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RFID tags are best simulated using
the frequency domain solver in CST
STUDIO SUITE. For these tags,
simulation can be used to calculate the
H-field and surface currents induced
in the coil (Figure 1), and to extract an
equivalent circuit for the tag.
HF RFID systems on the other hand
offer higher data rates and longer
ranges, making them suitable for
applications such as inventory
tracking and electronic toll collection.
In HF RFID tags, the coil acts as a
normal antenna, usually tightly folded
best position and orientation for the
RFID tags relative to the reader and,
using the built-in “Calculate RFID
Read Distance” macro in CST STUDIO
SUITE, the readable range of the tag
can be calculated over the range of
possible angles (Figure 3) given the
output power and sensitivity of the
reader antenna.
Most RFID tags include an integrated
circuit, which contains the data
associated with that tag. The chip
itself will have a characteristic
inductance and capacitance which will
affect the tuning of the antenna, and
may also include a matching circuit to
improve antenna efficiency. To allow
these to be taken into account by the
simulation, CST STUDIO SUITE also
includes a schematic circuit simulation
tool which is integrated into the 3D
design environment. The 3D model
can be treated as a block and included
in a circuit simulation or, using true
transient-circuit co-simulation, the
chip can be inserted into the 3D model
as a SPICE or IBIS fi le. A simulation
involving the complex chip impedance
is shown in Figure 5.
Figure 1: H-field (left) and surface current density
(right) for a typical LF RFID tag.
Figure 2: A high-frequency RFID tag (top) and its
farfi eld pattern at its
to reduce its area. This means that
the impedance matching in HF RFID
tags needs to be carefully optimized
to allow the small antenna to operate
efficiently.
HF RFID tags can be simulated
using the time domain solver or the
frequency domain solver, depending
on the antenna geometry and model
size (including the environment).
Useful results when dealing with HF
RFID tags include their S-parameters
and their farfields (Figure 2). The
farfields can be used to identify the
Figure 3: The read range of the RFID tag in Figure 2.
New-Tech Magazine Europe l 53