Design and fabrication of a compact branch-line hybrid coupler with a balanced phase using a new microstrip structure for GSM applications

A novel microstrip quadrature hybrid coupler is mathematically designed, optimized and fabricated in this work. The physical structure of the proposed coupler is new and it occupies a very small area of 0.01 λg2. The novel stubs embedded inside the conventional rectangle are connected parallel lines. The ends of these stubs are connected to a solid rectangle, which make the overall structure different from the previous works. Our branch-line coupler is designed to work at 1.8 GHz for GSM applications. The phase imbalance between S21 and S31 is only 0.01°, which is very small compared to the previous works. Another advantage of the presented coupler is its high isolation, which is about 40 dB. Meanwhile, other parameters such as common port return loss and coupling factors are desired. To design this coupler, two low-loss lowpass filters (LPFs) are designed and embedded in the main structure. A mathematical design method is presented to decrease the losses, where a 0.002 dB insertion loss for the LPF is achieved. We measured the presented structure. The measurement results verifies the simulations with high accuracy. Therefore, due to having a novel structure, very small size, the lowest phase shift, good isolation and good return loss the proposed coupler can be easily used in RF communication systems.