In some aspects, a downhole nuclear magnetic resonance (NMR) tool includes a magnet assembly and an antenna assembly. The NMR tool can operate in a wellbore in a subterranean region to obtain NMR data from the subterranean region. The magnet assembly produces a magnetic field in a volume about the wellbore. The magnet assembly includes a central magnet, a first end piece magnet spaced apart from a first axial end of the central magnet, and a second end piece magnet spaced apart from a second axial end of the central magnet. The antenna assembly includes a transversal-dipole antenna. In some cases, orthogonal transversal-dipole antennas produce circular-polarized excitation in the volume about the wellbore, and acquire a response from the volume by quadrature coil detection.