IDC Planar RF Sensors for Dielectric Testing and Applications

Abstract

In recent years, the design and implementation of various types of sensors in the radio frequency (RF) and microwave frequency region for material characterization and testing has gained a lot of attention from various researchers around the globe. The characterization and testing of dielectric materials in the RF and microwave frequency band primarily involves estimating the dielectric properties of materials in the specified frequency band for design and development of various types of modern RF devices and circuits. However, the RF dielectric testing procedure has also become quite attractive for several real-word applications apart from estimating the dielectric properties of materials due to the fact that it is basically a non-invasive and non-destructive process. The RF sensor is a generic term quite often used to represent the electronic device or the hardware, which is required to be designed and developed to facilitate the accurate testing of dielectrics in the RF and microwave frequency regime. In the earlier days, the conventional RF sensors were primarily based on the metallic waveguide or the coaxial line structures. The metallic waveguides or the coaxial lines are basically non-planar bulky structures, employed in the past to design various RF circuit components and devices. However, in modern times, these non-planar structures are mostly replaced with the planar configurations in order to realize the compact RF devices and components. It is mainly due to this reason that the planar RF sensor has emerged as the viable alternative to the conventional waveguide and coaxial sensors for the characterization and testing of dielectric materials in the specified frequency range. The planar RF sensors offer small size, lightweight, compact design, and ease of integration with other RF circuits based on the planar technology. Now, as mentioned earlier, apart from the direct applications of material testing, the planar RF sensors are now also being employed for certain indirect applications in the food, agricultural and bio-medical industries. For these industries, the major application emerging in recent years has been to detect the quality of various types of edible and medical products. It appears that in order to achieve accurate dielectric testing and to successfully realize various types of indirect industrial applications mentioned above, the sensitivity of the designed planar sensor should be reasonably high. As a matter of fact, the sensor with higher sensitivity is most suited to detect any type of adulteration in the edible or the medical grade product. In the last few decades, it has emerged that the planar RF sensors employing the concept of electrically small structures have reasonably higher sensitivity, thus being well suited to accurately estimate the complex permittivity of materials. These electrically small configurations are usually realized using the engineered planar structures in order to achieve excellent sensitivity due to the concentrated electric field, which are becoming quite popular in recent years for direct application in the RF industry to perform the dielectric testing of materials and media. The split ring resonator (SRR), the complementary split ring resonator (CSRR), and the interdigital capacitor (IDC) structures may be considered as a few of the basic engineered structures for these types of applications. However, the IDC structure is found to be more appropriate for certain dielectric testing applications than that of the CSRR and SRR due to its relatively high sensitivity and the overall compact size, as it can directly be etched on the main signal line, thus providing quite high sensitivity. This chapter is mainly focused on the IDC based planar RF sensor and its implementation for various dielectric testing applications. A detailed analysis, including the theoretical description involving the quasi static model and the electromagnetic model of a typical IDC based sensor structure, along with the associated dielectric sensing mechanism, is provided. Both the resonant and the non-resonant variants of the IDC structure including their working principle relevant to the dielectric sensing mechanism are discussed in detail. Finally, the use of the planar IDC RF sensors for various industrial, environmental, and biomedical applications based on their design configurations are discussed.