AN2866
Application note
How to design a 13.56 MHz
customized tag antenna
Introduction
RFID (radio-frequency identification) tags extract all of their power from the reader’s field.
The tags’ and reader’s antennas form a system of coupled inductances as shown in
Figure 1.
The loop antenna of the tag acts as a transformer’s secondary.
The efficient transfer of energy from the reader to the tag depends on the precision of the
parallel resonant RLC loop antennas tuned to the carrier frequency (usually 13.56 MHz).
The purpose of this application note is to give a step-by-step procedure to easily design a
customized tag antenna.
Figure 1.
RFID tag coupled to a reader’s magnetic field
Tag
Reader
ai15802
Figure 2.
An antenna designed for a specific chip and frequency
Antenna
Chip
ai15802
January 2009
Rev 1
1/24
www.st.com
Contents
AN2866
Contents
1
2
3
4
Simplified equivalent inlay circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Equivalent inlay circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Calculating the antenna coil inductance . . . . . . . . . . . . . . . . . . . . . . . . . 8
Designing the antenna coil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1
4.2
4.3
Inductance of a circular loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Inductance of a spiral coil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Inductance of an antenna with square coils . . . . . . . . . . . . . . . . . . . . . . . 10
5
Contactless measurement method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1
Antenna coil prototype verification with an analyzer . . . . . . . . . . . . . . . . 14
5.1.1
5.1.2
Preparing the equipment and connections . . . . . . . . . . . . . . . . . . . . . . 14
Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2
Antenna coil prototype verification without an analyzer (first
method) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.2.1
5.2.2
Preparing the equipment and connections . . . . . . . . . . . . . . . . . . . . . . 15
Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6
Non-contactless (contact) measurement method . . . . . . . . . . . . . . . . 18
6.1
Without an analyzer (second method) . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.1.1
6.1.2
6.1.3
Preparing the equipment and connections . . . . . . . . . . . . . . . . . . . . . . 18
Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Example using an LRI2K device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7
8
Frequency versus application: recommendations . . . . . . . . . . . . . . . 22
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2/24
AN2866
List of tables
List of tables
Table 1.
Table 2.
Table 3.
Antenna coil inductances for different C
tun
values at a given tuning frequency . . . . . . . . . . 8
K1 & K2 values according to layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3/24
List of figures
AN2866
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
RFID tag coupled to a reader’s magnetic field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
An antenna designed for a specific chip and frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Equivalent circuit of a chip and its antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Equivalent circuit of a chip, its antenna (modeled with a series
resistance) and connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Equivalent circuit of a chip, its antenna (modeled with a parallel
resistance) and connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Simplified equivalent circuit of a chip, its antenna and connections . . . . . . . . . . . . . . . . . . . 7
Antenna design procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Spiral coil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Square coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
User interface screen of the planar rectangular coil inductance calculator. . . . . . . . . . . . . 12
Rectangular planar antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Measurement equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Resonance traces of the prototype at different powers . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
ISO standard loop antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Without an analyzer: first measurement method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Oscilloscope views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Synthesis of resonance traces for different voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Measurement circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Determining the resonance frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Coil samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Coil characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
New coil samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Second coil characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Best antenna coil prototype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4/24
AN2866
Simplified equivalent inlay circuit
1
Simplified equivalent inlay circuit
The chip and its antenna can be symbolized using their equivalent electrical circuit.
Figure 3
shows the equivalent electrical circuit of the chip (parallel association of a
resistance which emulates the current consumption of the chip and a capacitance added to
the chip to ease tuning).
The antenna is a wire, so its equivalent electrical circuit is a wire with a resistance
symbolized by R
ant
. The antenna also has an inductance denoted by L
ant
. The capacitance
C
ant
is the representation of parasitic elements (produced by the bridge).
Figure 3.
Equivalent circuit of a chip and its antenna
A
R
ant
R
chip
C
tun
C
ant
L
ant
B
Chip
Antenna
ai15804
5/24
Embedded System
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EEWORLD
