(eBook - PDF - Electronics) Antenna Circuit Design for RFID Applications(Lee 2003)

Author: Youbok Lee, Ph.D. J3zb_
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Microchip Technology Inc. 5p>a]gp  
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50PAGE XI0O^[/n{  
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ENGLISH }C'z$i( y  
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INTRODUCTION 15zL,yo  
Passive RFID tags utilize an induced antenna coil ZNUV Bi  
voltage for operation. This induced AC voltage is gC3{:MC-G  
rectified to provide a voltage source for the device. As Qj=l OhM  
the DC voltage reaches a certain level, the device m
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starts operating. By providing an energizing RF signal, =thgNMDm"  
a reader can communicate with a remotely located wF%XM_M  
device that has no external power source such as a %'>. R  
battery. Since the energizing and communication kgQEg)A]!x  
between the reader and tag is accomplished through ^qDkSoqC"  
antenna coils, it is important that the device must be z!1j8o2  
equipped with a proper antenna circuit for successful 8'?e4;O  
RFID applications. $v^F>*I1
 
An RF signal can be radiated effectively if the linear /Ly%-py-$  
dimension of the antenna is comparable with the k&**f_b  
wavelength of the operating frequency. However, the -ZuzJAA  
wavelength at 13.56 MHz is 22.12 meters. Therefore, [$./'-I]  
it is difficult to form a true antenna for most RFID applications. e88JT_zrO  
Alternatively, a small loop antenna circuit that #e#8I7P  
is resonating at the frequency is used. A current 4@bL` L)  
flowing into the coil radiates a near-field magnetic field 0VbZBLe  
that falls off with r-3. This type of antenna is called a z><
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magnetic dipole antenna. s<5t}{x  
For 13.56 MHz passive tag applications, a few )sL:iGU  
microhenries of inductance and a few hundred pF of }ri"u;.R  
resonant capacitor are typically used. The voltage Y3-P*  
transfer between the reader and tag coils is accomplished _W!g'HP-D  
through inductive coupling between the two X+*| nvq]  
coils. As in a typical transformer, where a voltage in the & .#0jb1r  
primary coil transfers to the secondary coil, the voltage mr/^lnO  
in the reader antenna coil is transferred to the tag fEpY3od  
antenna coil and vice versa. The efficiency of the ,wq.C6;&  
voltage transfer can be increased significantly with high LHacHv  
Q circuits. tVe*J@i\$  
This section is written for RF coil designers and RFID Zj},VB*T  
system e .. N\vc<Zpn  
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