Characterisation of RF propagation in rectangular metal pipes for passive RFID systems

In this paper, we collect and extend the theory of Radio Frequency (RF) propagation within rectangular 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 rectangular metal pipes. The field components inside a rectangular metal pipe are derived to obtain the propagation and attenuation constants, modal cutoff frequency relationships, the different Transverse Electric (TE) and Transverse Magnetic (TM) modes. The cutoff frequencies, attenuation and angle of incidence are used within the theory to identify operating limitations of passive RFID systems in hollow rectangular metal pipes. This theory is combined with existing theories of propagation within cylindrical metal pipes to present a comprehensive general theory. Analytical evaluations are used to visualise the tags within the metal pipes and to validate the theory presented in this paper. It is shown that passive UHF RFID tags can be read even below the cutoff frequencies of these rectangular metal pipes when used under the guidance of the general theory.