White Paper: What Actually Creates The 90 Degree Coupler Phase Shift?

In a coupler made up of parallel coupled lines there is a phase relationship between the through port and the coupled port. The electrical phase of the coupled port is +90 degrees when the through port is referenced as 0 degrees. This +90 degree phase occurs at all frequencies for which the coupler has a good match. Parallel line couplers which have this characteristic are designed symmetrically with an odd number of sections.