Int. J. Radio Frequency Identification Technology and Applications, Vol. 3, No. 3, 2011 181 Copyright © 2011 Inderscience Enterprises Ltd. Variation of signal strength and electromagnetic field pattern in conveyor-based radio frequency identification systems Chongsun Oh UW RFID Lab, Department of Industrial and Systems Engineering, University of Wisconsin-Madison, 4158 Mechanical Engineering Building, 1513 University Avenue, Madison, WI 53706-1572, USA E-mail: chongsunoh@wisc.edu Dharmaraj Veeramani* Department of Industrial and Systems Engineering, University of Wisconsin-Madison, 4101 Mechanical Engineering Building, 1513 University Avenue, Madison, WI 53706-1572, USA E-mail: raj@cae.wisc.edu *Corresponding author Abstract: In this paper, we study the characteristics and variation of RF signal strength and electromagnetic (EM) field pattern in a conveyor-based RFID application, and their impact on readability of passive UHF RFID tags. Read-rate (number of reads per second) and received signal strength indicator (RSSI) are employed as a measure of the signal strength within the EM field. It is shown that EM field patterns significantly change depending on the position of tagged case(s) containing metal products, and the number of cases within the read zone. The presence of metal can create read null points in proximity distance to the reader resulting from multipath reflections from objects in the conveyor system environment. Understanding how such significant factors affect the signal strength and EM field pattern is essential for designing an optimised RFID system to achieve a desired level of performance. Keywords: passive UHF RFID; electromagnetic field pattern; read-rate; received signal strength indicator; RSSI; signal strength variation; conveyor based RFID. Reference to this paper should be made as follows: Oh, C. and Veeramani, D. (2011) ‘Variation of signal strength and electromagnetic field pattern in conveyor-based radio frequency identification systems’, Int. J. Radio Frequency Identification Technology and Applications, Vol. 3, No. 3, pp.181–194. Biographical notes: Chongsun Oh is a Project Assistant at the University of Wisconsin-Madison RFID Lab. She received her MSc in 2005 in Industrial Engineering from the University of Wisconsin-Madison, where she is currently a PhD candidate. Her research is focused on models for design, evaluation and operation of RFID in conveyor-based systems. She has an interest in RFID system design and optimisation, RFID applied supply chain, and ubiquitous networks.