The tag
chip is located at the end of the microstrip line between the microstrip line and a small
* This section is based on the work by Ng et al. (2007) # 2007 by IEEE with permission.
RFID Tags for Metallic Object Identi?¬?cation 253
square area that is connected to the ground plane through a via. The inset distance of the
feed and the length of the microstrip line are determined using simulations. Further details
on the design steps and tag dimensions can be found in Ng et al. (2007).
The tag antenna was fabricated and a practical read range measurement was carried out.
An RFID reader (Model ALR-9780-EA) suitable for operation in Australia was used in the
measurement. The RFID reader antenna used has a 6 dBi gain and the equivalent isotropic
radiated power (EIRP) from the antenna is 4 W. The tag is placed on a 1.5l31.5l
aluminum metallic plane and with the reader antenna radiating at normal incidence to
the metallic plane, the read range measured was 1.44 m.
Since it is always desirable to have a small tag size, further analysis on the tag antenna
has been performed by reducing the tag antenna size to ?¬?nd the smallest possible size
while still offering acceptable read range performance. The patch width Wpatch of the tag
antenna was reduced in steps of 10 mm from the original size of 99 mm to 19 mm. Changes
to the overall input impedance of the tag antenna have been observed through simulations
when Wpatch is reduced.
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