Recent developments in the field of microwave planar sensors have led to a renewed interest in industrial, chemical, biological and medical applications that are capable of performing real-time and non-invasive measurement of material properties. Among the plausible advantages of microwave planar sensors is that they have a compact size, a low cost and the ease of fabrication and integration compared to prevailing sensors. However, some of their main drawbacks can be considered that restrict their usage and limit the range of applications such as their sensitivity and selectivity. The development of high-sensitivity microwave planar sensors is required for highly accurate complex permittivity measurements to monitor the small variations among different material samples. Therefore, the purpose of this paper is to review recent research on the development of microwave planar sensors and further challenges of their sensitivity and selectivity. Furthermore, the techniques of the complex permittivity extraction (real and imaginary parts) are discussed based on the different approaches of mathematical models. The outcomes of this review may facilitate improvements of and an alternative solution for the enhancement of microwave planar sensors’ normalized sensitivity for material characterization, especially in biochemical and beverage industry applications.

This paper presents novel structures of planar microwave sensors for detecting and characterizing the dielectric properties in common solid materials which produce high Q-factors with capability to suppress undesired harmonic spurious. These sensors are based on novel planar symmetrical split ring resonator with spurlines filters by employing the perturbation theory, in which the dielectric properties of the resonator affect the Q-factor and resonance frequency. The proposed sensors achieve narrow resonance with low insertion loss and high-Q and sensitivity which peaked up to 652 at 2.22 GHz operating frequency. By using a specific experimental methodology, practical materials are applied as standards (Roger 5880, Roger 4350, FR4) to validate the sensitivity of the sensors for permitting potentially material characterization and detection. Accordingly, the mathematical equation is derived to extract the materials with unknown properties. The average accuracy percentage of the measured results for all cases of the designed sensors is found within 97%-98% compared with those in the literature for the same tested standard materials. It is believed that these sensors would lead for a promising solution of characterizing material particularly in determining material properties and quality, such as in food industry, bio-sensing medicine applications, and therapeutics goods detections.

In this study, a critical evaluation of analyte dielectric properties in a microvolume was undertaken, using a microwave biochemical sensor based on a circular substrate integrated waveguide (CSIW) topology. These dielectric properties were numerically investigated based on the resonant perturbation method, as this method provides the best sensing performance as a real-time biochemical detector. To validate these findings, shifts of the resonant frequency in the presence of aqueous solvents were compared with an ideal permittivity. The sensor prototype required a 2.5 µL volume of the liquid sample each time, which still offered an overall accuracy of better than 99.06%, with an average error measurement of ±0.44%, compared with the commercial and ideal permittivity values. The unloaded Q_u factor of the circular substrate-integrated waveguide (CSIW) sensor achieved more than 400 to ensure a precise measurement. At 4.4 GHz, a good agreement was observed between simulated and measured results within a broad frequency range, from 1 to 6 GHz. The proposed sensor, therefore, offers high sensitivity detection, a simple structural design, a fast-sensing response, and cost-effectiveness. The proposed sensor in this study will facilitate real improvements in any material characterization applications such as pharmaceutical, bio-sensing, and food processing applications.

This letter presents a high efficiency and accurate measurement of microwave resonator sensor using microstrip planar structural technology. The structure is designed based on the bridge split ring topology. The sensor has the ability to characterize liquid solvents using the extraction of polynomial fitting technique. The bridge equivalent circuit configuration allows the electromagnetic wave to focus on the sensing medium in order to gain high electric flux density, thus react as pushing effect for high Q factor. The bridge structure amplifies the electric field distribution. As demonstrated by the results, the unloaded Q _u -factor improves more than 400 over the narrow bandwidth at operating frequency of 2.3 GHz, and provides less than 10-dB insertion loss. By comparison, this design has an identical performance goal and provides excellent sensing capability which can be implemented in pharmaceutical, biomedical, and beverage industries application.

A new design of teeth gear-circular substrate integrated waveguide (TG-CSIW) sensor to extract complex permittivity of liquids is suggested in this study. The proposed sensor size is decreased up to 20% after the Angle Between Input and Output Ports (ABIOP) was introduced. As the outcome, there was a very excellent quality factor of 700 unloaded. The sensor operated at 2.45GHz resonant frequency, and it is progressively designed by the ANSYS program HFSS. The sensor’s simulated response was evaluated by different permittivity of liquid under test (LUT) with an amount of 0.11mL aqueous. A strong correlation is obtained between the simulated and experimental findings of complex permittivity of LUTs with a minimal error of less than 0.4%. In addition, the modelled sensor was employed to estimate the complex permittivity of unknown specimens. It has contributed to a miniaturized, low-cost, secure, non-contact, fast-detection system utilizing limited liquid volumes filled into Polypropylene tubes. The proposed sensor is practically useful for food or beverage products, fluids/tissues of organic and natural herbs industry applications.