In this article, a modified rectangular resonant cavity consisting of a rectangular waveguide (RWG) and two frequency selective coupled (FSC) end‐plates with embedded parallel metallic strips (PMS) is proposed. The proposed FSC end‐plate converts the semi‐constant coupling mechanism of the conventional iris‐coupled end‐plate to the tunable FSC characteristics. This is due to adding capacitive coupling properties to inductive coupling characteristics of the conventional iris‐coupled end‐plate by embedding PMS inside the iris‐hole. So, the proposed FSC end‐plate enhances output power of the resonant frequency of the cavity. Consequently, the proposed modified cavity can determine the permittivity properties of the high‐loss specimens. Also, the parametric study of the effect of variation of the physical dimensions and position of these strips to tune and trim the performance of the proposed FSC end‐plate is presented. Also, the present study proposes an approach for determining the maximum complex permittivity with the rectangular resonant cavity perturbation method. Based on WR‐229 RWG specifications, conventional and investigated cavities are fabricated and measured. The evaluated results of the proposed configuration and conventional model indicate about 184 determination enhancement in the imaginary part of the dielectric constant. Also, this design is validated by distilled water specimen and TiO2 films at a various partial pressure of oxygen that are deposited by utilizing radio frequency magnetron sputtering technologies and their complex permittivity are determined by manufactured cavities.
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