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New configurations of small-size shielded metallic Fabry-Perot (FP) antennas with improved performance over a large frequency band are presented in S-band for space missions. The bandwidth enlargement is obtained by stacking two FP cavities of different size, each of them presenting a low quality factor. Their radiating apertures measure around λ0 and 2 × λ0, respectively. Concentric corrugations are also introduced between both cavities to control the higher-order modes that are excited systematically in shielded small-size FP antennas due to lateral resonances. The obtained results are compared to those of a single-stage FP cavity antenna with the same aperture size. Several prototypes have been fabricated and measured. An aperture efficiency higher than 70%, a reflection coefficient smaller than -15 dB, and sidelobe levels lower than -20 dB have been obtained experimentally, over a wide frequency band (2.4-2.66 GHz). These characteristics make stacked FP cavity antennas very attractive to replace global coverage horn antennas, or to be used in feed clusters of multiple-beam antennas, especially in C- and S-bands, where they lead to more compact and less bulky solutions compared to classical feed horns.