Octave-bandwidth high-directivity microstrip codirectional couplers

It is a well-known problem with microstrip backward-wave couplers that directivity suffers due to the differing phase velocities of the even- and odd-mode waves. A fully-planar alternative is the forward-wave, or codirectional-coupler, but these structures have not yet been widely accepted, in part because they are complex and tedious to design. Here, we offer a simple quarter-arc geometry for loosely-coupled microstrip couplers with high-directivity that can be very quickly modeled and prototyped. A design example of a 50-110 GHz 12 dB coupler is shown. Measurements indicate that the coupler has better than 20 dB directivity over the full octave bandwidth.