RFID systems come in different flavors from the point of view of the frequency they use. Selecting the most adequate frequency is a function of two variables: the technological developments of systems at the different operating frequencies—directly related to the cost of systems—as well as the properties of electromagnetic waves at those different frequencies.
The initial users of RFID systems found systems operating in the low- frequency (LF) range followed a few years later by systems operating in the high-frequency (HF) range. Therefore, their selection of systems was limited to these two broad frequency ranges even if those were not the most adequate for their specific applications. Recent years have seen an increase in the number of available RFID systems operating in the UHF range as well as a reduction of their economic cost.
It is possible to expect a similar trend in upcoming years for RFID systems operating in the microwave range to become more available and more affordable. This will help the final users to make decisions on what type of RFID systems to deploy based on technical reasons rather than based on the availability and cost of these systems.
The LF band has been used by RFID systems for several years. This fact, together with the less stringent requirements of the electronics operating in this frequency range, has made these systems very cost-effective. Their major drawback is that due to the low frequency of the carrier, these systems can only communicate at low-frequency rates. The main advantage of LF RFID systems is that electromagnetic waves operating in the LF range are the least affected by the presence of metals due to the penetration depth of these frequencies.
Applications for Low-Frequency RFID Systems
This makes them ideal to be used in applications in which the transponders are surrounded by large metallic masses. Furthermore, LF waves can travel through water with minimal attenuation compared to waves of higher frequencies. This property makes them the frequency range of choice for the identification of animals due to their high water content. Most of these applications are based on implanting these transponders in livestock and pets.
RFID systems operating in the HF range can support higher read rates than LF systems. Although signals in the HF range are more affected by metal and have more attenuation when traveling through water, the transponders used for HF systems can be manufactured at a lower cost than the transponders operating in the LF range. This is due to the fact that the antennas for HF transponders can be made smaller.
The need for less conductive material to construct the antennas results in a lower cost. Finally, the global uniformity across the world in the frequency of 13.56 MHz makes HF systems able to operate in any country.
UHF RFID Systems
While HF RFID systems operate at the single frequency of 13.56 MHz, UHF RFID systems can operate at different frequencies in the UHF band. Moreover, different countries have established different frequency ranges for their UHF RFID operation, making the compatibility between systems more difficult. For example, in North America the assigned band is from 902 MHz to 928MHz, while in Europe it is from 860 MHz to 868 MHz and in Japan is from 950 MHz to 956 MHz.
Other countries and regions have other different allocated frequency ranges. Despite this challenge and due to the developments in microelectronics that allowed the decrease in cost for these systems, UHF systems have established themselves as a viable alternative to the existing LF and HF systems. One of the main advantages of UHF RFID systems is that their higher carrier frequency allows for a much faster read rate of the transponder information, thus allowing the transmission of higher amounts of data.
The communication between transponder and interrogator used in UHF systems is by electrical field instead of the magnetic coupling used by LF and HF systems. The antennas required by UHF systems can also be smaller than the antennas used for LF and HF systems, resulting in higher efficiency. These last two factors contribute to extended read ranges for UHF RFID systems. However, UHF systems also present several draw- backs compared to HF and LF systems. The antennas for transponders operating in the UHF range have very different physical dimensions depending on the parameters that the manufacturer wants to optimize in a given transponder.
Limitations of UHF
UHF systems do not work well around metals as these reflect their electromagnetic waves. In addition, water absorbs UHF waves, making them unusable for animal implantation. Furthermore, the extended read range of UHF transponders may become a drawback for those applications that want to keep the read of transponders confined to a certain area.
Although 2.45 GHz is within the UHF range (UHF extends from 300 MHz to 3 GHz), the RFID systems operating in the frequency range extending from 2.45 GHz to 5.8 GHz are commonly called microwave RFID systems. In this frequency range, the propagation is limited to line of sight. Microwave signals are also attenuated by water and reflected by metallic objects.
The cost of interrogators and transponders used in the microwave range is higher than for any other type of system. On the positive side, it is possible to achieve very high read rates, and with the help from spread spectrum techniques, it is possible to achieve very high reliability rates and noise immunity.