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With a direct integration of a field effect transistor (FET) into a C-shaped ring radiator, a novel miniaturized self-oscillating annular ring active integrated antenna is proposed and investigated in this paper. The direct integration approach provides a solid basis for the miniaturization while the slow-wave loading effect from the unstable transistor further enhances the reduction ratio. It makes the proposed design show an extraordinarily compact size of 0.041λg2 , which is 40% the size of the second smallest one in literature using printed circuit board technology. The proposed active antenna, providing overall comparable performance to the previous designs, has an EIRP of 7.30 dBm, DC-to-RF efficiency of 25.9%, phase noise of -91.9 dBc/Hz@100 kHz offset, and an oscillator FOM of -168.76 dBc/Hz. By introducing a reverse-biased varactor diode, this miniaturized active antenna also demonstrates promising voltage controllability with a high VCO gain of 48.7 MHz/V. The antenna geometry and modeling, design concept, simulation scheme, and experimental results are discussed thoroughly in this paper.