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We present a compact L-band orthomode transducer (OMT) with a new waveguide junction consisting of a quad-ridged square waveguide and four single-ridged triangular waveguides. Quad-ridged square and single-ridged triangular waveguides of various sizes were numerically analyzed to estimate the cutoff frequencies and characteristic impedances of the waveguides. An optimum junction structure for the OMT with a low cutoff frequency and specific characteristic impedance was found based on the numerical simulation. The junction has a relatively small size without waveguide branches, and provides good impedance matching with a 50 Ω coaxial feed line. In addition, Klopfenstein and linear tapers were employed to design waveguide ridges for minimizing mismatches at the OMT junction. Measurement showed a bandwidth of about 400 MHz from 1.0 to 1.4 GHz and an isolation level below -40 dB in the frequency range. The cross section of the OMT was reduced to 62% of the standard waveguide cross section, and the total length is only 56 cm (2.24 λ0) at 1.2 GHz.