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The Basics of RFID Technology

There are many different types of radio frequency identification technology. This article explains the difference between active and passive tags and between low-, high- and ultra-high frequency systems.
By Bob Violino
Passive UHF systems use propagation coupling. A reader antenna emits electromagnetic energy (radio waves). No electromagnetic field is formed. Instead, the tag gathers energy from the reader antenna, and the microchip uses the energy to change the load on the antenna and reflect back an altered signal. This is called backscatter.

UHF tags can communicate ones and zeroes in three different ways. They can increase the amplitude of the wave coming back (amplitude shift keying), shift the wave so it's out of phase (phase shift keying) or change the frequency (frequency shift keying). The reader picks up the signal and converts the altered wave into a one or a zero. That information is then passed on to a computer that converts the binary data into a serial number or the data stored on the tag.

Factors that affect performance
It's not necessary to understand the intricacies of the communication methods used, but end users do need to understand the basic characteristics of the different systems and what affects their performance.

Reading tags on liquid products is a challenge
Because low- and high-frequency systems use inductive coupling, the size of the reader field is smaller and can be more easily controlled. Ultra-high frequency systems that use propagation coupling are harder to control, because energy is sent over long distances. The waves can bounce off surfaces and reach tags you never expected them to reach; you might even read tags you don't want to read.

Low- and high-frequency systems also work better than UHF systems around metal and water. The radio waves don't bounce off metal and cause false reads. And they are better able to penetrate water; UHF radio waves are absorbed by water.

In fact, the problem with reading tags reliably is mainly an issue with UHF systems. Here are some additional issues that end users have to cope with.

Detuning the antenna: With propagation coupling, the antenna is tuned to receive waves of a particular frequency. When an antenna is placed on an object or product packaging that is not "RF friendly," the antenna can be detuned, making it difficult for the tag to receive enough energy to reflect back a signal. There are several ways to deal with this issue. Products with a lot of water and metal are particularly challenging to tag, and some antennas can be specially designed to be in tune when close to water or to couple with the metal to improve the ability to read the tag. Another way is to create an air gap between the tag and the object. In the case of metal, an air gap can increase performance if done correctly, because waves will reflect off the metal and provide more power to the tag.

Signal attenuation: Attenuation in RFID usually refers to the reduction in energy emitted by the reader or in the energy reflected back by the tag. If less energy is able to reach the tag, then the tag must be closer to the reader to be read. The energy emitted by the reader naturally decreases with distance; the rate of decrease is proportional to the inverse square of the distance. Passive UHF RFID tags (those with no batteries) reflect back a signal at very low power levels. A tag’s reflected signal decreases as the inverse fourth power of the distance between tag and reader. In other words, the signal emitted by the reader attenuates natural with distance, and the signal reflected by a passive tag attenuates at a much faster rate.

Signal attenuation can also be caused by the way a system is installed or external factors, such as the items tagged. Many readers have one or more external antennas that emit radio waves. These are connected to the reader by coaxial cables. As the energy travels from the reader, through the cable, to the reader antenna, the signal attenuates, so placing reader antennas too far from the reader can cause poor performance.

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