The width of the strip, the thickness of the substrate and the relative permittivity of the substrate determine the characteristic impedance of the strip which is a transmission line. As shown in the diagram, the central conductor need not be equally spaced between the ground planes.
The width of the strip, the thickness of the substrate, and the dielectric constant of the substrate all influence the characteristic impedance of a transmission line. The diagram illustrates that the central conductor does not have to be equally spaced from the ground planes.