We demonstrate experimentally the coupling of a THz beam into a laser-machined double-tapered thick slot waveguide operating between 0.3 and 2 THz. Simulation reveals that the THz field generated by a standard photoconductive antenna coupled to an undoped aspheric HRFZ-silicon focusing lens can be enhanced by 30 dB in the 40 μm taper waist, relative to a traditional Gaussian beam waist (at 1 THz). Analysis of the 8 dB insertion loss reveals positive implications for coupling between THz transceivers and in-plane-emitting antennas or low-loss THz waveguides.