An Ambient RF Energy Harvesting System Where the Number of Antenna Ports is Dependent on Frequency

We describe the design of a multiport rectenna system for ambient radio frequency (RF) energy harvesting where the number of ports utilized is dependent on frequency. A unique aspect of the design is the use of different numbers of antenna ports for harvesting RF energy at different frequencies. This allows the available area for the rectenna to be fully utilized at all frequencies. In particular, the proposed antenna is designed to have four ports for harvesting energy from the GSM-900 frequency band and 12 ports for the GSM-1800 frequency band in the same area. The design for the rectifiers for the GSM-900 and GSM-1800 frequency bands with direct current (dc) combining is provided and prototypes are demonstrated. Field-test measurements show that the proposed rectenna can provide an output dc voltage of more than 3.2 V, an output dc power of more than -10 dBm, and an RF-to-dc efficiency of greater than 42% when the power density is greater than 1400 mu W/m(2). Measurements in an ambient RF environment at a university campus with commercial GSM-900 and GSM-1800 systems in operation show that the proposed rectenna can achieve output dc voltages of up to 2.2 V and dc power up to -13.6 dBm.