Electric fields from RF tag interrogators underneath an urban rail train

RF tag interrogation systems are used for position determination in the automatic control of urban rail trains. Such systems consist of a Tag Interrogator Unit (TIU) which is attached to an antenna mounted underneath the rail car and transmits interrogation pulses to tags installed between the rails. The TIU operates in a complex electromagnetic environment that contains some components which are highly conductive, such as the rail car itself, the wheel bogie sets, the rails and rail spikes into the ground. Other components have lower but still significant conductivities and permittivities different from that of free space, such as the ground, the ballast, and the sleepers (ties). In the design of a positioning system it is important to gain an understanding of the influence of these components on the RF field distribution underneath the rail car, especially in the plane where the tags are located. This paper discusses the use of a computer simulation, using the RF module of the COMSOL Multiphysics finite element software package, to model the situation and to predict the electric field distribution, most importantly in the plane of the tag. The results of the simulation can be used to better understand the response of the TIU and the train position can be better determined.