Real-time data of high-voltage infrastructures collected by wireless sensors are the foundation of many smart grid applications. Energy harvesting can be an effective solution for autonomous, self-powered wireless sensors. In this paper, a coil with a novel helical core is proposed and optimized to scavenge the magnetic field energy efficiently near equipment carrying large currents. Due to the special design of the helical core, the path of the magnetic flux inside the core can be significantly increased, which leads to a reduction of the demagnetizing field. Therefore, the proposed core can generate a much higher flux density (hence more power) compared with conventional designs. The selection of the core material is studied and itis found that high permeability ferrite is the most suitable material. Experimental results show that the proposed helical coil with only 400 turns of wire can have a power density of 2.1 μW/cm 3 when placed in a magnetic flux density of 7 μTrm s. This value is bigger than any of the existing designs if placed in the same magnetic field. If more wires can be wound on the core, an even higher power density could be obtained. Therefore, the proposed design is a very efficient method for scavenging the magnetic field energy for a wide range of smart grid applications.