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We have studied the effects of concentration of cobalt and sintering temperature on the complex relative permittivity and permeability of MgFe1.98O4 ferrite ceramics. All samples were doped with 3 wt% Bi2O3 to improve the sintering properties of the Mg1-xCoxFe1.98O4 ceramics and thus attain a low dielectric loss tangent. X-ray diffraction data indicated that the ferrite samples all have a single-phase spinel structure and that there was no reaction between the ferrite and Bi2O3. In terms of microstructure development and grain growth, the effect of Bi2O3 is much more pronounced than that of Co. At a given concentration of Co, the magnetic properties can be fairly explained by a magnetic circuit model. The relationship between static permeability and the concentration of Co can be understood by using the single-ion model combined with the local uniaxial anisotropic model. Among the samples we prepared, Mg0.96Co0.04Fe1.98O4 sintered at 1000degC for 2 h has almost equal real permeability and permittivity-both approximately 10 - as well as sufficiently low dielectric and magnetic loss tangent over a range 3 to 30 MHz [the high-frequency (HF) band]. These properties that should be useful for miniaturization of HF antennas.