Diseño De Antenas

AN710
Antenna Circuit Design for RFID Applications
Author:

Youbok Lee, Ph.D.
Microchip Technology Inc.

REVIEW OF A BASIC THEORY FOR
RFID ANTENNA DESIGN

INTRODUCTION

Current and Magnetic Fields

Passive RFID tags utilize an induced antenna coil
voltage for operation. This induced AC voltage is
rectified to provide a voltage source for the device. As
the DC voltage reaches acertain level, the device
starts operating. By providing an energizing RF signal,
a reader can communicate with a remotely located
device that has no external power source such as a
battery. Since the energizing and communication
between the reader and tag is accomplished through
antenna coils, it is important that the device must be
equipped with a proper antenna circuit for successful
RFIDapplications.

Ampere’s law states that current flowing in a conductor
produces a magnetic field around the conductor. The
magnetic field produced by a current element, as
shown in Figure 1, on a round conductor (wire) with a
finite length is given by:

An RF signal can be radiated effectively if the linear
dimension of the antenna is comparable with the
wavelength of the operatingfrequency. However, the
wavelength at 13.56 MHz is 22.12 meters. Therefore,
it is difficult to form a true antenna for most RFID applications. Alternatively, a small loop antenna circuit that
is resonating at the frequency is used. A current
flowing into the coil radiates a near-field magnetic field
that falls off with r-3. This type of antenna is called a
magnetic dipole antenna.
For 13.56 MHzpassive tag applications, a few
microhenries of inductance and a few hundred pF of
resonant capacitor are typically used. The voltage
transfer between the reader and tag coils is accomplished through inductive coupling between the two
coils. As in a typical transformer, where a voltage in the
primary coil transfers to the secondary coil, the voltage
in the reader antenna coil is transferredto the tag
antenna coil and vice versa. The efficiency of the
voltage transfer can be increased significantly with high
Q circuits.
This section is written for RF coil designers and RFID
system engineers. It reviews basic electromagnetic
theories on antenna coils, a procedure for coil design,
calculation and measurement of inductance, an
antenna tuning method, and read range in RFIDapplications.

EQUATION 1:
µo I
B φ = -------- ( cos α 2 – cos α 1 )
4πr
where:
I = current
r = distance from the center of wire
µ0 = permeability of free space and given
as 4 π x 10-7 (Henry/meter)
In a special case with an infinitely long wire where:
α1 = -180°
α2 = 0°
Equation 1 can be rewritten as:

EQUATION 2:
µo I
B φ = -------2πr

2

( Weber ⁄ m )

FIGURE 1: CALCULATIONOF MAGNETIC
FIELD B AT LOCATION P DUE TO
CURRENT I ON A STRAIGHT
CONDUCTING WIRE
Ζ
Wire
α2

dL

α

I

R

α1
0

 2003 Microchip Technology Inc.

2

( Weber ⁄ m )

r

P
X

B (into the page)

DS00710C-page 1

AN710
The magnetic field produced by a circular loop antenna
is given by:

EQUATION 3:

FIGURE 2: CALCULATION OF MAGNETIC
FIELD B AT LOCATION P DUE TOCURRENT I ON THE LOOP

2

µ o INa
B z = --------------------------------2
2 3⁄2
2(a + r )

X

coil

2

=

µ o INa  1 
------------------ --- 3
2
r

2

for r >> a

2

I

α
a

where

R
r

y
I = current

Bz

a = radius of loop
r = distance from the center of loop
µ0 = permeability of free space and given as
4 π x 10-7 (Henry/meter)
The aboveequation indicates that the magnetic field
strength decays with 1/r3. A graphical demonstration is
shown in Figure 3. It has maximum amplitude in the
plane of the loop and directly proportional to both the
current and the number of turns, N.

P
z

V = V o sin ω t

FIGURE 3: DECAYING OF THE MAGNETIC
FIELD B VS. DISTANCE r
B
r-3

Equation 3 is often used to calculate the ampere-turn...