A phase shifter device comprising a substrate defining a slot line waveguide having first and second ends and being operably coupled to a microstrip waveguide and a shorting patch is described. The microstrip waveguide and shorting patch are moveable along the slot line waveguide so as to vary the distance between the first end of the slot line and the intersection of the slot line waveguide and microstrip waveguide whilst maintaining a substantially constant separation between the microstrip waveguide and the shorting patch. A separation between the microstrip waveguide and shorting patch equal to one quarter of the effective wavelength of the radiation carried by the device is described. The device is described for use in various phased array antenna systems.