Synthesis of duplexers with the common port matched at all frequencies

In this paper, we present a novel approach to the synthesis of duplexers with the antenna port matched at all frequencies without the use of 90° hybrids. The design is based on the synthesis of a three-port prototype network composed of coupled resonators, which models the duplexer in a normalized low-pass frequency domain. The polynomials characterizing this network are derived under the lossless assumption and imposing a particular condition on the polynomial determinant of the scattering matrix. Based on these polynomials, the synthesis of the canonical transversal prototype is performed. A new algorithm based on rotations of the coupling matrix is then introduced for the generation of new topologies, potentially more advantageous for the practical implementation. The proposed design approach is illustrated with several examples and by means of a test device, which has been designed and fabricated. The measurements have shown to be in excellent agreement with the expectations.