We present a resonant leaky-wave lens antenna, fed by a circular waveguide with annular corrugations in the ground plane. The proposed leaky-wave feed reduces the impact of the spurious TM0 leaky-wave mode in all planes over a wide bandwidth while reducing assembly complexity compared to previous methods. The proposed leaky-wave antenna has an aperture efficiency above 80%, a return loss below -15 dB, and a cross-polarization level below -20 dB over a bandwidth from 110-220 GHz (2:1). We have fabricated and measured a WR-5 band (140-220 GHz) antenna prototype with a lens diameter of 3 cm that achieves excellent agreement between measurement and simulation in terms of return loss, directivity, and gain.