Abstract: A fixture for determining RF characteristics of a dielectric medium comprises a first coupler having an RF through-window with input and output ports, the input port connectable to a waveguide input. A second coupler has an RF through-window with input and output ports, the output port connectable to an output waveguide. A plurality of rods are aligned with the axes of the input and output waveguides and connect the output port of the first coupler to the input port of the second coupler. Each rod is passable through the dielectric media and mates with the input port so as to form a continuous waveguide passing through the dielectric media.