This is a model for the coplanar wave, broadside parallel line coupler (BSCCPW). This can be TEM or quasi-TEM as per
homogeneous or non homogeneous dielectric distribution in a stripline case.
A reference load is required for calculation of the Quarter wave coupler characteristics (defaults to 50 Ohms).
For your 3db coupler you could start from: Substrate Er = 12.9 in air Er = 1, substrate 50u, plane gap 100u, track width 90u, shield spacing 10mm.