Small size dual-band coupled-line coupler for WiMAX applications

In this paper, a symmetric compact dual-band coupled-line coupler is proposed using microstrip transmission lines, circular resonators, and spiral lines for WiMAX applications. The proposed coupler provides two narrow operating bands, high coupling and isolation, and suitable return loss. The primary design consists of two parallel-coupled lines with a flat coupling over a wide frequency band. Then, the bandwidth, return loss, and isolation are optimized by adding circular resonators and spiral lines to achieve a compact size. In this design, the return loss is better than 30-35 dB in the bandwidth 3.4-5.79 GHz. Finally, this structure is fabricated using a Rogers-RT Duroid 5,880 substrate with a thickness of 20 mil, a loss tangent of 0.0009, a dielectric constant of 2.2, and an area of 18.8 x 14 mm2. The results of the measurements are in good agreement with those of the simulation.