4.2.1 Magnetic Field-Sensitive Antennas
A common example of a magnetic ?¬?eld-sensitive HF label suitable for operation in the HF
ISM band (13.56 MHz) is shown in Figure 4.1. The label is 42 mm in width and 47 mm in
length. The label is designed to have a suf?¬?cient number of turns to provide the resonating
inductance for the microcircuit input capacitance, as well as a ?¬‚ux collecting area in the
interior, which is as large as practicable and consistent with the size requirement for the label.
FIGURE 4.1
A magnetic ?¬?eld-sensitive antenna.
66 RFID Handbook: Applications, Technology, Security, and Privacy
Advantages of working in the near ?¬?eld at HF rather than at LF are that the number of
turns required to resonate the microcircuit capacitance is small enough for low-resolution
lithography to be used in antenna construction, and that no additional external resonating
capacitance is required.
When a larger space is available for the tag label, a larger coil area should be used. As
shown in Figure 4.2, fewer turns are then needed to obtain the required tuning inductance.
A ?¬?gure of merit for near-?¬?eld antennas is readily obtainable from the coupling volume
theory (Eshraghian et al., 1982; Cole et al., 2003; Ranasinghe, 2007) outlined in Chapter 12
of this book. It can be observed that the ?¬?gure of merit (the coupling volume) for a planar
coil operating in the near ?¬?eld varies as the third power of size, since the inductance of a
coil is dependent on the equivalent coil diameter.
Pages:
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152