Design of L-Band Small-Sized Cavity Filters

This paper introduces a third-order cavity filter operating at a frequency range of 1.2-1.4 GHz. The filter adopts a direct coupling topology, which is simple and easy to optimize design and process. Compared to a cross-coupling filter, the direct coupling structure is more widely used in practical industrial applications Additionally, a gradient optimization algorithm is utilized to optimize the coupling matrix, which in turn optimizes the transmission zero points. This results in the filter exhibiting better performance in terms of in-band insertion loss and out-of-band suppression.