Microwave oscillator injection locked at its fundamental frequency for producing a harmonic frequency output

A microwave oscillator comprising a waveguide having a cut-off frequency, the oscillator further comprising generating means, including a negative-resistance oscillator device, for generating microwave energy in the waveguide, wherein the generating means is operable to generate microwave energy simultaneously at a fundamental frequency which is below the cut-off frequency of the waveguide and at a harmonic frequency which is an integral multiple of the fundamental frequency and which is above said cut-off frequency, the generating means further comprising first means which in operation couples the oscillator device to the waveguide at said harmonic frequency and on which the values of said fundamental frequency and said harmonic frequency depend, characterised in that the oscillator further comprises second means, extending into the waveguide and being spaced from the first coupling means, for coupling microwave energy at said fundamental frequency from outside the waveguide to the first coupling means and thence to the oscillator device, to control the generation of microwave energy at said harmonic frequency.