Compact 3‐dB directional coupler design based on substrate integrated dual‐strip coaxial line

A novel compact 3‐dB directional coupler design based on the substrate integrated dual‐strip coaxial line (SIDSCL) is presented. The SIDSCL differs from the conventional substrate integrated coaxial line (SICL) in that the former possesses two central strips that are periodically connected through buried via‐holes and the latter has only one strip. In the coupled‐line section, the connection via‐holes are removed to form two coupled strips. The needed tight coupling is easily achieved by the strip's broadside‐coupling configuration. To shorten the coupled‐line section, both coupled strips are periodically and bilaterally loaded with open stubs, which provide the capacitances between the strips and the capacitances to the ground. In each SIDSCL feedline branch, a side patch is attached for neutralising the parasitic capacitance and inductance owing to the junction discontinuity that deteriorates the coupler's isolation and impedance matching. A sample coupler with a 25% reduction in the coupled‐section length is fabricated for verifying the circuit design.