In this study an improved model of small metallic post has been used in finite-difference time-domain (FDTD) method. This model has the advantages of wide range stability for different values of post radius, good accuracy and easy formulation. By using this model accurate results have been obtained in FDTD with large mesh size and therefore memory and calculation time have been reduced. Moreover circuit model has been used for the analysis and optimisation of substrate-integrated waveguide (SIW) branch line coupler. In this model a circuit is decomposed into several two or multi ports sub-circuits. Next, the proposed FDTD model is used to determine the scattering parameters of the sub-circuits. With a proper connection of the scattering parameters of these sub-circuits, the scattering parameters of the overall structure have been determined. Based on the improved post model in FDTD and using the circuit model a branch line SIW coupler with wider bandwidth has been designed and fabricated.

Keywords