RF energy harvesting Rectenna design

In this thesis, we propose a low power RF energy harvesting rectenna design. By the technological advances over the last twenty years, wireless communications continous to be a part of individual's life. Almost everyone has cell phone, tablet, PC and other electronics stuff. Ambient RF energy density increases as usage of those devices increases. Many new frequencies have arisen in RF spectrum because of GSM, 3G and Wi-Fi since the last two decades. Power density for those newcomers are between 0.01 mW and 0.1 mW per bandwidth. RF energy harvesting consept have arisen for utilization of ubiquitous ambient RF power.Rectenna is an electronic circuit which converts RF power into DC, properly. The term "rectenna" is composed of rectifier and antenna. Firstly, rectennas have been designed for wireless power transmission experiments in late 60s. Then it has been utilized in RFID systems. Finally, rectennas are used in RF energy harvesters. A rectenna incorporates receiving antenna, impedance matching network, input and output filters, rectifier and a load. Antenna is the first element of the system. It collects ambient RF energy for rectifier. Generally, single band microstrip antennas are used for RF harvesters due to its simplicity. Collected RF power is forwarded to rectifier unit. Impedance matching network between antenna and rectifier eliminates impedance mismatch and reduces power losses on the signal path. Input impedance of the rectifiers are strongly capacitive so distributed element matching networks are preferable. Harmonics are generated due to the nonlinearity of diode. Rectenna performance degrades drastically if harmonics reflected back to antenna. To avoid such a case, harmonic terminations are implemented. Generally, second harmonics are short-circuited, third harmonics are open-circuited. Input filter blocks DC, output filter blocks AC component of the signal. Schottky diodes are used as rectifier elements. Schottky diode is a semiconductor device which lets current pass over one direction and blocks on other direction. Diode does not let the current pass until bias voltage exceeds the threshold voltage because there is a potential barrier in between metal semiconductor junction. There are many rectifier topologies for energy harvesting rectennas. Most popular ones are half wave rectifiers, voltage clampers and voltage doublers. Half wave rectifier and voltage clamper contain just one diode and they are the most efficient harvester topologies among others. In ideal case rectification efficiency for these topologies is 100%.