This work presents the design of a tri-bandpass filter based on the side-coupled square ring structure. The modeling is conceived from the classic approach of coupled transmission lines. The geometry consists of a square structure of electrical length equal to twice the wavelength of the main operating frequency, and the laterally coupled lines through which the power supply occurs. Expressions for the admittance and transformation matrix were obtained based on the impedances and admittances that model the geometry. Analyzis were performed from EM simulations. A prototype was built to validate the modeling and adjusted for operation at 0.78, 1.5 and 2.30 GHz. All results are discussed and commented, and the comparisons between simulated and measured results are coherent and in agreement, thus proving the effectiveness of the applied method.
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