Electrical Engineering Department
BS in Electrical Engineering
Dean Y. Arakaki
Sustainability is one of today's primary engineering objectives. This principle involves system design that minimizes environmentally harmful energy emissions and resource consumption, and maximizes renewable energy practices . Communication antennas transmit wireless signals that can be converted into usable energy. The Rectenna system described in this report, shown in Figure 1, was designed to accomplish this energy conversion, with -5dBm (316µW) minimum power at the rectifier input. Since typical ambient signal power is in the -70dBm (0.1nW) range, the proposed system could only convert passive, relatively high-power microwave band AC signals to DC. The Rectenna system was designed for 1.9GHz signal reception; however, the greatest ambient 1.9GHz signal power measured in Cal Poly’s Microwave Lab was in the -75dBm (31pW) to -70dBm (100pW) range, shown in Table 1. The team provided an external 1.9GHz source (-20dBm to 3dBm) to verify the design.
An inset-fed microstrip patch is used as an energy harvesting antenna; the single patch was then arrayed into a 2x2 planar configuration. The designed patch antenna array has a 3dB larger gain, and 1% increased frequency bandwidth compared to the single patch. However, it is unable to harvest sufficient RF power for energy storage. When capturing multiple-source ambient RF signals, an omnidirectional antenna (captures energy in all directions) should be implemented, rather than a directional patch antenna array.
The Greinacher rectifier  converts RF energy into usable DC power which is multiple times the input RF peak voltage. Simulations show the Greinacher rectifier output voltage is a function of the number of stages and peak input voltage. The antenna and rectifier are matched with |S11| less than -21dB and -5dB, respectively, at 1.9GHz to mitigate power losses. A high-efficiency Main Boost Converter (BQ25504) increases rectifier output DC voltage to 3.1V for charge storage on a capacitor (battery). A Self-Oscillating Boost Converter (SOBC) handles startup when the capacitor is initially discharged. A passive switching circuit was developed to enable source-free switching from the SOBC to the Main Boost Converter. The system yields 29% and 12% maximum power efficiency with -1dBm (794µW) and -5dBm (316µW) input power to the rectifier, respectively.