Hybrid antenna design for an optically powered SHF RFID transponder applicable in metals

Abstract

This paper presents a hybrid antenna design for an optically powered super high frequency (SHF) radio frequency identification transponder applicable for the integration into metal. The key feature of the antenna is its ability to receive microwave signals at SHF for data communication and optical signals for the power supply of the transponder. The antenna design is based on a circular waveguide which is filled with a bundle of polymer optical fibers to guide light to the photodiodes. In addition, a transition is placed within the circular waveguide to transfer the waveguide mode of the SHF signal into a microstrip mode which is a more suitable structure for the integration of electronic transponder components. This paper discusses the constraints and solutions for the aforementioned combination of SHF microwave and light. The figures of merit of the optical power supply are presented, including considerations of the light distribution and the obtained power as a function of the incident angle and the used polymer optical fiber diameter. Furthermore, the measured gain and return loss of the SHF antenna structure is compared to the simulated results. I . I N T R O D U C T I O NDuring the last few years, radio frequency identification (RFID) applications in general have been significantly growing and especially in the field of electronic article surveillance they have become one of the most important techniques. However, commonly used RFID transponder antennas, typically dipole-like antennas, suffer from performance degradation under the influence of a metallic environment especially when they are placed directly on a metallic surface. For the use of transponders in machine tools e.g. a miller, it is desirable to integrate them into that tool, as a transponder placed on the surface can be damaged easily under mechanical stress. The usage of an integratable transponder equipped with memory is a very interesting task in the field of machine tool monitoring, providing the ability to trace the product life of the tools. Hereby, the range of application is mainly limited by the transponder size, as the integration of the transponder mechanically weakens the machine tool and the volume needed for the integration of the transponder is sometimes not available. Since the antenna dimensions and the carrier frequency are reciprocally proportional, the antenna is designed to work in the Industrial, Scientific and Medical (ISM) band at 24 GHz.Owing to the high operating frequency and therefore increased free space attenuation, a classical passive RFID transponder supply concept based on a rectifier [1] does not yield enough supply power. Thus, hybrid antenna concepts are necessary to overcome this limitation. These designs utilizing optical energy harvesting and data communication typically use solar cells directly as part of the antenna. For example, in [2], a patch antenna is designed using solar cells as ground plane of the patch or [3] uses a solar panel as a slot antenna. These solutions or similar ones are not favorabl...