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Do you want a fixed phase shift or a variable one? Do you want a mechanical version, such as a double stub tuner or an electronic solution using something like microstrips and PIN diodes? Whats the application?
The classical methods are branch line couplers and parallel line couplers. To get these wide bandwidths, you will have to use many elements and usually an analysis program with an optimizer. In each type the elements are a quarter wave long at the center frequency. In the branch line method they are of different impedances. In the parallel line method they are different distances. In both methods the system is symetrical from end to end.