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The complex dispersion diagram of an antisymmetrically biased ferrite-filled rectangular waveguide (RWG) is investigated, revealing that the waveguide supports a band-limited unidirectional magnetostatic surface wave mode. This mode is subsequently used to design a novel RWG isolator. A parametric analysis including loss is performed to characterize the isolation band. A prototype waveguide isolator has been fabricated using a small metallized ferrite substrate measuring 30-mm long, 6-mm wide, and 0.8-mm high. Measurements in the 10-20-GHz range confirm the theory and mark the first time that a ferrite isolator has been built using a completely filled RWG. The fabricated 6-mm-long waveguide isolator provides a peak isolation figure-of-merit (IFM) of 42.5 dB at 15.4 GHz. An IFM of over 16.2 dB is sustained from 15.2 to 18.3 GHz (18.5% bandwidth). The simplicity of this compact isolator makes it amenable to package-scale integration, such as in ferrite low-temperature co-fired ceramic, where it could be readily embedded within a multichip package.