Compact Microstrip Diplexer Based on CRLH Metamaterial Concept

In this paper, a compact microstrip diplexer based on resonant-type metamaterial transmission lines is proposed. The proposed diplexer is designed by cascading two bandpass filters with different center frequencies. The bandpass filter, which the proposed diplexer has been designed based on it, is realized by series of capacitive gaps with grounded stub on the upper metal layer and the complementary spiral resonators unit cell which etching on the ground plane. The proposed diplexer as a three-port device has been designed to operate at two arbitrary frequencies. The simulation results show that the three-port diplexer can separate two different frequencies effectively. In order to confirm the design procedure and the capability of the proposed diplexer in size reduction and selectivity, two prototype devices operative in the center frequencies at 2.4 and 3.5 GHz have been fabricated and tested. Good agreement between measurement and simulation results has been achieved. These one- and two-stage diplexers have the overall sizes of 0.19 lambda(g) x 0.05 lambda(g) and 0.34 lambda(g) x 0.06 lambda(g), respectively. The proposed diplexers have many advantages in terms of compact size, low loss, high isolation, ease of fabrication, and high selectivity.