volume | PIER Journals

Abstract

This research introduces a compact and highly selective tri-coupled line microstrip bandpass filter. The design features a narrow capacitive gap positioned at the midline to disrupt symmetry and facilitate bandpass functionality, as predicted through an even- and odd-mode image impedance framework. The split at the midline generates two modal capacitances (C_gg, C_gb), which influence Re (Z_i) and, in conjunction with geometric coupling, determine the passband and roll-off characteristics. Closed-form relationships for microstrip design are utilized to compute line widths and electrical lengths. A systematic parametric analysis demonstrates how the gap and interline spacing impact the fractional bandwidth and the steepness of the transition. Additionally, a substrate survey across dielectric constants ranging from 2 to 12.2 quantifies the trade-off between footprint and selectivity, indicating an area reduction of up to approximately 86% at higher dielectric constants. The selectivity is further enhanced by incorporating auxiliary shunt open stubs that introduce transmission zeros near the edges without necessitating additional resonator sections. A prototype fabricated on an FR-4 substrate operating at 2.4 GHz confirms the theoretical model: the measured |S₂₁| exhibits an insertion loss of approximately 0.58 dB, a fractional bandwidth at 3 dB of approximately 37.3%, a shape factor of 1.3, and two prominent TZs near 1.7 GHz and 3.1 GHz with rejection levels of 48-52 dB. Furthermore, the upper stopband maintains |S₂₁| < -35 dB within the frequency range of 3.10 to 3.20 GHz. These findings substantiate that a single TCL section, featuring a central gap and open stubs, can achieve sharp roll-off and low insertion loss while maintaining minimal layout complexity and enabling straightforward tuning on low-cost printed circuit board materials.

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Dr. Moretadha J. Kadhom