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Differential phase shift between two orthogonal TE11 waves in a circular hollow waveguide is achieved with a reflectionless array of capacitive pins. Using transmission-line theory, an analysis of such a structure is made and, under the assumption that the pin susceptance varies with frequency as jÂ¿C, a broad-band 3-pin array acting as a quarter-wave plate may be designed. Such an array, which is only one inch long at X-band, has been tested. A voltage ellipticity ratio of less than 1.1 and vswr less than 1.2 is maintained over a 12-per cent band.