Specifications

5.1. INTRODUCTION
Figure 5.1: Popular Texas Instruments Series 2000 Antennas
Full duplex systems do not require the tags to be charged before transmitting
their ID signature. As long as an interrogation signal is received, the tags will
transmit their ID code. Full duplex tags can thus be thought of as reflectors -
they reflect a received signal while modulating it with their own data stream.
This is known as backscattering.
Passive systems require strong transmitters in the reader device in order
to charge the tag’s capacitor. The system’s operational range is also limited
by the amount of energy the tags can store in their capacitors as, ultimately,
this determines the total energy in the retransmitted data stream. As a
result, communication is usually in the range of 2cm to 3m, depending on
the antennae, power supplies and frequency.
Support for multiple passive tags in the same field is thus also limited -
the tags all need to remain charged during the handshaking process, which
can take as long as three minutes for 20 tags [6]. The speed of movement
of the collection is thus limited to a few kilometres per hour in order to
guarantee all tags are read. This also means that readers must continually
transmit charging pulses to ensure reliable detection of mobile tags as they
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