An improved analytical model for broadside coupled transmission line used on planar circuit

Broadside coupled transmission lines have the advantage of more compact size and higher coupling factor compared to the traditional coupled microstrips in the realization of planar circuits such as substrate integrated baluns, directional couplers, and filters on printed circuit boards. A broadside coupled transmission line is a structure with interesting properties composed of two signal lines with strong coupling placed on the top and bottom of a dielectric substrate, surrounded by coplanar ground. This configuration allows the realization of the practical circuit with several hundred ohms impedance for even mode excitation, with the help of lower even mode capacitance. This leads to a high coupling factor and benefits to a high impedance transformation ratio balun or a wide bandwidth filter design. In this paper, an analytical model describing the symmetrical pair of conductors for broadside coupled transmission lines is presented. The analytical model based on the physical geometries of the coupled lines is verified by electromagnetic (EM) simulations and measurements. The results are in very good agreement with each other.