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Depending on their frequency, radio waves can be absorbed by water and biological tissues (Bottomley & Andrew, 1978). They can also be shielded by metals due to the eddy currents. These effects constitute a huge problem for the implementation of radio frequency identification (RFID) systems in real environments. RFID systems can be classified into three different groups depending on the physics involved: i) inductive coupling; ii) back-scattering; and iii) electrical coupling. Another classification can be done attending to the electromagnetic band used: i) LF (125-134.5 kHz); ii) HF (13.56 MHz); iii) UHF (~900 MHz); and iv) ISM (2.4 GHz). The last three systems (HF, UHF and ISM) require an environment without metals or water to work properly. On the other hand, LF systems are less affected by metals on the surroundings and can penetrate some materials like water, but they cannot work properly through metals due to the shielding or detuning of the electromagnetic signal. These problems could be avoided by reducing the working frequency. However, both LF and HF systems are based on the inductive coupling between tag and reader and this coupling become rapidly inefficient with the frequency reduction. Some different solutions have been proposed to solve these problems. In this chapter we explain the basics of the inductive coupling method and the detuning and shielding effects due to metals. Additionally, a new system that is able to work at ultra-low frequencies ULF (1-100 kHz) and through a metallic shielding is proposed. Finally we compare the properties of the LF and the new ULF systems.

RFID in Metal Environments: An Overview on Low (LF) and Ultra-Low (ULF) Frequency Systemsled