Design of an Implantable Rectenna for Wireless Power Transfer Application
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Implantable medical devices have been performing diverse diagnostic and therapeutic functions like monitoring, sensing or drug delivery. These medical devices require to communicate with the outer world but the major challenges for these devices are efficient power supply, reduction in size and to extend the operational time-span. In this thesis, an implantable antenna composed with a rectifier circuit (Rectenna) design is proposed and the performance of the rectenna is evaluated in terms of output voltage and efficiency. As rectenna is a combination of an antenna and a rectifier circuit, the report provides the design procedure and simulation results for both of these major components. A patch antenna was designed using the software tool CST Microwave Studio which is a very sophisticated software. The band chosen for antenna operation was in the Industrial, Scientific and Medical (ISM) band (902-928) MHz. For creating a real world simulation environment, a two layer human tissue model along with a biocase was designed and the antenna was encapsulated in between the skin and muscle layer. Different antenna miniaturization techniques were used to reduce the antenna size. To observe the antenna performance, the return loss S11 parameter was measured. From the simulation result, it is showed that at 915 MHz frequency, the antenna has a minimum return loss of -14.81 dB. For the rectifier circuit, same frequency band as antenna was chosen for operation. All designs and simulations were performed by using the software tool Advanced Design System (ADS). According to the standard, for RF power transmission, the input impedance of the loaded rectifier circuit should be around nonreactive 50 ohm. So the input impedance was tuned as close as possible to nonreactive 50 ohm. After that, a matching network was designed to transfer the maximum power from source to load. For RF to DC conversion, a double stage voltage rectifier was used. Different signaling approach such as single tone and dual tone were used to simulate the circuit. From simulation, an output voltage of 0.314 V for single tone and 0.915 V for dual tone was achieved. The overall efficiency of the rectifier was 39.39% at input power of -16 dBm.