A tunable power absorbing termination for a waveguide transmission line includes a section of waveguide having a front power feed end, a back power extracting end, and guidewalls extending between the front and back end thereof. A coolant circulating dielectric taper extends into the section of waveguide in an inclined orientation relative to a guidewall of the section of waveguide such that the point end of the taper extends substantially to the front power feed end of the waveguide section. The inclined orientation of the dielectric taper creates a free volume of space adjacent the taper behind the front power feed end of the waveguide section into which a tuner element is introduced which is position adjustable within the free volume of space to provide the power absorbing termination with a tuning capability.