Spinel ferrites of a nominal composition of Ni0.5Zn0.3Co0.2Fe2O4 (NiZnCo) with various B2O3–Bi2O3–SiO2–ZnO (BBSZ) glass additions were successfully synthesized at a relatively low temperature of 930 °C for their potential applications as magneto-dielectric antenna substrate materials. Measurements of magnetic and dielectric properties show that, with an increasing content of BBSZ from 0 to 2 wt. %, the permeability μ′ at low frequencies was increased from ∼4.7 to ∼8.9, whereas the permittivity ε′ was improved from ∼5.9 to ∼12.5. These tunable permeability and permittivity were proved to be closely related to the grain size and density, based on the magnetic circuit model and effective medium theory. Importantly, the sample with 0.5 wt. % addition demonstrated the optimal magneto-dielectric performance with nearly equivalent values of μ′ and ε′, which yields a large miniaturization factor of ∼6.5 and the normalized impedance of ∼1 over the frequency range of 10 MHz–1 GHz. Moreover, this sample exhibits low loss characteristics over a wide frequency range, where the dielectric loss tan δε is in the order of 10−3 and the magnetic loss tan δμ is less than 0.15. These properties make the ferrite material a promising candidate for miniaturized antennas with good impedance matched to free space and open a great potential for low-temperature co-fired ceramic applications toward the practical fabrication and high integration of the antennas.