Abstract
An ultra-thin differential patch antenna is proposed for head implants. Through the proximity coupling between the rectangular patch and the loop structure fed by a differential microstrip line, an antiphase mode is excited, which doubles the bandwidth of an aperture-coupled rectangular patch antenna. Moreover, the directional radiation allows maximum energy to be directed to the receiver and lowers the specific absorption rate (SAR). An antenna prototype with an overall size of $24 \times 10 \times 0.95\,\text{mm}^{3}$ exhibits the ${-} 10\,\text{dB}$ reflection bandwidth of $4.1 - 4.5\;\text{GHz}$ , directivity more than 5 dBi, and gain higher than ${-} 8\,\text{dBi}$ at the boresight. The maximum transmit power of 28 mW is permitted for the SAR limit of 1.6 W/kg across the impedance bandwidth.
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