Abstract: An oscillator formed in a rectangular waveguide (1) with both mechanical and electronic tuning comprises an oscillator device (4) spaced from a movable short-circuit termination (2) with a varactor diode (5) therebetween, the effective electrical spacing between the oscillator device (4) and the varactor (5) being approximately an integral number of half-wavelengths at the operating frequency. The varactor (5) extends into the waveguide (1) from one broad wall thereof and engages a transverse member (10) extending between the narrow walls so that only part of the height of the waveguide (1) at that region is obscured. This enables the operating frequency and hence the extent of coupling of the varactor (5) to the oscillator cavity, and thus the electronic tuning range, to be varied by adjusting the position of the short-circuit (2).