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Novel profiled irises for the design of waveguide polarizers are presented. The two degrees of freedom of their piecewise-circular profile allow one to better control the frequency behavior of the differential phase shift. Thanks to this feature, an optimum iris set can be found by selecting the discontinuities for which the differential-phase-shift deviation from the average value is minimum in the operative bandwidth. This optimum iris set can be identified for several values of the ratio 0 , where is the waveguide radius and 0 is the free-space wavelength at the central frequency. Hence, even if both the waveguide size and central frequency are assigned in the specifications, polarizers with a very low deviation from 90 of the total differential phase shift can be designed. In this way, the number of waveguide transitions in the antenna feed system can be reduced and the effect of spurious higher-order-mode coupling can be minimized. A Ka-band polarizer in circular waveguide with piecewise-circular-profile irises is described, showing a return loss better than 55 dB and a phase error of about 0.9 in a 10% bandwidth.