This paper presents a compact, single substrate, active substrate integrated waveguide (SIW) cavity backed patch antenna oscillator. In the last years, the fast development of wireless communications spurred a great demand for low profile antennas. Cavity backed antennas have been widely studied as they offer several design advantages such as increased efficiency due to surface wave suppression, and adequate metal surface to dissipate heat from active devices. Additionally, cavity backed active oscillator antennas have been used to improve oscillator phase noise performance. SIW technology allows one to obtain low cost, high performance waveguide-like structures using conventional fabrication techniques and permits their straightforward integration with planar circuitry, featuring good shielding, low loss and good power handling capability.