A tool that uses antennas to evaluate an environment surrounding a wellbore, where the tool can include a sleeve with an antenna mounted thereon, and the antenna including a ferrite assembly and a coil assembly, where the coil assembly can be mounted radially outward from the ferrite assembly, and the coil assembly can transmit electromagnetic signals into the environment and/or receive the electromagnetic signals from the environment. The electromagnetic signals can be evaluated to determine one or more parameters of the environment surrounding the wellbore. The tool can include one or more grooves disposed in a wall of a sleeve of the ferrite assembly, with one or more ferrite objects installed in the grooves, with a cross-sectional shape that can retain the ferrite objects in the groove. The ferrite objects can alter a trajectory of one or more of the electromagnetic signals.