(Grebenkemper 1987) describes a directional coupler that has become very popular, especially in commercial implementation.
The simplified circuit above from Grebenkemper’s article illustrates the key elements of the directional coupler.
An important detail of the design is that the primary of the right hand transformer appears in shunt with the antenna load, and the magnetising impedance of that transformer core compromises Insertion VSWR. It is important that the magnetising impedance is sufficiently high (or the admittance sufficiently low) to not cause significant Insertion VSWR.
Continue reading Grebenkember’s original Tandem match