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2000 LF Series
Application Note
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Texas Instruments 2000 LF Series Application Note

Series 2000 LF Antenna Design Guide
Application Note
11-06-21-068, March 2003
Radio Frequency Identification Systems
Contents
Contents................................................................................................................................ i
Edition 1 – March 2003 ........................................................................................................ i
About this Manual................................................................................................................ii
Abstract................................................................................................................................ 1
1 Why Custom Antennas may be Required.................................................................... 2
2 Standard Antennas........................................................................................................ 3
2.1 27 µH Inductance Antennas...................................................................................... 3
2.2 47 µH Inductance Antenna........................................................................................ 4
2.3 116 µH Inductance Antenna...................................................................................... 5
3 Fine Tuning Antennas................................................................................................... 5
3.1 Tuning to Resonance @ 134.2 kHz........................................................................... 5
3.1.1 The RFM-104B Module ......................................................................................................6
3.1.2 RFM-003B and RFM-007B Modules ..................................................................................7
3.1.3 The RFM-008B Tuning Board.............................................................................................7
3.1.4 The STU-MRD1 MicroReader. ...........................................................................................8
4 Antenna Design ............................................................................................................. 8
4.1 Determining Self Inductance ..................................................................................... 9
4.1.1 By Calculation .....................................................................................................................9
4.1.2 By Measurement...............................................................................................................10
4.2 Antenna Q............................................................................................................... 11
4.2.1 Determing the Antenna’s Q Value ....................................................................................12
4.2.1.1 By Measurement........................................................................................................................ 12
4.2.1.2 By Calculation ............................................................................................................................ 12
4.3 Controlling the Antenna’s Q .................................................................................... 13
4.3.1 Wire Selection...................................................................................................................13
4.3.1.1 Skin Effect.................................................................................................................................. 13
4.3.1.2 Litze Wire ................................................................................................................................... 14
4.3.1.3 Other Wires Used in Antenna Construction ............................................................................... 15
4.4 Antenna Size........................................................................................................... 16
4.4.1 Antenna Size vs. Inductance ............................................................................................17
4.4.2 Adapting a non 27µH Antenna .........................................................................................18
4.4.2.1 Using External Capacitance....................................................................................................... 18
4.5 Antenna Tails.......................................................................................................... 21
4.5.1 Tail Construction ...............................................................................................................21
4.6 Ferrite Cored Antennas........................................................................................... 22
5 Other Antennas ........................................................................................................... 22
5.1 Field Lines .............................................................................................................. 22
5.2 Opposing Antennas (In-Phase) ............................................................................... 23
5.3 Opposing Antennas (out-of-phase) ......................................................................... 24
5.4 Noise Canceling Antennas..................................................................................... 26
Appendix A – MicroReader Antenna Designs.................................................................. 27
Appendix B. Contacts........................................................................................................ 32
Figure 1.
Figures
Standard Antennas............................................................................................. 3
Figure 2. MicroReader Antenna (47 µH)............................................................................ 4
Figure 3. Mini-RFM Antenna (116 µH) ............................................................................... 5
Figure 4. The RFM-104B RF Module.................................................................................. 6
Figure 5. Inductance Fine-Tuning ..................................................................................... 6
Figure 6. RFM-003B and RFM-007B Modules ................................................................... 7
Figure 7. Capacitance Fine-Tuning................................................................................... 7
Figure 8. RFM-008B RF Module and ACC-008A Tuning Board ....................................... 8
Figure 9. The MicroReader ................................................................................................ 8
Figure 10. “ADU.EXE” Screen.......................................................................................... 10
Figure 11. LCR Meter ........................................................................................................ 10
Figure 12. High Q Vs. Low Q ............................................................................................ 11
Figure 13. LF System Spectrum....................................................................................... 11
Figure 14. Spectrum Analyzer Screen ............................................................................. 12
Figure 15. Litze Wire (3 sizes) .......................................................................................... 14
Figure 16. ‘Jumbo’ Oxygen Free Hi-Fi Wire..................................................................... 15
Figure 17. Road Loop Wire............................................................................................... 15
Figure 18. Transformer Wire ............................................................................................ 16
Figure 19. Reading Range Reduction due to Noise........................................................ 16
Figure 20. Windings Vs. Inductance ................................................................................ 17
Figure 21. Single Loop Vs. 27 µH Inductance ................................................................. 17
Figure 22. Out of Tune Conditions................................................................................... 18
Figure 23. Polypropylene Capacitor De-rating................................................................ 20
Figure 24. Polypropylene capacitors (0.01 µF & 0.47 µF)............................................... 20
Figure 25. Antenna Tail Construction.............................................................................. 21
Figure 26. Ferrite Cored Antenna..................................................................................... 22
Figure 27. Field Lines ....................................................................................................... 23
Figure 28. Antenna Field patterns.................................................................................... 23
Figure 29. Opposing Antennas (In-phase)....................................................................... 24
Figure 30. Two 54 µH Antennas Connected in Parallel (In-Phase) ................................ 24
Figure 31. Opposing Antennas (out-of-phase)............................................................... 25
Figure 32. Two 54 µH Antennas connected in Parallel (out-of-phase) .......................... 25
Figure 33. Noise Canceling Antenna ............................................................................... 26
Figure 34. MicroReader Antenna 1 .................................................................................. 28
Figure 35. MicroReader Antenna 2 .................................................................................. 28
Figure 36. MicroReader Antenna 3 .................................................................................. 29
Figure 37. MicroReader Antenna 4 .................................................................................. 30
Tables
Table 1.
RF Module Antenna Characteristics.................................................................. 9
Table 2. External Capacitance Values ............................................................................. 19
Table 3. MicroReader Antenna Designs.......................................................................... 27
Edition 1 – March 2003
This is the first edition of this LF Antenna Design Guide
It contains details on how to develop custom antennas for use with the following products:
RFM-003B, RFM-104B, RFM-007B, RFM-008B RF Modules and the MicroReader
(Note: The S2510 reader incorporates the RFM-007B)
This document has been created to help support Texas Instruments’ Customers in designing in and /or using TI*RFID products for their chosen application. Texas Instruments does not warrant that its products will be suitable for the application and it is the responsibility of the Customer to ensure that these products meet their needs, including conformance to any relevant regulatory requirements.
Texas Instruments (TI) reserves the right to make changes to its products or services or to discontinue any product or service at any time without notice. TI provides customer assistance in various technical areas, but does not have full access to data concerning the use and applications of customers’ products.
Therefore, TI assumes no liability and is not responsible for Customer applications or product or software design or performance relating to systems or applications incorporating TI products. In addition, TI assumes no liability and is not responsible for infringement of patents and / or any other intellectual or industrial property rights of third parties, which may result from assistance provided by TI.
TI products are not designed, intended, authorized or warranted to be suitable for life support applications or any other life critical applications which could involve potential risk of death, personal injury or severe property or environmental damage.
TIRIS and TI*RFID logos, the words TI*RFID™ and Tag-it™ are trademarks or registered trademarks of Texas Instruments Incorporated (TI).
Copyright (C) 2001 Texas Instruments Incorporated (TI)
This document may be downloaded onto a computer, stored and duplicated as necessary to support the use of the related TI products. Any other type of duplication, circulation or storage on data carriers in any manner not authorized by TI represents a violation of the applicable copyright laws and shall be prosecuted.
Page (i)
PREFACE
Read This First
About this Manual
This LF Antenna Design Guide Application Note (11-06-21-068} is written for the sole use by TI*RFID Customers who are engineers experienced with TI*RFID and Radio Frequency Identification Devices (RFID).
Regulatory and safety notes that need to be followed are given Section XX.
Conventions
Certain conventions are used in order to display important information in this manual, these conventions are:
WARNING:
A warning is used where care must be taken or a certain procedure must be followed, in order to prevent injury or harm to your health.
CAUTION:
This indicates information on conditions, which must be met, or a procedure, which must be followed, which if not heeded could cause permanent damage to the system.
Note:
Indicates conditions, which must be met, or procedures, which must be followed, to ensure proper functioning of any hardware or software.
Information:
Indicates conditions, which must be met, or procedures, which must be followed, to ensure proper functioning of any hardware or software.
If You Need Assistance
For more information, please contact the sales office or distributor nearest you. This contact information can be found on our web site at: http://www.ti-rfid.com
Page (ii)
.
J.A.Goulbourne
This document describes how to design and develop custom antennas suitable for attaching to Texas Instruments’ Low Frequency (LF) Radio Frequency (RF) modules and readers. It looks at the matching circuits of the standard RFMs and details the antenna requirements for each one.
The issues of reader inductance and Q are examined, together with wire selection, tail construction and the use of external capacitance in bringing an antenna to resonance.
Lit Number 11-03-21-004
LF Antenna Design Guide
Abstract
Page (1)
1 Why Custom Antennas may be Required
There are many reasons why custom built antennas may be required:
• special sized antennas are needed
• the antennas have to be built into structures/ equipment e.g. doors
• very large antenna are required e.g. road loops
• small antennas are needed (for localized reading).
• the antenna is for the microreader
A further reason may be to get increased read distance but the reader antenna is just one factor amongst many that dictates reading distance. In order of importance these factors are:
• The size and shape of the transponder’s antenna.
• The size and shape of the reader’s antenna
• The electrical noise in the environment
• The transmitter power (limited by legislation)
• Metal in the environment
Warning:
Increasing the antenna size doesn’t automatically lead to an increase in a tag’s reading performance – it may reduce.
The tag’s signal must always be 6 dB stronger than any electrical noise to ensure a successful read.
As an reader’s antenna size increases, more ambient noise is picked up and a tag may have to move closer to the antenna to make sure its signal is still the strongest.
Result – shorter reading distance
Texas Instrument’ antennas are optimized and, size for size, a custom antenna design is unlikely to give a greater read range.
Page (2)
2 Standard Antennas
Because different RF Modules require antennas with different inductances, Texas Instruments have three categories of antennas available:
2.1 27 µH Inductance Antennas
These antennas are used with the RFM-104B, RFM-007B and RFM-008B RF modules.
RI-ANT-G02E
RI-ANT-G04E
RI-ANT-G02E
RI-ANT-S02C
Figure 1. Standard Antennas
Page (3)
The RI-ANT-G01E, RI-ANT-G02E antennas have 1m tails and are nominally 27 µH and when connected to the appropriate RF Module can be tuned to resonate at 134.2 kHz. The RI-ANT-G04C antenna is provided with no tail and is nominally 26 µH. If
2.5 mm
2
(14 SWG) wire is used, a 4m (12’) tail can be added and still be capable of
being tuned to resonance.
Information:
The antenna tail is an integral part of the RF Module’s matching circuit. Changing the length of the tail changes the performance. This topic is dealt with in a later section
2.2 47 µH Inductance Antenna
The MicroReader requires an antenna with a self inductance of 47 µH. The following antenna is available:
Figure 2. MicroReader Antenna (47 µH)
RE-LNA-DLXK-NO
Page (4)
2.3 116 µH Inductance Antenna
The RFM-003B module requires an antenna with a self inductance of 116 µH and the following antenna is available:
RI-ANT-P02A
Figure 3. Mini-RFM Antenna (116 µH)
Historically, the Mini-RF Module was intended for hand-held readers and so the antenna is supplied with a 100 mm (5”) tails.
3 Fine Tuning Antennas.
The antenna and feed cable are all part of an LC antenna matching circuit on Series 2000 readers. Changing any part has an impact on the total system, e.g. lengthening the feeder cable. Each module requires antennas of a certain Inductance to ensure the matching circuit is correct and, because of manufacturing tolerances, each antenna must be fine tuned in its final positions before a system is commissioned,. Each module has provision for this tuning.
3.1 Tuning to Resonance @ 134.2 kHz
Texas Instrument’s LF RFID system operates at 134.2 kHz and any antenna must be fine-tuned to resonate at that frequency for optimum performance.
1
ƒ
(134.2 kHz)
=
2
π LC
Page (5)
[1]
Equation [1] is the formula that determines at what frequency the antenna circuit
(L)
(L)
resonates and you can see how either the Capacitance (C) or the Inductance (L) can be varied to arrive at the required frequency (ƒ). Some RF modules tune to resonance by varying the capacitance, whilst the RFM-104B and the Remote Antenna Tuning Boards both vary the inductance.
3.1.1 The RFM-104B Module
The RFM-104B (standard) RF module is shown in Figure 4
Figure 4. The RFM-104B RF Module
RFM-104B modules use a variable inductor to fine tune antennas. A representation of the circuit is shown in Figure 5.
Capacitance (C)
Antenna
Inductance
Variable Inductor
Variable
Inductance
Figure 5. Inductance Fine-Tuning
Page (6)
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