1038 M.T. JLANI, W.P. WEN, ET AL., EQUIVALENT CIRCUIT MODELING OF THE DIELECTRIC LOADED MICROWAVE BIOSENZOR Equivalent Circuit Modeling of the Dielectric Loaded Microwave Biosensor Muhammad Taha JILANI 1 , Wong Peng WEN 2 , Lee Yen CHEONG 3 , Mohd Azman ZAKARIYA, Muhammad Zaka Ur REHMAN 4 1 Dept. of Electrical and Electronics Engineering, 3 Dept. of Fundamental and Applied Sciences, Universiti Teknologi Petronas, Perak, Malaysia 4 Dept. of Physics, COMSATS Institute of Information Technology, Islamabad, Pakistan mtaha.jilani@gmail.com, wong_pengwen@petronas.com.my Abstract. This article describes the modeling of biological tissues at microwave frequency using equivalent lumped elements. A biosensor based on microstrip ring resonator (MRR), that has been utilized previously for meat quality evaluation is used for this purpose. For the first time, the ring-resonator loaded with the lossy and high permittivity dielectric material, such as biological tissue, in a partial overlay configuration is analyzed. The equivalent circuit modeling of the structure is then performed to identify the effect of overlay thickness on the resonance frequency. Finally, the relationship of an overlay thickness with the corresponding RC values of the tissue equivalent circuit is established. Simulated, calculated and measured results are then compared for validation. Results agreed well while the observed discrepancy is in an acceptable limit. Keywords Microwave biosensor, microstrip ring resonator, equivalent circuit modeling, effective permittivity, dielectric constant, meat dielectric characterization, overlay thickness. 1. Introduction Over a decade, electrical properties of materials have gained significant research interest. The measurement of these properties is considered as most promising research area in the field of medical, agriculture and materials engi- neering [1]. The main advantage of these methods is the non-invasive and non-destructive evaluation, which is desirable for many in-line industrial applications. It is due to the fact that the interaction of electromagnetic fields with a material can be investigated to understand the un- derlying physical properties of the material. Whereas, this interaction is mainly governed by the relative permittivity, loss factor and conductivity of a material [2]. When a di- electric material is subjected to the electromagnetic field, its behavior can be defined by [3] * ’ j       (1) where j = √െ1 , İ′ is the real part called dielectric constant and the imaginary part is a loss factor. The dielectric con- stant describes the ability of a material to store energy, whereas, the loss-factor defines energy dissipation of a material. Another, important property of a material to conduct an electric current is known as conductivity σ and it is given as [2] 0 tan / r      (2) where tanį is tangent-loss which is the ratio between loss factor and dielectric constant, ω is the angular frequency, İ 0 is the free space permittivity and İ r is the dielectric constant of the material. These properties can be utilized effectively, once their relationship can be established with the physical attributes of a material. In food industry, the application of dielectric proper- ties is evident into various products, such as agriculture, edible-oil, dairy, and meat. Particularly in meat industry, due to increasing demand for high quality products and strict regulations for health and safety. The need for an ef- fective, online and non-destructive dielectric technique is become obvious. Several efforts have been attempted to identify the relationship of dielectric properties with the quality attributes, like: moisture content, composition, freshness, aging, discrimination of frozen & thawed prod- ucts and microbial activities [4]. Similarly, various new techniques have been proposed and adopted to determine the complex permittivity. Since, biological tissues are het- erogeneous, anisotropic and semi-solid in nature, practi- cally only few techniques are preferred for the measure- ments. The most widely used technique is the coaxial probe, which exhibits high accuracy for broadband meas- urements [2]. It is based on reflection principle, and it is successfully utilized for determination of meat-aging [5], frozen detection [6], fat analysis [7], and to study the tem- perature effect [8]. Although, this method has higher accu- racy but still is subject to errors. The main source of errors are sample thickness (at least semi-infinite for probe) and the air-gap between the sample and a probe, it is reported that they can introduce errors of up to 20% [9]. The irregu- larities on a sample surface can reduce accuracy, therefore to decrease error rate MUT should be smoothen. However,