TABLE OF CONTENTS Tag antennas operate on the same principles as reader antennas, but face some very different practical challenges.
Cost: the total cost, including IC, substrate, antenna, adhesive, die attach, and testing, must be less than US$1 for most applications, and for high-volume supply-chain applications the long-term goal is to reduce total tag cost to less than US$0.05. In contrast, a moderate-quality patch antenna for a reader application has a purchase price around US$150.
Size: in supply chain applications tags must fit onto a 4-inch- (100 mm) wide label. Since the natural resonant size, half a wavelength, is about 16 cm, many tag antennas must reduce their size. In addition, many applications require total thickness < 1 mm, eliminating many potential structures from consideration.
Polarization: in many applications the orientation of tags, or the objects to which they are attached, cannot be controlled. A trade-off must be made between the use of circularly polarized reader antennas, sacrificing range, or the use of polarization-diverse tag antennas, adding cost and size to the antenna structure.
Matching to the IC load: recall from Chapter 5 that tag IC's consume very little current, and need reasonable input voltage (at least enough to turn the charge pump diodes on) that is, the IC's have a high (parallel) input impedance. Antennas and associated matching structures need to provide as high an output voltage as possible from a given incident electric field, despite size constraints,...
