Electromagnetic properties of nickel-zinc ferrites based materials make them potential candidates for applications linked to telecommunications. In the present study nanosized particles of spinel ferrite Ni0.5Zn0.3Co0.2Fe2O4 were prepared by co-precipitation method. An optimized material is obtained after adequate heat treatment and partial filling of the porosity by epoxy resin. This material lies between ceramic and composite medium (with porosity close to 40%), and shows almost constant complex permeability and permittivity in the frequency range from 0.1GHz to 0.7 GHz, and equal to ~3.5-j0.15 (loss tangent~0.04) and ~4-j0.2 (loss tangent ~0.02) respectively. The refractive index n is close to 3.75. These electromagnetic properties, in particular the low levels of losses, show that this material could be useful to the design of miniaturized antennas in the VHF-UHF (300MHz-700MHz) range of frequency.