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In this paper, an L-band wideband quad-ridged waveguide orthomode transducer (OMT) for the Five hundred meter Aperture Spherical radio Telescope (FAST) is presented with a simple design principle. By designing two critical parts of the OMT separately and introducing matching rings into two orthogonal probes, an improved wideband performance has been realized successfully. The OMT is designed to operate across the 0.95GHz-1.9GHz band, and the simulation shows a return loss better than -20 dB for both polarizations, cross-polarized isolation levels over 45 dB and insertion loss lower than 0.15 dB over the entire bandwidth. The measured results are in good agreement with the simulations. [ABSTRACT FROM AUTHOR]