An efficient compact rectenna based on a novel asymmetric loop-shaped planar inverted-F antenna (PIFA) is proposed for portable devices and wireless sensors. The antenna element, or the loop-shaped PIFA, has a compact uniplanar structure and exhibits higher radiation efficiency. The rectifier used is a half-bridge boost converter, which can double the output dc voltage. In between the rectifier and loop-shaped PIFA is a matching circuit with harmonic suppression capability, further enhancing the conversion efficiency. For the 2.45-GHz prototype design, the peak conversion efficiency of 61.4% is achieved as the incident power density is 7.4 mW/m2 and the load resistance is 510 Ω.