User's Manual

MPR series User’s Manual: Draft version 0.95 11/4/04 page 28
Tag antennas are also sensitive to their local environment, a fact that is of particular import since tags are
meant to be attached to objects. Many common materials, such as paper and most plastics, have little effect
on microwave propagation; tags can be attached readily to cardboard or plastic boxes or containers without
affecting their operation. However, large metal objects have important effects both on the local electric
fields and the impedance of nearby antennas. Tag antennas cannot be attached directly to metal plates or
boxes without suffering degraded performance. Tag antennas spaced 5 mm to 1 cm (0.2 to 0.4 inch) from a
metal surface can perform acceptably, particularly if designed for near-metal service. Aqueous fluids
(water and water-containing materials such as milk, juices, most cleaning fluids, etc.) also have a strong
effect on local field intensity and may affect tag antenna impedance as well, depending somewhat on the
tag design. Again the best operation of a tag will be obtained if it is kept at least 1-2 cm from bodies of
aqueous fluid.
The received signal from a tag antenna is connected to an integrated circuit. Tag IC’s are very small (to
keep the cost of manufacturing low), and are typically embedded in a plastic coating for mechanical
protection. The IC contains a rectifying circuit to convert the received 900 MHz signal to a DC voltage
used to power the remainder of the IC. Variations in the received power are converted to variations in a
DC voltage, providing the IC with a method of sensing information transmitted by the reader. The IC can
also modify the impedance it presents to the antenna, by using a transistor as a switching element, thus
causing a variation in the signal reflected back to the reader and enabling the tag to communicate back to
the reader without needing its own radio transmitter.
The necessity of powering the tag is the primary limitation on the read range. Tags require a few 10’s of
microwatts of RF power to operate, limiting the range to about 3-6 meters with an isotropic antenna, or
about 10-15 meters with a directional antenna. When linearly-polarized reader antennas are used, read
range may be degraded by misorientation of the tag. Most indoor environments have very complex
propagation characteristics, with the transmitted signal reflecting off numerous obstacles such as walls,
floors, other tagged objects, people, vehicles, desks, tables, etc. As a consequence, the signal strength can
vary by a factor of 10 or more between two neighboring locations separated by about a half-wavelength (16
cm or 5 inches): this phenomenon is known as fading, and is encountered in most wireless communications
systems. A tag with the misfortune to find itself in a fade may fail to power up, while a tag farther from the
reader but happily located in a region of maximum signal strength responds readily. Thus there is no
reliable simple correlation between tag location the likelihood of reading a tag. The exact signal strength
configuration is sensitive to the positions of all reflecting / diffracting objects in proximity to the read
region (including people and their tools and toys) to an accuracy of much less than a wavelength, and thus
in practice is impossible to predict or control.
The best approach to deal with fading is the use of diversity: intentional variations in the propagation
environment to ensure that each tag finds itself in a region of decent signal strength at some point.
Diversity can be achieved by alternately employing two antennas in slightly different positions (displaced
by at least a half a wavelength); the MPR6000 or MPR7000 can be operated in this fashion by alternately
addressing antennas A and B. Alternatively, the location of the tags relative to the reader antenna(s) can be
varied; this beneficial effect occurs naturally when the tags to be read are moving on a conveyorized belt,
or are rotated as a pallet of boxes is wrapped with plastic in preparation for transport.
1.5.3.4 System integration
An RFID reader can collect large amounts of data, often much more than would have been obtained by a
human being employing a bar code reader. To convert this data into knowledge may require considerable
filtering. For example, if a fork lift driver moves a pallet out of a door, then returns to the facility to correct
an error in some paperwork, and finally drives out through the door to the truck again, the reader may take
three inventories of the same pallet, but it is rarely desirable to treat the resulting information as suggesting
that the same items were shipped three times. On the other hand, if the pallet is returned by a hand truck,
and the operator’s colleague stands in front of the reader antenna during the transfer, the reader may fail to