It is presently possible to obtain multi-octave bandwidth from a single coupler which employs several cascaded quarter-wavelength sections of uniformly-coupled line. A comprehensive bibliography covering progress in this field may be found in Reference 1. Unfortunately, the physical junctions between various quarter-wave sections in the usually employed TEM conductor geometries contribute reactive discontinuities which significantly degrade coupler directivity.
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C.P. Tresselt (2005) studied this question.
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