Abstract: A directly modulated microwave source for use in communications equipment is disclosed. The arrangement, fabricated in an open ended waveguide comprises an oscillator section operating in the TE.sub.101 mode and an impedance matching section providing adjustable coupling to a load. A suitable active element is a Gunn diode, post mounted in proximity to the closed end of the waveguide for H field coupling. Modulation in frequency or phase is achieved by a varactor diode, also post mounted, positioned approximately 2/3 of the length of the waveguide from the closed end. A vertically adjusted post for the load coupling adjustment, is placed adjacent the varactor post, but nearer the opening. The two adjacent posts define the boundary between sections and provide H field coupling to the varactor diode. Temperature stabilization means and frequency tuning means are located respectively on the upper and lower surfaces of the waveguide at anti-nodial portions of the oscillator section.