Also in my junior year, I began focusing on a practical problem many people experience in their daily lives, i.e., when we are out and about and a battery in one of our devices requires a recharge, it is often the case that no electrical socket is available. If we were able to recharge our devices wirelessly, the issue would become much less complicated. Therefore, I joined Prof. Shih-Yuan Chen as a research assistant to work on a wireless charging project aimed at resolving the cable issue. I proposed four kinds of rectennas at the Antennas, Microwave and Millimeter-Wave Circuits Lab at NTU. I enjoyed being the primary contributor to a paper on this project entitled “A Compact Size Slot Loop Rectenna for Dual-Band Operation at 2.4- and 5.8-GHz ISM Bands,” which has yet to be completed. Traditional dual- or multi-band slot loop antennas typically operate at resonances on integral multiples of a guided wavelength. However, the proposed radiator of the prototype antenna measures 12mm × 12mm, which is only 0.096λ0 × 0.096λ0, where λ0 is the free-space wavelength at 2.45GHz. The simultaneous impedance matching on the two frequency bands is realized by adding a simple L-C matching circuit to the feed line and a microstrip open stub shunt at its end.
Another project in which I took part, entitled “Miniaturized SRRs-Loaded Loop Structure for Enhanced Wireless Power Transmission,” was accepted for the 2014 International Symposium on Antennas and Propagation. In this project, compact, metamaterial-inspired broadside-coupled split-ring resonators (BC-SRRs) were embedded within a small loop (49mm × 49mm) fed by a coplanar strip (CPS) to increase the effective permeability of the near zone of the loop to concentrate the magnetic flux. Moreover, the proposed structure embedded with the inner SRRs exhibited greater efficiency over a considerable frequency range. In particular, the PTE was found to improve up to 58% around 13.56MHz. A third paper, entitled “A 2.45-GHz High-Efficiency Loop-Shaped PIFA Rectenna for Portable Devices and Wireless Sensors,” was acceptedfor the 2015 IEEE Antennas and Propagation Society International Symposium held in Vancouver. As the primary author, I had the honor of presenting it at the symposium to a group of peers and mentors. Traditional rectennas are formed by patch antennas/arrays due to the higher gain and efficiency. However, their relatively large size makes it difficult to integrate such rectennas into portable devices or wireless sensor nodes. I proposed a rectenna based on a novel asymmetric loop-shaped planar inverted-F antenna (PIFA) achieving efficiency of almost 61.4% and a compact size of 0.066λ0 × 0.28λ0. I designed simulated antenna in ANSYS HFSS, optimizing matching circuits and rectifiers in Keysight (Agilent) ADS, implemented this on a printed circuit board, and measured the results in an anechoic chamber.
Moreover, the proposed asymmetric PIFA has not only two resonances but also several other notable characteristics, which became the basis for a follow-up project. The first rectenna was a single band model with a comparatively large size (0.066λ0 × 0.28λ0), containing a large number of elements. For this follow-up project, however, a compact, efficient dual-band model with a flexible size of 0.037λ0 × 0.155λ0 was proposed. This model not only featured harmonic suppression, but could also easily share or integrate with a Bluetooth antenna to keep devices compact, as the location of the proposed asymmetric loop-shaped PIFA is changeable, i.e., it can be located at the center or the periphery, depending on design specifications. Last but not least, a dual-band rectifier providing double voltage drop across the load was used in our rectenna design. Between the antenna and rectifier, there was a dual-band, filter-like matching circuit, including three lump elements with harmonic suppression capability for the harmonic resonances generated by the nonlinear diodes in the rectifier, further enhancing conversion efficiency. Based on its abundant, harmonious characteristics, we drafted a paper entitled “High-Efficiency Dual-Band Loop-Shaped PIFA Rectenna with Compact Adjustable Size for Portable Devices and Wireless Sensors” and submitted it to IEEE Transactions on Antennas and Propagation. As the primary author, I wrote most of the paper and gained a better understanding of how to meticulously and comprehensively explain physical phenomena involving electromagnetic theory, which differs greatly from writing a regular conference paper.
Meanwhile, through my participation in a project aimed at inventing a greenhouse sensor module system for Taiwan’s Industrial Technology Research Institute (ITRI), I was again able to experience the excitement of translating theory into practical use. Our team built the module using an RFID system, temperature sensor, microcontroller, and energy harvesting units. I designed and built a system for energy harvesting with an antenna, rectifier, and measurement setup. The system does not rely on cables or batteries, which is fully accounted for by the aforementioned high-efficiency dual-band loop-shaped PIFA rectenna. I also proposed a method to predict rectenna efficiency before trials started. We reached the open voltage and input impedance of the antenna used in the rectenna by changing the radiated antenna into an absorbing antenna in ANSYS HFSS and utilized Thevenin's theorem to implement the antenna model in Keysight (Agilent) ADS in which we designed matching circuits and rectifiers. By developing such a predictive system, which had been tested and verified in the aforementioned project “A Compact Size Slot Loop Rectenna for Dual-Band Operation at 2.4- and 5.8-GHz ISM Bands,” we were able to effectively design rectennas.