A Stripline Directional Coupler Utilizing a Non-Homogeneous Dielectric Medium

In the conventional 3 db directional coupler, shown schematically in Figure 1a, equal output signals appear at ports 2 and 3 when port 1 is used as the input. No signal appears at port 4. The effective dielectric constant for the even mode (/spl epsi/re) must be approximately equal to the effective dielectric constant for the odd mode (/spl epsi/ro) to assure equal propagation velocities which are essential for this type of coupler.