文档介绍：AN710
Antenna Circuit Design for RFID Applications
REVIEW OF A BASIC THEORY FOR
Author: Youbok Lee, .
Microchip Technology Inc. RFID ANTENNA DESIGN
INTRODUCTION Current and ic Fields
Passive RFID tags utilize an induced antenna coil Ampere’s law states that current flowing in a conductor
voltage for operation. This induced AC voltage is produces a ic field around the conductor. The
rectified to provide a voltage source for the device. As ic field produced by a current element, as
the DC voltage reaches a certain level, the device shown in Figure 1, on a round conductor (wire) with a
starts operating. By providing an energizing RF signal, finite length is given by:
a reader municate with a remotely located
EQUATION 1:
device that has no external power source such as a
battery. Since the energizing munication
µ
between the reader and tag is plished through oI 2
Bφ= --------- ()cosα– cosα()Weber ⁄ m
antenna coils, it is important that the device must be 4πr 2 1
equipped with a proper antenna circuit for essful
RFID applications. where:
An RF signal can be radiated effectively if the linear
I = current
dimension of the antenna parable with the
r
wavelength of the operating frequency. However, the = distance from the center of wire
µ
wavelength at MHz is meters. Therefore, 0 = permeability of free space and given
it is difficult to form a true antenna for most RFID appli- as 4 π x 10-7 (Henry/meter)
cations. Alternatively, a small loop antenna circuit that
In a special case with an infinitely long wire where:
is resonating at the frequency is used. A current
α
flowing into the coil radiates a near-field ic field 1 = -180°
-3
that falls off with r . This type of antenna is called a α
2 =0°
ic dipole antenna.
Equation 1 can be rewritten as:
For MHz passive tag applications, a few
microhenries of inductance and a few hundred pF of EQUATION 2:
resonant capacitor are typically used. The voltage