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An original mechanism is here proposed to achieve polarization conversion from linear to circular with the use of full-metal polarizing screens. Such screens are self-supported and conceived from the periodic arrangement of 3-D unit cells. They are built from sections of rectangular waveguides operating below the cutoff frequency and loaded with slotted discontinuities. The polarizer operates in transmission mode; the discontinuities are responsible for both its high return losses and the conversion of the impinging linear polarization to circular. Two types of 3-D cells are presented, and both of them are analyzed and designed through equivalent circuit models (CMs). These models have been thoroughly built in order to capture all the phenomena underlying the discontinuities' behavior. The characterization of the first cell is fully done analytically, whereas the second cell needs reduced help from a full-wave solver. Furthermore, the CMs allow simple design guidelines to be identified for this type of polarizer. Two designs are performed operating in Kaband, proving that an extension of the operation bandwidth (axial ratio and S 11 ) to 11% is possible by employing the second cell. Index Terms-Analytical circuit modeling, circular polarization (CP), metallic polarizer in transmission, 3-D unit cells. I. INTRODUCTION T HE conception of directive radiating systems for modern satellite communications is facing several bottlenecks, one of them related to the generation of circular polarization (CP) [1]-[7]. The most extensive solution consists in the integration of waveguide polarizers within the beamforming network (BFN) and before the radiating elements (the recent examples can be found in [2], [6], and [8]). Typical polarizing components are implemented in dual-polarized waveguides (supporting two degenerate modes) which are loaded with discontinuities or perturbations, such as stepped septums, ridges, irises, or corrugated grooves [1]-[3], [9]-[11]. The design strategy consists in obtaining the required 90 • phase shift between the two signals and high return loss (RL) for both polarizations. In this paper, compactness and sim-

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