Characterization of RF Propagation in Helical and Toroidal Metal Pipes for Passive RFID Systems

In this paper we extend the theory of radio frequency (RF) propagation within helical and toroidal metal pipes. This work is motivated by the need to understand the operation of UHF radio frequency identification (RFID) systems when the tags are placed within toroidal metal pipes. The field components inside a helical metal pipe are derived to obtain the propagation and attenuation constants and the different transverse electric and transverse magnetic modes. The cutoff frequencies, attenuation and angle of deployment are used within the theory to identify operating limitations of passive RFID systems in hollow helical metal pipes. The helical model is then simplified to describe the case of a toroidal metal pipe. This theory extends existing theories of propagation within cylindrical metal pipes to form a comprehensive general theory. Analytical evaluations are used to visualize the tags within the metal pipes and to validate the theory presented in this paper. This evaluation presents the propagation of RF waves within toroidal, bent and cylindrical metal pipes. The general theory is shown to predict, and simulations are revealed to confirm that UHF RFID tags can be read under certain conditions within narrow pipes whose cutoff is below UHF frequencies.