Active tags may incorporate sensors.
With active tags, battery life is critical as an active tag cannot harvest energy from the
reader signal as a passive or semipassive tag can.
Passive tags are the cheapest of the four but have the least capability. Passive tags are
commonly used to track items functioning as a wireless barcode. Semipassive tags
are more expensive than passive tags, but cheaper than active tags, with capabilities falling
in the middle as well. Because of their cost, semipassive tags are ideal in cases where a
large number of assets must be monitored (with sensors), or in situations where the tag
cannot be reused. Active tags are the most expensive, but offer the greatest capabilities.
Currently, the military and highway drivers (EZ-Pass) are the primary users of active tags.
The remainder of the chapter focuses on far-?¬?eld passive RFID tags.
1.2 Backscatter Communication
All objects re?¬‚ect radio waves, where RF energy and these re?¬‚ected waves are the basis for
pulsed radar systems. Pulsed radar allows distance and direction to be determined. Two
basic types of antennas exist, an omnidirectional and a directional antenna. An omnidirectional
antenna emits RF energy in all directions, whereas a directional antenna emits RF
energy in a speci?¬?c direction.
Backscatter takes advantage of the re?¬‚ection of radio waves.
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