Simulation and analysis of an antenna in a transponder for the electronic toll collection system of Expressway in Thailand

Thailand Expressway implements an electronic toll collection system (ETC) that automatically deducts tolls from prepaid account. ETC system uses the dedicated short-range communications (DSRC) technology by semi-passive RFID at frequency of 5.8 GHz in order to communicate between the transceiver called the road side unit (RSU) and the transponder called the on board unit (OBU). The lifetime of OBU depends on a build-in battery and the signal strength of communication. Therefore, the lifetime may be expanded by replacing a build- in battery or increasing the signal strength of OBU. It is not feasible to replace a build-in battery. The best way to expand the lifetime of OBU is to increase the signal strength. This paper presents the study of OBU in order to analyze the relation between the physical components and the necessary design parameters of an antenna. The results illustrate that the parasitic elements of an antenna play an important role in the design parameters. It is shown that the antenna return loss (at operated frequency of 5.8 GHz) is improved from -5.44 dB to -19.90 dB after including the bottom parasitic element with the loop antenna. By adding both bottom and top parasitic element with the loop antenna. The directivity of antenna is increased from 2.52 dB to 8.03 dB. Simulation results show that both top and bottom parasitic element have impact on the design parameter (return loss and directivity of antenna). The performance of OBU can be improved by adding a top and a bottom parasitic element.