Two Multiband Circular Polarization Conversion (CPC) structures using FSS have been introduced for practical applications in this article. An elliptical patch wideband antenna operating from 5-13.8 GHz is integrated with a novel single layer metal backed modifed I shaped FSS based reflection mode circular polarizer (CP).This polarizer operates at multiple frequencies 7.2, 8, 8.2, 8.3 and 10.4 GHz respectively. Further, a single layer transmission mode FSS based polarization converter comprising double P shaped unit cells is realized which accomplishes CP generation at 10.2, 10.4, 10.6, 10.8, 11 and 11.2 GHz. An axial ratio of less than 3 dB and phase difference of 90° (or an odd multiple of 90°) is obtained between the TE and TM transmissions which ratifies the generation of CP at multiple frequencies. The radiation performance of the microstrip patch antenna is undegraded when used in conjunction with either of the LP-CP polarization converters at the operating frequencies as validated through full wave simulations in HFSS. The formation of CP is illustrated through E field distribution and polarization ellipses. © 2020 IEEE.