# A Sensor for Characterisation of Liquid Materials with High Permittivity and High Dielectric Loss
###### Abstract
[23] is too low and much noise can be observed during in vivo measurement. The CSRR structure can be found in Refs. [24; 25; 26; 27; 28; 29; 30]. The substrate integrated waveguide structure reported by [PERSON] et al. [24] can only measure the real part of the permittivity. The sensor developed by [PERSON] et al. [25] measured permittivity through the influence of liquid on the operating frequency of an antenna. However, the measurement was greatly influenced by its environment and the deviation is as high as 11.0%. Good sensitivity can be obtained using the sensors in Refs. [26; 27; 28], but the measurement range of the loss tangent is narrow, and they cannot measure corrosive liquids for direct contact between the sensors and samples. In Ref. [29], liquid samples pass through the central hole of the CSRR sensor, which maximizes the sample's exposure to the electric field. However, it is still difficult to distinguish small changes in permittivity. Another CSRR-based liquid sensor working at 200-330 MHz is reported for dielectric measurement of different density ethanol [30]. Again, it is not able to measure corrosive liquids. For the resonance-based method, the resonant structure acts as an LC resonator. The general description of this type of sensor can be found in Ref. [21] and a good review on this topic can be found by [PERSON] et al. [31]. When materials are placed on top of the resonator, interaction between fields and samples will modify the resonant frequency \(f_{\rm r}\) and the corresponding quality factor \(Q\). The permittivity can be retrieved from the variation in \(f_{\rm r}\) and \(Q\). However, the difficulty in the measurement of high-permittivity high-loss measurement lies in its dilemma with high sensitivity. Being inspired by two pioneered designs in Ref. [29] and Ref. [32], we recognize that two approaches may be beneficial to handle this dilemma. One is to increase the field intensity in the resonating area under the condition of unit power input. The other is to use partial filled sample. By combining these methods, the sensitivity and measurement range can be simultaneously improved. To enable measurement of high-loss polar liquids, a multi-element CSRR structure is proposed. In this design, three circular split rings are used, which will prove that ahigher regional electric field can be created. A glass tube passes through the central hole of the coaxial rings, see Figure 1. The glass tube is made of low loss fused quartz, functioning as a liquids holder and partial filling the resonator. These methods enable high sensitivity measurement for high-permittivity and high-loss liquids. In the following sections, the performance of the circular CSRR structure and the rectangular CSRR structure will be compared in detail in Section 2. It is concluded that the circular CSRR structure has higher sensitivity than the rectangular CSRR structure. In Sections 3 and 4, a theoretical model is established based on the simulation results, and extensive simulation analysis is carried out. Measurements and results are presented in Section 5. Section 6 summarizes this work.