Antenna Active
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 Magellan Maestro 4700 Automotive GPS Receiver US $64.99
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 CrimeStopper SP 400 1 way Combo Alarm Remote Start Keyless Entry US $99.99
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 Sony NV U94T Automotive GPS Receiver US $60.00
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 GlobalSat SDIO GPS Receiver US $4.73
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Passive RFID Tags Vs. Active RFID Tags
An RFID tag is a small device that can be attached to or incorporated into a product and transmit data to a reader using radio waves. RFID tags have antennas that allow them to receive and respond to radio-frequency queries from an RFID transceiver.
RFID tags come in two varieties: active and passive. Active RFID tags have a built-in power supply, such as a battery, as well as electronics that perform specialized tasks. By contrast, passive RFID tags do not have a power supply and must rely on the power emitted by an RFID reader to transmit data. Thus, if a reader is not present, the passive tag cannot communicate any data. Active tags can communicate in the absence of a reader.
The remainder of this article discusses other differences between the two types of tags that may influence your decision to choose one type over the other when deploying your RFID application.
Passive RFID Tags
Passive tags are comprised of three elements: an integrated circuit or chip, an antenna, and a substrate.
The RFID chip stores data and performs specific tasks. Depending on its design, the chip may be read-only (RO), write-once, read-many (WORM), or read-write (RW). Typically, RFID chips carry 96 bits of memory but can range from 2-1000 bits.
Attached to the chip is the antenna, whose purpose is to absorb radio-frequency (RF) waves from the reader's signal and to send and receive data. Passive RFID tag performance is strongly dependent on the antenna's size: the larger the antenna, the more energy it can collect and then send back out. Larger antennas, therefore, have higher read ranges (although not as high as those of active tags).
Antenna shape is also important to the performance of the tag. Low- and high-frequency (LF and HF, respectively) antennas are usually coils because these frequencies are predominantly magnetic in nature. Ultrahigh-frequency (UHF) antennas, on the other hand, look similar to old-fashioned TV antennas ("rabbit ears") because ultrahigh frequencies are solely electric in nature.
The third component of a passive RFID tag is called a substrate, which is commonly a Mylar or plastic film. Both the antenna and the chip are attached to the substrate, which may be thought of as the "glue" that holds all of the tag's pieces together.
In contrast to active RFID tags, passive tags are usually smaller and less expensive.
Active RFID Tags
Like passive RFID tags, active tags have both a microchip and an antenna. The chips, however, are usually larger in size and have greater capabilities than the RFID chips in passive tags.
Active tags have two additional components that differentiate them from passive tags: an on-board power supply and on-board electronics.
The power supply is usually a battery, although it can also be solar. The built-in power supply allows the tag to transmit data to a reader on its own, without the need to draw power from the reader itself like passive tags do. In addition, active tags can be read from distances of 100 feet or more, whereas passive tags can only be read from up to about 20 feet.
On-board electronics may consist of sensors, microprocessors, and input/output ports, all of which are powered by the tag's on-board power source. The electronics allow active RFID tags to be used in a wider range of applications than passive tags. For example, perishable food products may be tagged with sensors that collect data that can then be used to determine expiry dates and warn the end user that the item may be spoiled. Even though many products have expiration dates printed on them, these dates are valid only if the product is stored under the optimal conditions (temperature, humidity, exposure to light, etc.) for that type of product. Thus, the product may expire before the printed date if it is not stored properly. An RFID tag equipped with a temperature sensor might be able to predict the actual expiration date of a carton of milk, for example, which may be very different from the printed date.
Whether you choose to use active or passive tags in your RFID system will likely depend on both your particular application and your budget.
About the Author
Gary Randall is an RFID Strategist at American Barcode and RFID, Inc. in Phoenix, AZ. With the background and experience to see business issues from your perspective, American Barcode & RFID is committed to providing the right combination of technical innovation, performance, and cost effectiveness. For more information, visit ABRFID.com.