Improved miniaturization method for coupled-line filters

A miniaturization method for coupled-line filters is demonstrated that relies on the equality of the two most important parameters for filter synthesis, the propagation constant and the image impedance, before and after miniaturization. The method enables to maintain the bandwidth intact as opposed to the previous miniaturization method based on the same structure. This proves that bandwidth reduction in miniaturized filters is not due to miniaturization itself but rather due to the mathematical formulation of miniaturization. As compensation of reduced bandwidth is not necessary, the proposed method provides much simpler miniaturization procedure.