Determination of FR-4 Dielectric Constant for Design of Microstrip Band-Stop Filter Purposes

In this research, the design and practical realization of microstrip Band-Stop (BS) filter using FR-4 substrate is presented. FR-4 is low-cost substrate with varying dielectric constant, especially at higher frequencies. Therefore, the measurement of FR-4 dielectric constant based on open resonant cavity method is used. Various samples of Printed Circuit Boards (PCB) were used for measurement. The results acquired from measurements of PCBs were different for each piece of PCB. Finally, it turned out that precision of measurement method is dependent on geometric dimensions of PCBs. Obtained frequency dependent dielectric constant improves accuracy of mathematical model of BS filter created in MATLAB. This model also includes attenuation losses in both copper and substrate materials. For verification of filter model created in MATLAB, scattering parameters of various filter models were compared with scattering parameters of same models obtained from full-wave Electro-Magnetic (EM) simulator. One of these BS filter configurations was practically realized. Scattering parameters of MATLAB filter models, filter models created in full-wave EM simulator and practically realized BS filter are in very good agreement.