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

Author: Youbok Lee, Ph.D. ,J^Op   
Microchip Technology Inc. yR!>80$j  
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INTRODUCTION W% [
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Passive RFID tags utilize an induced antenna coil _~kcr5  
voltage for operation. This induced AC voltage is LZVO9e]  
rectified to provide a voltage source for the device. As p(G?  
the DC voltage reaches a certain level, the device GN<I|mGLJK  
starts operating. By providing an energizing RF signal, Rv-o__C!  
a reader can communicate with a remotely located NIfc/%  
device that has no external power source such as a FCWphpz  
battery. Since the energizing and communication =gIYa  
between the reader and tag is accomplished through twtDyo(\  
antenna coils, it is important that the device must be |-fx 0y   
equipped with a proper antenna circuit for successful K)?^
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RFID applications. jveRiW@  
An RF signal can be radiated effectively if the linear @\y7 9FX  
dimension of the antenna is comparable with the 6&Dvp1`m  
wavelength of the operating frequency. However, the *LRGfk+h  
wavelength at 13.56 MHz is 22.12 meters. Therefore, 6!nb)auVi  
it is difficult to form a true antenna for most RFID applications. yjq )}y,tF  
Alternatively, a small loop antenna circuit that `9+EhP$RS  
is resonating at the frequency is used. A current 2z\F m/Z.  
flowing into the coil radiates a near-field magnetic field *K(xES!b  
that falls off with r-3. This type of antenna is called a [1C#[Vla  
magnetic dipole antenna.
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For 13.56 MHz passive tag applications, a few ge[i&,.&z  
microhenries of inductance and a few hundred pF of ?5Fj]Bk]  
resonant capacitor are typically used. The voltage ;]Aa  
transfer between the reader and tag coils is accomplished zEW:Xe)  
through inductive coupling between the two oc(bcU  
coils. As in a typical transformer, where a voltage in the h 3&:"*A2  
primary coil transfers to the secondary coil, the voltage UjaC( c  
in the reader antenna coil is transferred to the tag DR /)hAE  
antenna coil and vice versa. The efficiency of the CnH R&`  
voltage transfer can be increased significantly with high N::;J  
Q circuits. qM0MSwvC=  
This section is written for RF coil designers and RFID OK\%cq/
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system e .. ^H4iHjg  
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