Directional Multiport Ambient RF Energy-Harvesting System for the Internet of Things

Ambient radio-frequency (RF) energy harvesting can extract energy from RF signals in the ambient environment and has potential for applications in the Internet of Things. However, the power density of the ambient RF energy is low and therefore methods to maximize the average output direct current (dc) power are required. In this work we show that average output dc power in ambient RF energy harvesting is nonlinearly dependent on antenna directivity and linearly dependent on antenna port number. To maximize average output dc power it is therefore necessary to utilize directional multiport rectennas. To demonstrate the enhancements possible, the design for a directional 4-port pixel patch rectenna system to harvest ambient RF energy from the GSM-1800 frequency band is provided. The design has an average antenna size for each port of 0.3λ×0.3λ, and realized gains of 5.5 dBi. Measurement results show that the proposed rectenna can increase average output dc power by up to 6.2 and 4.5 dB compared to omnidirectional and directional single-port rectenna designs of similar size, respectively. Measurement in a real ambient environment is also conducted, showing that the proposed rectenna can achieve an output dc power of 11.2 μW which is five times higher than the two reference rectennas.