1. Introduction
Reconfigurable radio system represents a potential solution for interference, cost, size, and power consumption problems encountered by wireless communication industry [1]. The three-port diplexer as a key component of front-end is widely used in RF/microwave transceivers to direct signals of two different frequencies to appropriate outputs and to prevent mutual interference between channels. Thus, a duplexer can be essentially developed by connecting two filters (different center frequencies) in parallel and each is designed with high selectivity to ensure channel isolation.