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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.
Active tags have a read range of up to 300 feet (100 meters) and can be read reliably because they broadcast a signal to the reader (some systems can be affected by rain). They generally cost from $10 to $50, depending on the amount of memory, the battery life required, whether the tag includes an on-board temperature sensor or other sensors, and the ruggedness required. A thicker, more durable plastic housing will increase the cost.
Passive RFID Systems
Passive RFID tags have no power source and no transmitter. They are cheaper than active tags (20 cents to 40 cents) and require no maintenance, which is why retailers and manufacturers are looking to use passive tags in their supply chains. They have a much shorter read range than active tags (a few inches to 30 feet).
A passive RFID transponder consists of a microchip attached to an antenna. The transponder can be packaged in many different ways. It can be mounted on a substrate to create a tag, or sandwiched between an adhesive layer and a paper label to create a printable RFID label, or smart label. Transponders can also be embedded in a plastic card, a key fob, the walls of a plastic container, and special packaging to resist heat, cold or harsh cleaning chemicals. The form factor used depends on the application, but packaging the transponder adds significantly to the cost.
Radio waves behave differently at each of these frequencies, which means the different frequencies are suitable for different applications. We'll explain a little bit about the different frequencies, but it's useful to think of low frequency waves as the waves that reach your radio. They can penetrate walls well, but can't go through metal. Low-frequency tags are ideal for applications where the tag needs to be read through material or water at close range (more about read range in a minute).
As you increase the frequency of radio waves they start to behave more like light. They can't penetrate materials as well and tend to bounce off many objects. Waves in the UHF band are also absorbed by water. The big challenge facing companies using UHF systems is being able to read RFID tags on cases in the center of a pallet, or on materials made of metal or water.
Inductive vs. Propagation Coupling
So why are companies eager to use UHF passive systems in the supply chain, rather than low-frequency and high-frequency systems? One reason is some vendors in the UHF market have offered simple, low cost tags. Another reason is read range. Companies need to be able to read tags from at least 10 feet (3.3 meters) for RFID to be useful in a warehouse. That's because there is no way to read a tag on a pallet going through a dock door from less than 10 feet. At closer distances, the reader begins to interfere with the normal operation of forklifts and other equipment. Low-frequency tags can usually be read from within 12 inches (0.33 meter). High frequency tags can be read from up to 3 feet (1 meter), and UHF tags can be read from 10 feet or more.
Read range is determined by many factors, but one of the most important is the method passive tags use to transmit data to the reader. Low- and high-frequency tags use inductive coupling. Essentially, a coil in the reader antenna and a coil in the tag antenna form an electromagnetic field. The tag draws power from the field, uses the power to run the circuitry on the chip and then changes the electric load on the antenna. The reader antenna senses the change in the magnetic field and converts these changes into the ones and zeros that computers understand. Because the coil in the tag antenna and the coil in the reader antenna must form a magnetic field, the tag must be fairly close to the reader antenna, which limits the read range of these systems.
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