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The dielectric permittivity and magnetic permeability of nano-sized magnetic powders are studied for a wide microwave frequency range from 2 to 40 GHz. These powders are studied for their potential application as soft microwave absorbers in wireless systems as well as for use in the manufacture of small magnetic recording tape, transformers and circulators. A newly developed and precise transmission-reflection based waveguide technique has been used to carry out measurements across the eight frequency bands. These materials have been accurately characterized for the first time across this frequency range. In this paper, we present equations for simultaneous calculation of permittivity and permeability across a wide frequency band using the vector network analyzer. To account for variable sample thickness in the in-waveguide measurements, the algorithm for transmission and reflection calculations has been modified. The results show that the dielectric permittivity depends on the particle size and density of the sample used for measurement.