To cater for the impedance changes and maintain a conjugate
match of both real and imaginary parts between the antenna and the chip, the inset
feed distance and the microstrip line length were adjusted slightly for each decrement
of Wpatch.
RFID tags corresponding to different patch widths were fabricated. Read range
measurements of these tags are performed using the same RFID reader and reader antenna
as mentioned earlier. The plot of read range versusWpatch is shown in Figure 13.3 for the tag
in free space and when the tag is attached to a 1.5l31.5l aluminum metallic plane. It can be
observed from Figure 13.3 that there is a pattern in the reduction of read range whenWpatch
is reduced. The read range of the smallest size tag (with Wpatch??19 mm) is about half
that of the full size tag (with Wpatch??99 mm). However, despite the read range reduction,
the read range for the smallest tag is still acceptable considering the amount of tag size
reduction compared with the full size tag. Hence, the smaller tag can be suitable for use
in applications that do not require a maximum possible read range but do require a smaller
tag size to attach the tag to a limited space or area on the metallic object.
Wpatch
Lpatch
Lpatch + 12h
Wpatch + 12h
Via
Patch
Substrate
Chip
FIGURE 13.2
Structure of the RFID tag consisting of a patch antenna (top view).
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