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This letter presents the design and implementation of a single-layer, cavity-backed antenna oscillator based on substrate integrated waveguide (SIW) technology. The proposed antenna topology is particularly suitable for two-dimensional array configurations. The active circuit and the radiator are placed on opposite sides of the substrate and within the cavity area, allowing for a compact implementation. An X-band prototype was designed and fabricated, and it features frequency-tuning capability introduced by employing a varactor diode in the active circuit. The measurements showed 11.87-12.36 GHz tuning range, phase noise better than - 107 dBc/Hz at 1 MHz offset, and 7% DC-to-RF conversion efficiency.