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Diagram of the proposed double-folded substrate integrated waveguide (DFSIW) re-entrant cavity: (a) full view; (b) cutaway view (without feedline); (c) exploded view.
Source publication
In this study, an ultra-compact humidity sensor based on a double-folded substrate integrated waveguide (SIW) re-entrant cavity was proposed and analyzed. By folding a circular re-entrant cavity twice along its two orthogonally symmetric planes, the designed structure achieved a remarkable size reduction (up to 85.9%) in comparison with a conventio...
Contexts in source publication
Context 1
... is, the RH condition of the ambient air can be easily retrieved by measuring the corresponding resonant frequency variation of the sensor. Figure 1 illustrates the geometrical structure of the proposed DFSIW re-entrant cavity humidity sensor, which is generated by folding a conventional circular re-entrant cavity twice along its two orthogonally symmetric planes. The structure consists of four dielectric substrate layers sandwiched between multiple metallic layers. ...
Context 2
... experimentally demonstrate its sensing performance, the proposed DFSIW re-entrant cavity humidity sensor was fabricated using standard printed circuit board (PCB) technology. The four layers (shown in Figure 1c) were separately manufactured in the fabrication process. Both the capacitive posts on substrates 1 and 4 were built using a copper plate together with a metallized vias array embedded in the substrate. ...
Context 3
... experimentally demonstrate its sensing performance, the proposed DFSIW re-entrant cavity humidity sensor was fabricated using standard printed circuit board (PCB) technology. The four layers (shown in Figure 1c) were separately manufactured in the fabrication process. Both the capacitive posts on substrates 1 and 4 were built using a copper plate together with a metallized vias array embedded in the substrate. ...
Citations
Here, a six-port network, consisting of power splitter/combiner, two hybrid couplers and a composite right-/left-handed (CRLH) transmission-lines (TLs) voltage-controlled phase shifter (CRLH-TLs-VC PS), is loaded with the reference and sensing re-entrant cavity resonators (RECRs) in substrate integrated waveguide (SIW) configuration to implement a tunable microwave interferometer for high sensitivity and resolution humidity sensing. The humidity sensitive material PEDOT:PSS is deposited only in the sensing SIW RECR to form a phase difference between the reference and sensing RECRs. Then, the phase difference is loaded into the transmission branch of the six-port network and can be easily compensated to 180° by a positive phase provided by the CRLH-TLs-VC PS. Besides, the interferometer implemented in differential configuration not only avoids the use of an additional attenuator, but also can suppress the temperature effect. A series of LabVIEW-based real-time measurement experiments are performed using a vector network analyzer to demonstrate the good humidity response characteristics, such as the high sensitivity of 678.85 kHz/%RH, the high resolution for the discriminable minimum RH change in the range from 0.37% to 5.5%, and the low temperature coefficient of frequency (TCF) of -10.699 ppm/°C.
In this article, a back-to-back (BTB) substrate integrated waveguide (SIW) re-entrant cavity resonator (RECR) is proposed to perform volatile organic compound (VOC) gas detection at room temperature. First, the structural parameters are optimized by finite difference time domain (FDTD) simulation to obtain high
${Q}$
-factor and sensitivity. The strongly induced
${E}$
-field can be further highly concentrated between two BTB capacitive posts, which can effectively improve the filling factor and enhance the interaction of the induced
${E}$
-field with the material under test (MUT). Second, polydimethylsiloxane (PDMS) as a VOC gas-sensitive material is deposited in the gap sensing area, where strongly induced
${E}$
-field is highly concentrated, to achieve high sensitivity to VOC gas when VOC gas penetrates into the pores distributed in the PDMS, resulting in the change of the permittivity of the PDMS due to the swelling of the PDMS. Finally, by injecting VOC gases, such as isopropyl alcohol (IPA), ethanol, and acetone gas, into the sensing bottle, it is experimentally verified that the proposed sensor demonstrates good stability, reliability, and repeatability during the VOC gas absorption and recovery process. The sensitivities demonstrated by the sensor for IPA, ethanol, and acetone gases are 7.82, 4.58, and 9.78 kHz/ppm, respectively, and the limit of detection (LOD) for IPA, ethanol, and acetone gases are 292, 498, and 233 ppm, respectively. The measured results indicate that the sensor demonstrates a great potential for highly sensitive detection of VOC gas concentration in air at room temperature and variable relative humidity (RH).
In this paper, flexible substrate integrated waveguide (SIW) resonators have been designed and fabricated on polyimide substrates for humidity sensing applications. The proposed SIW resonant cavity allows the resonator to obtain the maximum humidity sensitivity and meet the demand for flexible microwave sensing detection. Meanwhile, the humidity response performance can be further significantly enhanced by introducing nanodiamond (ND) sensing material. Three prototypes of ND-coated SIW sensors with different bending radii are measured to analyze their humidity sensing performance. The experimental results demonstrate that the proposed ND-coated SIW sensor with the minimum bending radius can achieve a maximum humidity sensitivity of 1.09 MHz/% relative humidity (RH) in the high RH region (>75.3% RH) and a low humidity hysteresis of 1.8% in the range of 11.3–97.3% RH. This study provides a promising candidate to realize flexible microwave sensors with excellent sensing performance.
This paper fabricated a high-performance chitin nanofibers-coated interdigital electrode-piezoelectric quartz crystal (IDE-PQC) humidity sensor. Firstly, this paper investigated the influence of each Butterworth-Van Dyke equivalent parameter of the piezoelectric quartz crystal on the resonance characteristics of the IDE-PQC humidity sensor. Then, the results of MATLAB calculations revealed that the resonant frequency of the IDE-PQC humidity sensor changes larger (i.e., higher sensitivity) by increasing the PQC electrode area and decreasing the quartz wafer thickness. Next, the theoretical result was proved using advanced design system (ADS) simulation. Finally, the moisture-sensitive performances of IDE-PQC humidity sensors with different electrode areas and quartz wafer thicknesses were tested using chitin nanofibers as the moisture-sensitive material. The experimental results showed that the IDE-PQC humidity sensor with a larger electrode area and thinner quartz wafer thickness has higher sensitivity. This paper provides a new solution for building a high-performance series piezoelectric quartz crystal sensor.
This work presents a differential humidity sensor by longitudinally stacking the sensing and referencing substrate integrated waveguide (SIW) re-entrant cavity resonators (RECRs) for relative humidity (RH) detection. Three different technologies, such as air-filled substrate, metal grid holes (METGH) and sensitive material, are integrated to enhance the humidity sensing response. Firstly, introduction of air-filled substrate and METGH can effectively improve the flowability of the humid air, and can reduce the response time. Secondly, PEDOT:PSS is used as a humidity sensitive (HS) material and deposited in the highly concentrated E-field distribution area of sensing SIW RECR, and the modulation of the dielectric constant of HS material by humid air can achieve high humidity sensitivity. Furthermore, the proposed differential structure itself can suppress common-mode perturbations to prevent temperature cross sensitivity. Several different sensors are fabricated and a series of experiments are performed to investigate the dependence of humidity sensing response on the device structure and the deposited amount of HS material. When a specific amount of HS material is deposited in the HS area, excellent humidity sensing performances such as high sensitivity, low hysteresis and fast response can be successfully achieved. Among them, the sensor
$\text{B}_{{1}}$
(300-uL HS material and with METGH) demonstrates the best humidity sensing performance in the range of 32.8% to 97.3% RH, and shows high sensitivity of 209.3 kHz/%RH and low hysteresis of 0.592% RH, and the corresponding response time is reduced to 1/6 of that without METGH. Moreover, the sensor
$\text{B}_{{1}}$
has excellent suppression on the temperature variation.
In this article a wireless passive temperature sensor is designed, fabricated, and tested. The sensor uses a contactless air-filled substrate-integrated waveguide (CLAF-SIW) resonator that consists of an air cavity surrounded by a low-impedance electromagnetic band gap (EBG) structure. The contactless feature of the sensor mitigates the fabrication complexity issue of air-filled SIW (AF-SIW) while it benefits from the high-quality factor of an air-filled resonator. The resonant frequency of the CLAF-SIW resonator is dependent on the permittivity of the substrate and the dimensions of the structure which are both temperature-dependent. Change in temperature results in a variation in the resonant frequency of the sensor. As such, we measure temperature by measuring the resonant frequency of the sensor. The developed CLAF-SIW sensor has a measured unloaded quality factor of 1340 that allows for long-range wireless measurements. The resonant frequency of the sensor is measured using a ringback-based wireless interrogation system. The interrogation system energizes the sensor using radio frequency (RF) pulses and determines the frequency of the ringback signals radiated by the sensor using frequency domain analysis. The sensor has a sensitivity of 300 kHz/
$^\circ \text{C}$
and an interrogation-to-sensor distance of 80cm.
Interferometric methods of optical sensing based on the phase shift of the Bloch surface waves (BSWs) and guided waves (GWs) supported by a one-dimensional photonic crystal are presented. The photonic crystal, composed of six SiO 2 /TiO 2 bilayers with a termination layer of TiO 2 , is employed in the Kretschmann configuration. Under resonance condition, an abrupt phase change is revealed, and the corresponding phase shift is measured by interferometric techniques applied in both the spectral and spatial domains. The spectral interferometric technique employing a birefringent quartz crystal is used to obtain interference of projections of p-and s-polarized light waves reflected from the photonic crystal. The phase shifts are retrieved by processing the spectral interferograms recorded for various values of relative humidity (RH) of air, giving the sensitivity to the RH as high as 0.029 rad/%RH and 0.012 rad/%RH for the BSW and GW, respectively. The spatial interferometric technique employs a Wollaston prism and an analyzer to generate an interference pattern, which is processed to retrieve the phase difference, and results are in good agreement with those obtained by sensing the phase shift in the spectral domain. In addition, from the derivative of the spectral phase shifts, the peak positions are obtained, and their changes with the RH give the sensitivities of 0.094 nm/%RH and 0.061 nm/%RH for the BSW and GW, respectively. These experimental results demonstrate an efficient optical sensing with a lot of applications in various research areas.
This research investigates a microwave transduction-based humidity sensor that is a promising candidate for real-time clinical healthcare applications and green miniaturized wearable electronic devices. Optimization of sensing material, sensing platform, and device fabrication techniques produces a carbon dots (CDs)-decorated metal organic framework (MOF)-derived porous Co3O4 (CDs-Co3O4) microwave resonator-based sensor with excellent real-time humidity detection. Inspired by the water absorption component polyacrylamide in baby diapers, the acrylamide is adopted to synthesize CDs for microwave humidity sensor. Combining CDs with MOF-derived porous Co3O4 enhances humidity sensitivity under microwave excitation, with a frequency shift of 3.40 MHz/% RH and a loss variation of 0.15 dB/% RH between 5% and 99% RH. These values are 49.7% (for frequency shift) and 20.5% (for return loss) higher than Co3O4 sensor. Moreover, CDs-Co3O4 exhibits high selectivity towards water vapor against other volatile organic compounds, and the response or recovery time are both less than 5 s. Fabricated by an integrated passive device technology, the sensing platform is miniaturized at 0.98 × 0.80 × 0.22 mm³ with superb device stability and reliability. The CDs-Co3O4 sensor remarkably monitors respiratory patterns of breathing or apnea, as well as subtle changes in the humidity levels of an approaching finger. A charge transfer process and microwave interactions are the mechanisms for improved humidity sensitivity.
This letter presents a miniaturized differential microfluidic sensor by longitudinally integrating two substrate integrated waveguide (SIW) re-entrant cavity resonators (RECRs) for characterization of dielectric liquid materials. The mutual coupling between these two RECRs is prevented by the stacked metal wall between them. The feedline implemented by a microstrip-to-stripline-to-SIW transformer in the stacked metal wall is proposed to excite these two RECRs simultaneously. The dependence of complex permittivity of calibrated samples on the resonance frequency shift and quality factor of the sensor is used to build a predictive model of complex permittivity of liquid. The ability of this sensor to accurately measure liquid complex permittivity is verified by practical measurements.
In this paper, a modified substrate integrated waveguide (SIW) re-entrant microwave sensor is implemented by an annular slot loaded SIW re-entrant cavity resonator (RECR) to evaluate complex permittivity of liquid under test (LUT). The fringe electric field (E-Field) around metal post of the SIW RECR is suppressed by loading the plane disk. And then, by loading a narrow annular slot, the corresponding fringe non-linear capacitor is transferred as an annular capacitor which has linear response to real permittivity of LUT and is connected in parallel with the gap parallel plate capacitor of the RECR, and then the corresponding physical equivalent circuit model is accurately established. The dependence of the sensitivity of the proposed sensor on the dimension of the annular slot is discussed. The unique quantitative function relationship, instead of a mathematical fitted one, between the resonant frequency and the real permittivity of LUT is obtained to characterize the real permittivity predictive model by reasonably neglecting the ratio term of annular-slot capacitance to LUT effective capacitance. Additionally, an improved algorithm for more accurately extracting imaginary permittivity is proposed. A prototype of the proposed sensor is fabricated and measured. The predictive model of the complex permittivity is calibrated by using of methanol-water mixtures with known complex permittivity and is further verified by ethanol-water mixtures. The result of experiment verification, with the maximum measurement errors of real and imaginary permittivity of 4.07% and 7.79%, demonstrates that the complex permittivity of the liquid can be accurately characterized by using the proposed sensor.
