Energy surrounds us, which becomes ever-more crucial as demands for energy grow rapidly. Because of the global extent of wireless technologies and almost unlimited amount of electromagnetic (EM) energy being used for wireless communications, energy-harvesting techniques are being explored as ways to recapture some of the energy originally invested in those wireless systems.
Due to the differences in polarization for the many EM waves, energy harvesting relies on the appropriate receiving antenna, such as a circularly polarized (CP) antenna to capture CP EM waves. In that vein, innovators from Singapore have developed a tapered-slit CP patch antenna with a compact rectifier for energy-harvesting purposes. The antenna prototype shows more than 5.8 dBic gain across a bandwidth of 894 to 901 MHz with a 3-dB axial ratio (AR) beamwidth of more than 180 deg. A compact composite-right/left-handed (CRLH) rectifier operating at 900 MHz was designed for use with the antenna for energy-harvesting purposes.
Such a CP antenna is a good fit for energy-harvesting applications, since a CP antenna can harvest RF energy regardless of device orientation while also being relatively insensitive to multipath effects. With a wide AR beamwidth, the antenna can cover a large area and be made as small as practical while still capturing a large amount of available RF energy at 900 MHz. The rectifier converts the received RF energy to reusable dc energy. Ideally, the rectifier provides high RF-to-dc conversion efficiency so that most of the captured RF energy can be reused as dc energy and is not lost as heat.
By using a tapered-slit microstrip radiator patch printed on good-quality, high-frequency substrate material, RO4003 circuit material from Rogers Corp., eight tapered slits with length differences of 1.3% were implanted correspondingly on a patch radiator in the octagonal directions from the patch center. This was done to achieve miniaturization and an effective CP radiation pattern.
Computer simulations were performed with the aid of CST Microwave Studio from Computer Simulation Technology, which generated, for example, the surface current distributions of the proposed antenna. Most of the surface currents were found to travel around the tapered slits, and the size of the slit can be used to increase the path of the current as needed and to ultimately tune the antenna structure for frequency and gain. Measurements of a prototype design and the computer simulations were quite close, with the simulated AR 3-dB bandwidth from 898 to 903 MHz (5 MHz wide) in comparison to the measured AR bandwidth from 894 to 901 MHz (7 MHz wide).
See “A Wide-Angle Circularly Polarized Tapered-Slit Patch Antenna with a Compact Rectifier for Energy-Harvesting Systems,” IEEE Antennas & Propagation Magazine, April 2019, pp. 94-100.