This work includes formulation of mechanically flexible magnetic composites and application to a quarter-wavelength microstrip patch antenna benchmarking structure operating in the lower UHF spectrum (∼300-500 MHz) to investigate capability for miniaturization. A key challenge is to introduce sufficiently low magnetic loss for successful application. Particles of NiZn ferrite and BaCo ferrite, also known as Co₂Z, were characterized. Flexible magnetic composites comprised of 40 vol% NiZn ferrite or BaCo ferrite particles in a silicone matrix were formulated. Effects of treating the particles with silane in the formulation process were not detectable, but larger particle size showed to increase ϵ* and μ*. By comparing ϵ* and μ* of the composites, BaCo ferrite was selected for the antenna application. Antennas on the developed magnetic composite and pure silicone substrates were electromagnetically modeled in a full-wave FEM EM solver. A prototype of the antenna on the magnetic composite was fabricated. Good agreement between the simulated and measured results was found. Comparison of the antennas on the magnetic composite versus the pure silicone substrate showed miniaturization capability of 2.4X and performance differences of increased bandwidth, reduced Q, and reduced gain. A key finding of this study is that a small amount of permeability (μ_r∼2.5) can provide relatively substantial capability for miniaturization, while sufficiently low magnetic loss can be introduced for successful application at the targeted operating frequency. The magnetic composite showed the capability to fulfill this balance and to be a feasible option for RFID applications in the lower UHF spectrum.