Transmit power & frequency. Active tags transmit at higher power levels than passive tags can. The higher power allows effective operation in difficult installations. Such installations include metal, liquid, or areas with RF interference. For instance, active tags are less susceptible to interference than passive tags are. Metal environments include vehicles and shipping containers. Liquid-base installations include biological implants in animals and people. (Animals and people are 70 pecent water.) In difficult installations, tag frequency also affects performance. For example, low frequencies (LF) pass through obstructions that reflect or block high frequencies (HF). A metal or liquid installation would probably require a LF tag.

Range, life, size & cost. Compared to passive tags, active tags offer a much longer transmit distance (some 1,000 yards). With nothing between the tag and reader, the range is usually farther outdoors. Active tags have larger memories than passive tags. Active tags can also store information from the reader. The smallest active tags are about the size of a grape. The largest active tags compare in size to a rolled-up newspaper. One disadvantage of active tags is that they cost more than passive tags do. Active tags cost a few dollars apiece. Because an active tag's battery wears out, a passive tag might outlast an active tag. Yet an active tag's battery can keep going for a decade.

Table of Contrasts Between Active & Passive Tags

Passive Tags

No onboard power supply. Passive RFID tags are transponders without onboard power supplies. Inbound radio frequency signals induce a tiny electrical current in the device antenna. The circuit rectifies and filters this AC current. The resulting direct current runs a low-power CMOS integrated circuit in the tag. When the chip comes close to a reader, the chip transmits data. The same antenna that collects power from the incoming signal also transmits the return signal. Passive tags offer read distances from several yards down to a few inches.Transmission range depends on three factors...

Deluxe RFID tags can write and store data on a non-volatile EEPROM. The cheapest tags strictly contain read-only memory. After the operator programs these inexpensive devices, the data is permanent. The chip won't accept revised data.

Small size. Eliminating the power supply keeps passive RFID cards small. Some ot these tags are about the size of a grain of rice. Operators can embed these devices in printable stickers. Zebra® RFID printers both program the tags and print the stickers.

Human implants. Low-frequency RFID tags can even reside under the skin. VeriChip™ manufactures a 134-kHz chip and promotes such subcutaneous RFID implants. To safeguard the body against reactions with its chip, VeriChip encapsulates the chip in glass. The chips are injectable. The usual injection point is the upper right triceps. Unfortunately, a hacker can clone a VeriChip device. (See our links for a site that tells how.) Also, some MRI scanners might cause hazardous heating of the implanted devices.

Size & cost. EPC (Electronic Product Code) RFID tags can be reasonably inexpensive. In quantity, the cheapest ones cost a nickel apiece. Big-box stores and the US Department of Defense both use such tags. With the antenna, tag size varies from post card size down to pinhead size.

Chipless Passive tags. A fairly new concept in RFID is the chipless tag. Inside a chipless tag are materials that reflect part of the reader signal. The return signal is unique. With this signal, the reader can identify a particular tag. Chipless tags offer advantages over chipped tags. For instance...

• RF interference isn't the problem that it is with chipped tags.

• Chipless tags work over a broad temperature range.

Printable technology. Silicon tags allow manufacturers to use conductive ink and print tag antennas. The technology for printing tag circuitry also exists. The circuitry for such tags resides on a polymer substrate instead of silicon. Of course, silicon is rigid. The polymer is flexible. The same web-fed (roll) presses that print magazines can print polymer tags. These polymer tags are in development now. One day, the printing process will make polymer tags cheaper than silicon tags.

Catalog of Uses

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