Abstract: Microwave phase-shifting apparatus includes a section (20) of waveguide of circular cross-section having a closed end (23) and an open end. A fixed phase-shifting member (20) is located within the section to convert plane-polarized incident energy to circular polarization. A rotatable shaft (27) of dielectric material projects through the end wall of the section along the longitudinal axis, and carries a rotatable element (28). This element is of such a form and in such a position that linearly-polarized microwave energy entering the waveguide section is reflected out of the section with a reflection coefficient having a phase angle component which may be continuously adjusted over a range of 360.degree. by rotation of the shaft, in either direction.