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A miniaturized circularly polarized implantable antenna operating at ultrahigh frequency band (902-928 MHz) for radio frequency identification biomedical monitoring is first presented and experimentally validated in this article. The proposed antenna features a compact volume with a dimension of π × (6) 2 × 1.27 mm 3 by employing an extended ring with meandered lines for size reduction. Moreover, adjusting the length of symmetrical meandered lines can introduce two orthogonal modes, which makes for good performance of circular polarization. Superb impedance matching between the chip and tag antenna is well implemented by applying a modified T-match stub. In the simulation, the antenna achieves a −10-dB impedance bandwidth of 42 MHz (902-944 MHz) and 3-dB axial-ratio bandwidth is 53 MHz (892-945 MHz). Finally, the specific absorption rate is also calculated for human safety and the measured reading range reaches the maximum distance of about 87 cm. K E Y W O R D Scircular polarization, implantable antenna, miniaturized tag antenna, radio frequency identification

This article presents a stacked dipole antenna operating at industrial, scientific, and medical‐band (902‐928 MHz) for ingestible temperature monitoring applications. To date, the proposed capsule system has a smaller dimension of 11 mm (diameter) × 16 mm (length) compared to other existing capsule systems. To achieve size reduction of the capsule system, the conjugating matching method is utilized and the matching circuits are intended to be removed. The whole capsule system is simulated in a one‐layer muscle phantom for initial parameters optimization, and the impedance bandwidth ranges from 908 to 924 MHz, which is sufficient for data transmission requirements and also brings robustness on the performance stability whatever the surrounding organs is. A wireless data transfer link is established between the integrated capsule and external receiver. An in‐vivo measurement is also conducted to validate the system practicability, and besides, the temperature date of a rat is monitored in real time and shown at a display screen.