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This paper is an investigation of the feasibility of applying a mechanically flexible magnetic composite material to radio frequency identification (RFID) planar antennas operating in the lower ultrahigh-frequency (UHF) spectrum (~300500 MHz). A key challenge is that the magnetic loss introduced by the magnetic composite must be sufficiently low for successful application at the targeted operating frequency. A flexible magnetic composite comprised of particles of Z-phase Co hexaferrite, also known as Co2Z, in a silicone matrix was developed. To the authors' knowledge, this is the first flexible magnetic composite demonstrated to work at these frequencies. The benchmarking structure was a quarter-wavelength microstrip patch antenna. Antennas on the developed magnetic composite and pure silicone substrates were electromagnetically modeled in Ansoft High-Frequency Sounder System full wave electromagnetic software. A prototype of the antenna on the magnetic composite was fabricated, and 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 2.4 times and performance differences of increased bandwidth and reduced gain, both of which were attributed in part to the increase in the dielectric and magnetic losses. A key finding of this paper is that a small amount of permeability (mur~2.5) can provide a substantial capability for miniaturization, while sufficiently low-magnetic loss can be introduced for successful application at the targeted operating frequency. This magnetic composite shows the capability to fulfill this balance and to be a feasible option for RFID, flexible wearable, and conformal applications in the lower UHF spectrum.