A convenient size for an antenna for labeling most cases is approximately a credit
card size label (86 mm354 mm).
. The frequency range of operation required is 902??“928 MHz.
. While using a reader radiating 4WEIRP to meet general application requirements,
tags should have read range of not <2 m.
. Since tag orientation can be ?¬?xed on boxes, there are no constraints on the
directivity of the antenna.
. Losses in the antenna should be kept small.
Given the earlier requirements, copper was chosen as the material of choice for the
antenna due to its superior conductivity. Considering the skin depth of copper, copper
sheets of thickness 32 mm should be used. The substrate considered needs to be
?¬‚exible, thin, and have a low dielectric loss. Polyesters with low dielectric constants
such as polyethylene terephthalate (PET) of thickness 50 mm are considered for the
application.
The RFID straps used will be those from Alien Technologies. The straps have a Class
I Generation 1 chip where the input impedance of a strap is typically 18.95155.8j V based
on the parallel input impedance values of R??1300 V and C??1.1 pF (refer to Figure 4.8a) at
915 MHz.
4.5.2 Antenna Type
It is possible to extend the analysis and use the results of Brown and Woodward (1952) to
analyze bow-tie antennas. Then, the Ansfot HFSS simulation tool (Ansoft, 2005) can be
used to ?¬?ne-tune the antenna impedance properties to form a match to the label IC??™s input
impedance.
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