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Coupling of focused terahertz radiation into tapered metal parallel plate waveguides with sub-wavelength gap widths is presented. A line-focus terahertz time-domain spectroscopy setup with flexible foci is used to investigate coupling of broadband terahertz radiation from free-space into waveguides for different gap widths. Various waveguide lengths are compared in experiment and calculation in terms of occurring loss and divergence phenomena. Amplitude coupling ratios of 80% from free-space into the waveguides are obtained.