliter liquid in the microfluidic channel, which further results in a change in the effective dielectric constant. First, the EMSIW antenna is designed, and then the microfluidic channel is designed for nondestructive chemical sensor applications. The S-parameters of the proposed microfluidic EMSIW antenna are simulated and measured. The resonant frequency is successfully switched from 4.2 to 4.6 GHz when the concentration of ethanol is changed from 0% to 100%. The possibility of the proposed antenna to be used as an ethanol chemical sensor is demonstrated from the relationship between the resonant frequency and the concentration of ethanol.

In this study, a high-Q circular substrate-integrated waveguide (SIW) cavity resonator is proposed as a non-contact and non-invasive radio frequency (RF) sensor for chemical sensing applications. The design of the structure utilizes SIW technology along with a circular shape to achieve a high unloaded Q factor, which is one of the important requirements for RF sensors. The resonant frequency of the proposed circular SIW cavity sensor changes when a liquid material or a chemical (microliters) is inserted in the sensitive area of the structure. The sensing of liquid materials with different permittivities is accomplished via the perturbation of the electric fields in the SIW configuration. When a microwell that is 4 mm in radius is installed vertically through the center of the bare circular SIW cavity, the operating frequency varies from 5.26 to 5.34 GHz. Similarly, when the microwell contains ethanol, the frequency shifts from 5.26 to 5.18 GHz, and the amplitude of reflection coefficient is shifted from −29 dB to −17 dB; when the microwell contains mixing deionized (DI)-water, the frequency moves from 5.26 to 4.98 GHz (which is also 0% Ethanol in our study), and the amplitude of reflection coefficient is shifted from −29 dB to −8 dB. A high unloaded Q factor is maintained throughout all experimental results. To demonstrate our idea, different concentrations of ethanol are tested and recorded. The experimental validation yields a close agreement between the simulations and the measurements.

This research proposes a substrate-integrated waveguide (SIW) cavity sensor to detect several chemicals using the microwave frequency range. The frequency response of the presented SIW sensor is switched by filling a very small quantity of chemical inside of the fluidic channel, which also causes a difference in the effective permittivity. The fluidic channel on this structure is either empty or filled with a chemical; when it is empty the structure resonates at 17.08 GHz. There is always a different resonant frequency when any chemical is injected into the fluidic channel. The maximum amount of chemical after injection is held in the center of the SIW structure, which has the maximum magnitude of the electric field distribution. Thus, the objective of sensing chemicals in this research is achieved by perturbing the electric fields of the SIW structure.

A frequency-tunable compact antenna based on quarter-mode substrate integrated waveguide (QMSIW) technology to reduce the size of the antenna is proposed. The resonant frequency of the antenna is electronically controlled by a dc bias voltage of a varactor loaded between the QMSIW and an additional parasitic patch on the top plane of the structure, with the bias network designed on the bottom plane. The performance of this antenna is demonstrated experimentally, with its resonant frequency varying from 2.28 to 2.50 GHz by increasing the dc bias voltage from 0 to 30 V.

Microwave resonators working as sensors can detect only a single analyte at a time. To address this issue, a TE₂₀-mode substrate-integrated waveguide (SIW) resonator is exploited, owing to its two distinct regions of high-intensity electric fields, which can be manipulated by loading two chemicals. Two microfluidic channels with unequal fluid-carrying capacities, engraved in a polydimethylsiloxane (PDMS) sheet, can perturb the symmetric electric fields even if loaded with the two extreme cases of dielectric [ethanol (E), deionized water (DI)] and [deionized water, ethanol]. The four layers of the sandwiched structure considered in this study consisted of a top conductive pattern and a bottom ground, both realized on a Rogers RT/Duroid 5880. PDMS-based channels attached with an adhesive serve as the middle layers. The TE₂₀-mode SIW with empty channels resonates at 8.26 GHz and exhibits a −25 dB return loss with an unloaded quality factor of Q ≈ 28. We simultaneously load E and DI and demonstrate the detection of the four possible combinations: [E, DI], [DI, E], [E, E], and [DI, DI]. The performance of our proposed method showed increases in sensitivity (MHz/ε_r) of 7.5%, 216%, and 1170% compared with three previously existing multichannel microwave chemical sensors.

Evaluation and non-destructive identification of stress-induced cracks or failures in metals is a vital problem in many sensitive environments, including transportation (steel railway tracks, bridges, car wheels, etc.), power plants (steam generator tubing, etc.) and aerospace transportation (landing gear, aircraft fuselages, etc.). There are many traditional non-destructive detection and evaluation techniques; recently, near-field millimeter waves and microwave methods have shown incredible promise for augmenting currently available non-destructive techniques. This article serves as a review of developments made until now on this topic; it provides an overview of microwave scanning techniques for crack detection. This article summarizes the abilities of these methods to identify and evaluate cracks (including describing their different physical properties). These methods include applying filters based on dual-behavior resonators (DBRs), using complementary split-ring resonators (CSRRs) for the perturbation of electric fields, using waveguide probe-loaded CSRRs and using a substrate-integrated-waveguide (SIW) cavity for the detection of sub-millimeter surface and subsurface cracks.