In this article, a method is proposed for the characterisation of epoxy resin thermal and hygrothermal ageing based on microwave resonator with interdigital-capacitor-shaped defected ground structure (IDCS-DGS), which can realise rapid and simple non-destructive testing of the ageing degree of epoxy resin materials. The resonant frequency of IDCS-DGS was proved to decrease with the increase of dielectric constant of the around material by means of simulation. The breakdown test at high voltage showed the deterioration of epoxy resin after hygrothermal ageing, which is caused by moisture absorption and hydrolysis process. In addition, thermal ageing also caused a decline in the insulation performance of epoxy resin samples. Accelerated oxidation of epoxy resin samples at high temperatures produces polar molecules that increase the dielectric constant of the samples. As for the hygrothermal ageing samples, the entrance of polar water molecules will increase the dielectric constant of epoxy resin, and thus make an effect on the resonant frequency of IDCS-DGS. The tests of transmission performance show that after the hygrothermally aged epoxy resin samples were put on the microwave sensor, the resonant frequency decreased continuously with hygrothermal ageing time, and the deviations of resonant frequency are highly consistent with the weight variation rate of the samples. 

Hygrothermal ageing characterisation of epoxy resin based on transmission characteristics of microwave resonator with interdigital‐capacitor‐shaped defected ground structure

Extensive research has been devoted recently to the design of metasurfaces. Design strategies based on impedance theory stand out for power flow regulation. The designed impedance matrix is characteristic of a given pair of incident and modulated wave fields, hence providing opportunities for encrypted information storage. We consider acoustic vortices propagating along cylindrical shell waveguide as signal carriers. Within the shell waveguide, an annular acoustic impedance metasurface is introduced for encrypted information storage. Information stored in the metasurface would only be retrieved when interrogated with the correct input. Hidden messages are successfully read in numerical simulation when the three-layer model is used to implement the metasurface. A realistic structure based on Helmholtz resonant unit cells is further designed to verify experimentally the encryption storage process. This work is expected to spur more applications of impedance metasurface to acoustic holography and encrypted acoustic communications.

Annular acoustic impedance metasurfaces for encrypted information storage

Moisture ingress significantly accelerates the aging of oil-paper insulated equipment, making accurate assessment of moisture content critical for the operation and maintenance of oil-immersed insulated power equipment. In this paper, a novel method for rapid moisture content evaluation is proposed based on a microstrip ring resonator (MRR) designed with a fundamental frequency of 1.16 GHz. A calculation model for the dielectric response analysis of MRR is established to characterize dielectric properties of the material superimposed on the substrate. ANSYS HFSS electromagnetic simulation software was used to study the impact of dielectric material thickness and the substrate on the MRR test results. Experimental measurement demonstrated that the maximum deviation in resonance frequency was merely 0.542 %. The dielectric properties of uniformly and non-uniformly damp insulating pressboard in the frequency band 1 ∼ 6 GHz were analyzed. The MRR test results were compared with those of the standard resonant cavity method and frequency domain dielectric spectroscopy (FDS) test. Employing the regression algorithm and curve analysis, moisture assessment models for both uniformly and non-uniformly damp oil-paper insulation were established. The experimental results show that the MRR test can clearly distinguish between the pressboard samples with varying moisture levels under uniform conditions, with a maximum moisture evaluation error of 8.19 %. The results demonstrate that the MRR test method, requiring only 56 s, can quickly and effectively evaluate the moisture content of insulating pressboard. 

Dielectric response characteristics and moisture exposure evaluation of oil-paper insulation based on microstrip ring resonator

Moisture ingress is one of the major causes of aging in oil–paper insulated transformers; the measurement of the moisture content of the insulating material is thus necessary during transformer disassembly for maintenance. One critical engineering consideration is the need for rapid and nondestructive moisture content measurement of the oil–paper insulation to minimize the power-outage time. Herein, a rapid characterization method for the moisture content of oil–paper insulation based on the dielectric response obtained using a microstrip petal-like ring resonator (MPRR) is proposed. The geometric parameters of the resonator and dielectric response model were established based on simulations and theoretical analysis. The geometry of the resonant ring was optimized by modifying the originally circular ring structure of the microstrip ring resonator (MRR) into a petal-like ring structure; this increases the number of resonant peaks from five to seven in the frequency range of 0.5–6.0 GHz for the same electrode area. The effects of the coupling mode, coupling gap, and microstrip ring size on the resonance characteristics of the MPRR were simulated and analyzed. Compared with the MRR, the MPRR has the advantages of a reduced fundamental resonant frequency and a smaller resonant-ring area that improves radiation efficiency. The impact of moisture content on the dielectric response of the pressboard obtained using the MPRR was studied. A moisture-calculation equation based on the measured dielectric response for a selected type of pressboard was established by the Lasso regression. The results of verification experiments show that the error of the proposed method is sufficient for practical applications. 

Improved microstrip ring resonator for dielectric response tests and moisture evaluation of oil-impregnated pressboard.

This study investigates the sensitivity enhancement of Complementary Split-Ring Resonators (CSRR) for detecting ammoniacal nitrogen (NH3-N) in water quality applications. Detecting NH3-N at low concentrations is critical for water quality monitoring, necessitating the development of highly sensitive NH3-N sensors. A comparative analysis was performed on the performance of circular and rectangular single-ring CSRRs. These resonators were designed on an FR-4 substrate with a resonant frequency of 2.4 GHz, a relative permittivity of 4.4, and a dielectric constant of 4.3. The evaluation involved analyzing the S-parameters' responses for both CSRR configurations under unloaded and NH3-N loaded conditions. The measurement results indicated that the circular single-ring CSRR exhibited a higher quality factor compared to the rectangular counterpart. Moreover, the circular single-ring CSRR demonstrated superior sensitivity in detecting NH3-N. These findings lay the groundwork for the development of highly sensitive sensors, essential for safeguarding water resources and ensuring water quality across various applications, including environmental monitoring and public health.

Enhancing Ammoniacal Nitrogen Detection in Water Quality Applications Using Complementary Split-Ring Resonators: A Comparison Between Rectangular and Circular Configurations

A novel characterization method for water trees based on the transmission characteristics of complementary split ring resonators

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A novel characterization method for water trees based on the transmission characteristics of complementary split ring resonators