We demonstrate experimentally the resonant extinction of terahertz (THz) radiation by a single plasmonic bowtie antenna, formed by two n-doped Si monomers with a triangular shape and facing apexes. This demonstration is achieved by placing the antenna at the output aperture of a conically tapered waveguide, which enhances the intensity of the incident THz field at the antenna position by a factor of 10. The waveguide also suppresses the background radiation that is otherwise transmitted without being scattered by the antenna. Bowtie antennas, supporting localized surface plasmons, are relevant due to their ability to resonantly enhance the field intensity at the gap separating the two triangular elements. This gap has subwavelength dimensions, which allows the concentration of THz radiation beyond the diffraction limit. The combination of a bowtie plasmonic antenna and a conical waveguide may serve as a platform for far-field THz time-domain spectroscopy of single nanostructures placed in the gap.