hf antenna design notes technical application...

11-08-26-003 May 2001

Radio Frequency Identification Systems

HF Antenna Design Notes

Technical Application Report

Literature Number: 11-08-26-003


Contents

Edition 1 – May 2001 ............................................................................................................. iAbout this Manual ............................................................................................................... ii

Abstract ................................................................................................................................ 11 Reader Requirements.................................................................................................... 22 Tools Required .............................................................................................................. 2

2.1 VSWR Meter ............................................................................................................. 22.2 Antenna Analyser...................................................................................................... 32.3 Oscilloscope.............................................................................................................. 42.4 Charge Level Indicator .............................................................................................. 42.5 Software Tools .......................................................................................................... 5

3 Antenna Design Considerations .................................................................................. 53.1 What is the Read Distance Required?....................................................................... 53.2 What is the Tag Orientation?..................................................................................... 53.3 At What Speed is the Tag Travelling? ....................................................................... 63.4 What is the Tag Separation?..................................................................................... 63.5 How Much Data is Required?.................................................................................... 6

4 Environmental issues.................................................................................................... 74.1 What are the Governmental (PTT/FCC) limits? ......................................................... 74.2 Is there Electrical Noise?........................................................................................... 74.3 Is there Metal in the Environment?............................................................................ 74.4 Proximity of other Antennas ...................................................................................... 7

5 Materials......................................................................................................................... 86 Loop Antennas .............................................................................................................. 9

6.1 Loop Antenna Resonant Theory.............................................................................. 106.2 Inductance Measurement........................................................................................ 11

6.2.1 Calculation........................................................................................................................11Example: 50cm x 50cm loop, 15mm diameter copper tube.............................................. 11

6.2.2 Resonance Capacitance ..................................................................................................126.2.3 Determining the Q ............................................................................................................12

6.3 Measuring the Quality Factor .................................................................................. 147 Antenna Matching ....................................................................................................... 16

7.1 Gamma Matching.................................................................................................... 167.2 T-Matching .............................................................................................................. 197.3 Transformer Matching ............................................................................................. 217.4 Matching Transformer ............................................................................................. 227.5 Baluns..................................................................................................................... 237.6 Capacitance Matching............................................................................................. 27

8 Coupling between Antennas....................................................................................... 318.1 Reflective Antennas ................................................................................................ 318.2 Two Antennas on a Splitter (in Phase) .................................................................... 318.3 Two Antennas on a Splitter (Out-of-Phase) ............................................................. 318.4 Rotating Field Antennas .......................................................................................... 328.5 Null Neighboring Antennas...................................................................................... 33



Appendix A Return Loss .................................................................................................. 34Appendix B Reactance & Resonance Chart.................................................................... 35Appendix C Coax-cable Splitter ....................................................................................... 36Appendix D Component Suppliers List ........................................................................... 37Appendix E Resistor Capacitance Values....................................................................... 38

FiguresFigure 1. VSWR Meter ........................................................................................................... 3Figure 2. Antenna Analyser .................................................................................................. 3Figure 3. Charge Level Indicator .......................................................................................... 4Figure 4. Tag Coupling..........................................................................................................5Figure 5. Tag Reading Zones................................................................................................ 6Figure 6. Capacitor Types ..................................................................................................... 8Figure 7. Field Densities for different sized antennas vs. distance ................................... 9Figure 8. Close-up of Figure 7 showing Theoretical Read Distances ................................ 10Figure 9. Antenna Q............................................................................................................... 12Figure 10. Spectrum Analyser screen................................................................................. 14Figure 11. Gamma Matching Circuit.................................................................................... 17Figure 12. A Gamma Matched Antenna .............................................................................. 17Figure 13. Gamma Matched Antenna.................................................................................. 18Figure 14. T-Matching Circuit .............................................................................................. 19Figure 15. Using the Analyzer to Determine the Matching Points .................................... 19Figure 16. T-Matched Tape Antenna ................................................................................... 20Figure 17. Transformer Matching........................................................................................ 21Figure 18. Toroid Matching Transformer ............................................................................ 23Figure 19. 1:1 BALUN........................................................................................................... 24Figure 20. Toroid BALUN..................................................................................................... 24Figure 21. Pig's Nose BALUN .............................................................................................. 24Figure 22. Generic Transformer Matching Network .......................................................... 25Figure 23. Transformer matched Example ......................................................................... 26Figure 24. Unbalanced Capacitor Tapped Matching .......................................................... 27Figure 25. 50cm x 50cm Capacitor matched Antenna ....................................................... 28Figure 26. Capacitance matching........................................................................................ 29Figure 27. Capacitor Matched Circuits................................................................................ 29Figure 28. TI*RFID Capacitor Matching Board.................................................................... 30Figure 29. Miniature Capacitance Matching Circuit ........................................................... 30Figure 30. Field Lines for In-Phase Antennas on a Splitter ............................................... 31Figure 31. Out-of-phase Magnetic Field Pattern................................................................. 32Figure 32. Rotating Field Antennas..................................................................................... 32

TablesTable 1. Return Loss Figures .............................................................................................. 34



Page (i)

Edition 1 – May 2001

This is the first edition of the HF Antenna Design Notes.

It contains a description of designing and tuning HF Antennas for use with S6000readers for use with the following products:

All Tag-it™ inlays, the S6000, S6500 Readers and any third party reader.

This document has been created to help support Texas Instruments’Customers in designing in and /or using TI*RFID products for theirchosen application. Texas Instruments does not warrant that itsproducts will be suitable for the application and it is the responsibility ofthe Customer to ensure that these products meet their needs, includingconformance to any relevant regulatory requirements.

Texas Instruments (TI) reserves the right to make changes to itsproducts or services or to discontinue any product or service at any timewithout notice. TI provides customer assistance in various technicalareas, but does not have full access to data concerning the use andapplications of customers’ products.

Therefore, TI assumes no liability and is not responsible for Customerapplications or product or software design or performance relating tosystems or applications incorporating TI products. In addition, TIassumes no liability and is not responsible for infringement of patentsand / 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 besuitable for life support applications or any other life critical applicationswhich could involve potential risk of death, personal injury or severeproperty or environmental damage.

TIRIS and TI*RFID logos, the words TI*RFID™ and Tag-it™ are trademarks orregistered trademarks of Texas Instruments Incorporated (TI).

Copyright (C) 2001 Texas Instruments Incorporated (TI)

This document may be downloaded onto a computer, stored and duplicated asnecessary 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 aviolation of the applicable copyright laws and shall be prosecuted.



Page (ii)

PREFACE

Read This First

About this ManualThis application note 11-08-26-003 is written for the sole use by TI*RFID Customerswho are engineers experienced with TI*RFID and Radio Frequency IdentificationDevices (RFID).

ConventionsCertain 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 certainprocedure must be followed, in order to prevent injury orharm to your health.

CAUTION:This indicates information on conditions, which must be met,or a procedure, which must be followed, which if not heededcould cause permanent damage to the system.

Note:

Indicates conditions, which must be met, or procedures, whichmust be followed, to ensure proper functioning of any hardware orsoftware.

Information:

Information about setting up and procedures, that make the use ofthe equipment or software easier, but is not detremental to itsoperation.

If You Need Assistance

For more information, please contact the sales office or distributor nearest you. Thiscontact information can be found on our web site at: http://www.ti-rfid.com.

http://www.ti-rfid.com/

Literature Number 11-08-26-003


Page (1)

HF Antenna Design NotesAllan Goulbourne

Abstract

This document describes how HF (13.56MHz) antennas can be built and tunedso that their characteristics match the requirements of the Texas Instrumentshigh performance Readers S6000 and S6500 and third party RF modules.

Tag-it Inlays

In general, the distance at which a Tag-it transponder inlay (tag) can be readis related to the size of the Readers antenna system and its associatedmagnetic field strength the larger the antenna the greater the range.However, as the antenna size increases other issues emerge:

The Signal to Noise (S/N) ratio will reduce. Shielding may be required to remain within regulatory legal limits. Magnetic flux holes may develop and the tag stop being read. Matching the antenna to the reader will become more difficult and if the

inductance gets too high, may prove impossible.



Page (2)

1 Reader RequirementsTexas Instruments Tag-it system operates in the High Frequency band at 13.56MHz and antennas utilize the magnetic (H) field to transfer power to the battery-lesstag during reading, writing or locking operations. (The associated electrical (E) field isnot used). The reader expects an antenna to be tuned to a centre frequency of13.56MHz, have 50Ω impedance and when connected to a reader have a (loaded) Qfactor of 30. For optimum performance, the reader matching should have a VSWRratio of less than 1:1.2. Feeder coax cables can be up to 8 m in length.

Note:

The transponders are downlink limited, i.e. providing that thepowering signal at the tag reaches a field strength of152dBµV/m, the tag will always have more than sufficientperformance to respond back over the same distance.

2 Tools RequiredFor antenna development, the following equipment is recommended:

VSWR meter Antenna Analyzer Oscilloscope (20 MHz) Charge level Indicator

2.1 VSWR Meter

The Voltage Standing Wave Ratio (VSWR) meter, see Figure 1, is used in-linebetween the reader and the antenna and indicates the efficiency of the matching byshowing the ratio of the forward signal against a reflection. If the antenna is matchedcorrectly then the VSWR should read 1:1, i.e. no reflections or return loss (VSWR issometimes expressed as the Return Loss. See Appendix A). The VSWR meter canalso indicate the output power in Watts. The VSWR meter should:

Have a full scale range of 5 Watts Operate at < 1 Watt Indicate power output.



Page (3)

Figure 1. VSWR Meter

2.2 Antenna Analyzer

An antenna analyzer can determine the characteristics of an antenna without having tohave a reader connected. It is a variable frequency signal generator that shows thematching frequency, impedance and VSWR - of the connected antenna. It can beused, together with an oscilloscope to calculate the loaded quality (Q) factor of theantenna under test.

Figure 2. Antenna Analyzer



Page (4)

2.3 Oscilloscope

The oscilloscope is used in conjunction with the antenna analyzer to allow the QualityFactor (Q) of an antenna to be calculated. It is also used in multi-antenna systems tocheck for minimum coupling between adjacent antennas. It should have:

1) 20MHz (min) Bandwidth2) Dual channel

2.4 Charge Level Indicator

This tool can be made from a Tag-it inlay and a few components and is used toshow the charge-level at any point inside the Readers antenna system.

Figure 3. Charge Level Indicator

V

1~10

pF

1N 414 8

100

nF IN L AYAN T ENN A

+

1MΩ

104 J

REM OVE CHIP



Page (5)

2.5 Software Tools

A software application can be used to determine antenna characteristics:

'ADP.exe' can be used to calculate the inductance of wire or tube antennas. For tubeantennas it will also give the capacitance values to tune the antenna to resonance.This program can be down loaded from the web site http://www.ti-rfid.com

3 Antenna Design ConsiderationsA number of questions need answers before the design of an antenna can begin:

3.1 What is the Read Distance Required?

A single antenna 500mm x 500mm has a reading range of about 500mm at 800mW ora pair of similar antennas (connected to the same reader) can cover distances greaterthan 1m. With larger antennas and greater power outputs, longer reading distancescan be achieved. The writing distance is approximately 70% of the reading range.

Integrators are advised that they should build a margin of safety into their designswhen specifying maximum read performance

3.2 What is the Tag Orientation?

Tags receive power by magnetic coupling with the antenna and will receive maximumpower when in the best orientation.

Figure 4. Tag Coupling



Page (6)

Figure 4 shows the magnetic field lines associated with an antenna and how, whenthese lines are orthogonal to the tag, best coupling results. Thus when a tag is facingan antenna it reads well but move the tag to the side of the antenna where the linesare now at right angles to the tag and no coupling results. In this location the tag willread when at right angles to the antenna. The reading zones are shown in Figure 5.

Figure 5. Tag Reading Zones

In practice though, the tag can be rotated around ± 40º either side of its optimalposition and will still be read

3.3 At What Speed is the Tag Travelling?

At a baud rate of 19,200 the S6000 Reader will read a single block, approximately 30times a second, with this rate doubling at 115,000 baud. Writing, reading multipleblocks and the Simultaneous ID, have different timings and the designer must ensurethat, whatever speed the tag is moving at, it is in the magnetic field long enough forthe complete transaction to take place. High-speed operations may require alengthened antenna.

3.4 What is the Tag Separation?

The ability to read closely separated tags depends on the width of the antenna but isclosely tied in with tag reading speed.

3.5 How Much Data is Required?

The more data that is required from the tag, the greater the time the tag must be withinthe field of an antenna, which in turn will be related to the speed the tag is travelling.

TAG

TAG

ANTE

NN

A



Page (7)

4 Environmental issues

4.1 What are the Governmental (PTT/FCC) limits?

Systems Integrators should consult their local Governmental Agencies / Test housesto determine the legal limits for the RF field generated from an antenna. Where largeantennas are required, they may need to be shielded to meet local regulations.

ETSI Regulations can be found in EN 300 330 or FCC Regulations can be found inFCC CFR47 Part 15.

4.2 Is there Electrical Noise?

Problems with electrical noise are rare but it is wise to perform a site survey beforecommencing antenna design, than struggle to solve a problem later. In generalelectrical noise tends to reduce the receive performance and result in reduced readingranges. Slight changes in antenna orientation to the noise source, additionalgrounding or shielding can all help to reduce the effects. If the problem is commonmode noise, fitting BAlanced UNbalanced (BALUNs) transformers will help.

4.3 Is there Metal in the Environment?

The presence of metal close to an antenna will reduce its performance to some extent.As the antenna size increases, so does the minimum separation distance from metalbefore de-tuning effects are noticed. Much of the effect can be 'Tuned out' but whenthe metal is close, e.g. less than 200mm, the metal will absorb some power and theread distance will drop. Antennas will always have to be tuned in situ.

4.4 Proximity of other Antennas

The presence of other antennas will alter the way a system performs because ofcoupling between antennas. In some cases, e.g. reflective passive antennas (coveredlater), the coupling will be deliberate but in most cases, this effect will have to beminimized and will be covered in a later section.



Page (8)

5 MaterialsWhilst antennas can be made from almost any conductive material, the use of coppertube or wide strip offers the best results. Aluminum is a suitable alternative to copperbut is more difficult when creating joints. For larger antennas, where the inductancemay be getting too high, 22mm (¾) copper tube can provide a self-supportingantenna. Smaller antennas can be made with 15mm (½) tube. Wide copper strip(35mm) is a suitable alternative to 22mm tube but if the antenna is small, RG405 rigidcoax (use the outer copper sheath) is all that is required. Capacitors should be micaor NP0 ceramic devices. Resistors should be manufactured from carbon film.

Figure 6. Capacitor Types

In Figure 6, above, three types of capacitor are illustrated. A) Is a fixed value silveredmica capacitor. This type of capacitor is readily available and exhibits high stability foruse in RF circuits. B) Is multi-turn precision and used for fine-tuning and C) is anothervariable mica type and is available in a number of ranges.

One important consideration when selecting capacitors is that their voltage ratingshould be suitable for the high voltages of the resonant loop antennas.



Page (9)

6 Loop AntennasLoop antennas are recommended as the most suitable for generating the magnetic (H)field that is required to transfer energy to the battery-less tag.

The field strength is a measure of the output power and for the tag to operatesuccessfully, magnetic field strength of 100mA/m (or 152dBµA/m) is required. Thefollowing graphs (Figures 7 and 8) illustrate how small sized antennas have higherfield strengths closer to the antenna than larger sizes but fall-off more quickly.Antennas much larger than 1.2m will start to have lower field strength.

Figure 7. Field Densities for different sized antennas vs. distance

Note: H(x) is the field strength generated by a loop antenna.

H(x)r equates to the diameter of the loop antenna whose field strength is beingmeasured.

2.001.901.801.701.601.501.401.301.201.101.000.900.800.700.600.500.400.300.200.100.00

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 .0

H(x)r = 0.5 mH(x)r = 0.4 mH(x)r = 0.3 mH(x)r = 0.2 m

Metres

H(x)

[A/m

]



Page (10)

Figure 8. Close-up of Figure 7 showing Theoretical Read Distances

6.1 Loop Antenna Resonant Theory

A loop antenna is a tuned LC circuit and for a particular frequency, when the inductiveimpedance (XL) is equal to the capacitive impedance (XC) the antenna will be atresonance. This relationship is expressed as follows:

As can be seen from the equation, the relative values of the inductance (L) andcapacitance (C) are interrelated. For this frequency though, if the antennas are madetoo large, the inductance rises to a point were very small capacitor values arerequired. Once the inductance exceeds 5 µH, capacitance matching becomesproblematic. Two techniques can help to limit the rise of inductance:

1. Use low resistance copper tube in place of wire.

2. Connect two antennas in parallel thus halving the inductance.

= 12π√LC

[1]

0.5 0.6 0.7 0.8 0.9 1.0Metres

0.00

0.10

0.20

0.30

0.40

H(x)

[A/m

]H(x)r = 0.5 mH(x)r = 0.4 mH(x)r = 0.3 mH(x)r = 0.2 m

152 dBµV/m(100 mA/m)



Page (11)

6.2 Inductance Measurement

Fundamental to many of the equations used in the design process is the accuratemeasurement of the inductance of a loop (L). This can be done in a number of ways:

6.2.1 Calculation

This is the least accurate way and although equations may be accurate for roundantennas, they are an approximation for rectangular designs.

The formula below is reasonably accurate for square antennas made from tube:

Where

Side = Centre to centre length of antenna side (cm)Diameter = Tube diameter (cm)

Example: 50cm x 50cm loop, 15mm diameter copper tube

Information:

The program ADU.EXE is recommended for inductance calculation.

1. Measurement at 1 kHz (LCR Meter)

Using an LCR meter is again an approximation but accurate enough for calculating theresonating capacitor.

2. Accurate measurement of LCR parameters.

Using an Impedance Analyzer, for example HP4192A or Agilent Technologies 4294Ayou can set the desired frequency, in this case 13.56MHz and the instrument willmeasure the Capacitance, Resistance and Inductance of the Antenna. This will allowthe correct tuning components to be selected for the antenna.

LµH = [2]Side x 0.008 LN + 0.379Side x 1.4142 x Diameter

LµH = 48.5 x 0.008 LN + 0.37948.5 x 1.414 2 x 1.5

= 1.36 µH



Page (12)

6.2.2 Resonance Capacitance

Equation [1] can be rearranged to calculate the capacitance needed to bring anantenna to resonance at 13.56 MHz:

Example:50cm x 50cm Antenna, 15mm Tube, L = 1.36µH

6.2.3 Determining the Q

The performance of an antenna is related with its Quality (Q) factor.

Figure 9. Antenna Q

1CRES = (2 x 3.142 x 13560000)2 x 0.00000136

= 1.0129 x 10 -10

= 101 pF

1

Where: ω = 2π

[3]CRES =ω2L

13.56 MHzfc + 424.75 kHzfc - 424.75 kHz

fc

Q too high

Q <= 30

fc + 484.29 kHzfc - 484.29 kHz

-3dB Antenna Band-passcharacteristic



Page (13)

In general, the higher the Q, the higher the power output for a particular sizedantenna. Unfortunately, too high a Q may conflict with the band-pass characteristicsof the reader and the increased ringing could exceed the 11µs pause in the protocolbit timing. For these reasons the Q or the antenna when connected to a 50Ohm load(i.e. the reader) should be 30 or less.

The total ac resistance at resonance is difficult to calculate or measure withoutsophisticated equipment. It is easier to assume a Q value and work backwards.

L

First it is necessary to calculate the total parallel resistance of the finished antenna,having the required unloaded Q of 60.

Example if L = 1.36µH, = 13.56 MHz & Q = 60

Rpar = 2 x 3.142 x 13560000 x 0.00000136 x 60= 6,646 Ohms

Then assume a value for the present Q and repeat the calculation

e.g. L = 1.36µH, = 13.56 MHz, Q = 120

Rpar, antenna = 2 x 3.142 x 13560000 x 0.00000136 x 120 = 13,291 Ohms

Rpar = 2πLQ [5

Rpar

Q

Q = Rpar [4 2ππππƒL



Page (14)

The required resistance can be calculated using formula [6]

e.g. Rpar = 6,646, Rpar, antenna = 13,291

R = 13KΩ (13,293 Ohms)

Then when you measure the Q and it needs changing, it is relatively easy to adjust theresistor value (Rpar) to give the required Q by using formula [7].

6.3 Measuring the Quality Factor

The Quality factor (loaded) of an antenna can be readily measured if you have aninstrument capable of generating frequencies between 13 MHz and 14 MHz. (e.g.MFJ-259B HF/VHF SWR Analyzer, refer to page 37 for supplier details) and aspectrum analyzer or oscilloscope.

Figure 10. Spectrum Analyzer screen

-3dB

012

Qpresent x Qrequired x 2πLQpresent - Qrequired

[7]Rpar =

1R = 1Rpar

1Rpar, antenna

[6]



Page (15)

The spectrum analyzer should be set to the 1dB/div scale and the frequency isadjusted until the maximum amplitude of the signal is seen. By lowering and raisingthe frequency, the upper and lower -3dB points can be found and recorded.

The three frequencies (f1, f0, f2) from Figure 10 can be used in the following formula:

If an oscilloscope is used the method is slightly different. In this situation then themaximum voltage is recorded as the frequency is adjusted and the value multiplied by0.707 in order to obtain the equivalent -3dB value. The frequency is then raised andlowered to get the 1 and 2 values.

Q = [8] 02 - 1

e.g. Q = = 37 13.5613.76 - 13.39



Page (16)

7 Antenna MatchingFor optimum performance, the antenna and its feeder coaxial cable must haveimpedance of 50 Ohms. Matching changes the impedance of a resonant loop to 50Ohms and the accuracy of the matching is checked by the Voltage Standing WaveRatio (< 1:1.2) on the VSWR meter.

There are numerous matching techniques but this document will detail only fourmethods:

Gamma Matching T-Matching Transformer Matching Capacitance Matching

WARNING:

High voltages exist at the antenna terminals due to theresonance behaviour. You should not modify the matchingcircuits with the equipment switched on.

7.1 Gamma Matching

This method must be the easiest and cheapest method. The equivalent circuit isshown in Figure 11.

Where L is the InductanceCpar is the parallel capacitanceRpar is the damping resistance.

L Rpar Cpar



Page (17)

Figure 11. Gamma Matching Circuit

Note:

Figure 11 shows an unbalanced system. If a 1:1 BALUN is usedbefore the coax cable, then the system will once again be inbalance.

Once the Inductance of the antenna has been determined, the resonant capacitor iscalculated and fixed across the open ends of the loop. The coax cable shield isconnected to the centre of the antenna, opposite the capacitor position, with the coaxcentre wire attached to the matching tap tube, which is in turn connected to the mainloop at the correct impedance point.

L = 1.36µH, Rpar = 15K, Q = 37C par = 103pF (82pF + 10pF + 5 ~ 30pF)

Figure 12. A Gamma Matched Antenna

CPAR

R1

15 mm ØOD TUBE

5 mm ØOD TUBE

220 mm



Page (18)

Figure 12 & 13 show a 50cm x 50cm square loop antenna, constructed from 15mmcopper pipe. The matching tap is made from 5mm copper / nickel brake pipe.

A 15KΩ resistor is fixed in parallel with the capacitor to reduce the Q.

The point on the loop's perimeter where the matching tap is fixed depends on the Q ofthe antenna - the higher the Q - the closer the tapping point moves to the centre.

Note:

In practice a variable capacitor with a large range e.g. 12 80pF isused, together with a fixed value to achieve the desired range.Once matching has been achieved though, it is removed andmeasured. A larger fixed capacitor is then used together with asmaller variable capacitor, to allow for fine tuning.

Figure 13. Gamma Matched Antenna



Page (19)

7.2 T-Matching

This method, like the Gamma-matching, taps the antenna loop for the matching points.Figure 14. shows the equivalent circuit.

Figure 14. T-Matching Circuit

This type of matching is 'balanced' as both screen and centre core of the coax cableare tapped and not, as in Gamma matching, just the core. Once again, as for Gammamatching, the tapping can be external or internal to the loop. Figure 15 shows how theantenna analyzer can be used to find the tapping points.

Figure 15. Using the Analyzer to Determine the Matching Points

L Rpar Cpar



Page (20)

As for Gamma matching, the matching arms need to have at least 20mm separationfrom the loop to prevent capacitive coupling. The exact matching points will vary withthe Q; with a higher Q, the closer together are the matching points - the lower the Q,the farther apart the matching points are.

Figure 17 shows part of a large T-matched antenna made from 34mm wide coppertape.

Figure 16. T-Matched Tape Antenna

Note:

The proportions between the size of the main loop and thematching arms should be approximately (3:1). In this casethe loop is 34mm wide tape and the matching arms 12mmwide.

WARNING:




Page (21)

7.3 Transformer Matching

The equivalent circuit for transformer matching is shown in Figure 18. Withtransformer matching, the galvanic de-coupling means that the antenna inductor hasno DC connection to the reader. This can sometimes be required or help to overcomegrounding problems

Balun Matching AntennaTransformer

Figure 17. Transformer Matching

The transformer matching circuit comprises two elements

The matching transformer. A Balanced / Unbalanced (Balun) transformer.

Determining the matching is very similar to that of the Gamma matched antenna:

Measure or calculate the inductance of the antenna loop (L) Calculate the value of the parallel resonant capacitor (Cpar) Determine the parallel resistor (Rpar) necessary for Q

Cpar Rpar L



Page (22)

7.4 Matching Transformer

It is now necessary to calculate the turns ratio for the matching transformer usingformula [9] below, with the value for Rpar using formula [5] with Q = 30.

Wheren/m is the transformer ratio (set m = 2 initially)Rin is 50 OhmsRpar is from [5] but using the required Q value of.30

e.g. Rpar = 3,323 Ohms, Rin = 50 Ohms, L = 1.36µH, m = 2

n = 8 (7.4)

Note:

This formula is not exact. In practice, the number ofwindings depends on the antenna Q; the higher the Q, themore turns required. The actual number of windings requiredfor the example (Figure. 14) is n = 8.

WARNING:


nm

2 Rinx Rpar

(2πL)2= [9]

n m x Rin x Rpar

(2πL)2=



Page (23)

Figure 18. Toroid Matching Transformer

Figure 19 shows a 5:2 matching transformer. The ferrite toroid must be the correctgrade for this frequency and we recommend the Philips 4C65 grade. Part number:RCC 23/7 4322 020 9719.

7.5 Baluns

The Balun converts an unbalanced load to a balanced load and is primarily used toremove common mode noise problems associated with multiple antennas that havedifferent ground potentials. It is also used to connect balanced antennas to(unbalanced) coax. These differences may set up common mode currents that candisturb the receive circuits.

The Balun is a trifilar winding of 1:1 ratio and it is important to keep the three wirestight together but the sets of three wires can be evenly spaced around the toroid.

Again the correct grade of ferrite is important and we again recommend the Philips4C65 grade (or the equivalent Siemens K1). Figure 20 shows a schematic of thewindings.

Note:

It is important to note that although ferrite manufacturers datastates a Philips equivalent in practice this could be incorrect. Builda Golden Balun using Philips 4C65 grade ferrite and test othersagainst this.



Page (24)

Figure 19. 1:1 BALUN

Figure 21 uses 3 colors to indicate how a toroidal Balun is wound. In practice 1mmpolyurethane insulated transformer wire is used.

Figure 20. Toroid BALUN

The coax core is connected to 'A' and 'B' to the coax screen. Wires 'C' and 'D' are notpolarized and connect to the antenna / matching circuit. It is also possible to use'Pig's Noses' (twin holed ferrites) to produce the Baluns.

Figure 21. Pig's Nose BALUN

A

C

B

D

A

BC

D

SIX TURNS

PIG'S NOSE

UNBALANCED

BALANCED

1:1



Page (25)

Note:

Integrators are advised to check the efficiency of their Balunsbefore use. Connect the Balun to a reader via an SWRmeter and terminate wires 'B' and 'D' with 50 Ohms. TheVSWR meter should show 1:1.

For development work, it is recommended produce a generic transformer-matchingnetwork. This unit should have 120pF parallel capacitance, with a 12 ~ 90pF coarsevariable capacitor and a 2 ~ 16pF fine adjuster. The matching transformer should betapped so that the various ratios can be quickly selected.

Figure 22. Generic Transformer Matching Network

WARNING:




Page (26)

L = 1.36µH, Cpar = 102pF (12 ~ 90pF + 2 - 12pF)R = 13KΩ, Q = 50, Primary winding 8, Secondary 2

Figure 23. Transformer matched Example

WARNING:


TRANFORMER MATCHED

C2

R1



Page (27)

7.6 Capacitance Matching

Capacitance matching is perhaps the more difficult to develop of the three methods ofantenna matching. Small changes in capacitance can make large differences to thematching, so it is easy to miss the 'window' when using trial and error methods. AnExcel spreadsheet has been developed to make this task easier but without expensivemeasuring equipment to provide the correct inputs, the output can only be a startingpoint for trial and error.

To calculate the required capacitance: Calculate or measure the Inductance (L) [2] e.g. 1.36µH Calculate the total resonant capacitance (C) [3] e.g. 101pF Calculate the equivalent resonator resistance (Rpar) [5] e.g. 6646Ohms

Figure 24. Unbalanced Capacitor Tapped Matching

1) From the total capacitance [3], calculate values C1 and C2

LRpar

C1

C2

C

[10]C2 = C x Zout

Zin

Where Zout = Total parallel resistance [5] e.g. 6646 OhmsZin = 50 OhmsC = Total Capacitance [3] e.g. 101 pF



Page (28)

and

Figure 26 is the same 50cm x 50cm Antenna but this time capacitance matched.

L = 1.36, Rpar = 22 K, Q = 54Cpar = 2 ~ 12pF, C1A / C1B = 156.8pF, C2 = 1000pF

Figure 25. 50cm x 50cm Capacitor matched Antenna

The actual matching circuit for the antenna above is shown in Figure 26.

CAPACITOR MATCHED

C2

C1A C1B

R1

CPAR

[11]

C - C2 1 1

1C1 =



Page (29)

Figure 26. Capacitance matching

Texas Instruments' produce a capacitor matching board (which is currently built intoantenna of the Discovery Kit). This board has fixed and variable capacitors that allowtrial and error adjustment without constantly having to change capacitors with asoldering iron. The board can be configured as 'Balanced' or 'Unbalanced'

Figure 27. Capacitor Matched Circuits

Once capacitive matching has been completed, the values can be measured and builtusing fixed capacitors, as in Figure 26

UNBALANCED

Antenna

C2C4

A2 A1R1

J20(IN)

SA2 SA3

BALANCED

Antenna

C2C4

A2 A1R1SA2 SA3

C3



Page (30)

WARNING:High voltages exist at the antenna terminals due to theresonance behaviour. You should not modify the matchingcircuits with the equipment switched on.

Figure 28. TI*RFID Capacitor Matching Board

One big advantage of capacitance matching is that for small antennas, the voltagesare low enough for surface mount components to be used which can significantlyreduce the matching circuit size.

Figure 29. Miniature Capacitance Matching Circuit

C2C4

SMA CONNECTOR

UNBALANCED MATCHING 2200 pF 1500 pF 680 pF 470 pF 220 pF 100 pF 47 pF 47 pF

100 pF220 pF470 pF680 pF

SA2 SA3

47 pF100 pF220 pF470 pF680 pF1500 pF2200 pF

A2 A1

R1

C3



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8 Coupling between AntennasWhere multiple antennas are operating close to one another, the coupling betweenantennas can enhance or degrade the performance of individual antennas.

8.1 Reflective Antennas

If a matched (but unconnected) antenna is positioned opposite a driven antenna, theperformance of the driven antenna is enhanced. Such an antenna is sometimescalled a reflective antenna, which suggests how it helps to increase the range. Thistype of coupling can be face-to-face or with the side lobes of the driven loop poweringthe reflective loop.

8.2 Two Antennas on a Splitter (in Phase)

More than one antenna can be connected to the same reader by means of a splitter(see Appendix C for supplier). Although splitters introduce losses, the readingdistance of two opposing driven antennas can be greater than double that of singleantenna and are particularly effective when writing to tags. With a single antenna thefield strength drops off with distance but when two opposing antennas are used, thefield is 'fed' from both sides, allowing writing a greater distances. This arrangement isalso useful when detecting items travelling on a conveyor, when the tag can be oneither side of the item.

Figure 30. Field Lines for In-Phase Antennas on a Splitter

8.3 Two Antennas on a Splitter (Out-of-Phase)

When two opposing antennas are out of phase with each other, the magnetic fieldpattern changes. A tag parallel to the antennas will now have a reading hole in themiddle but a tag at 90º to the antennas will read all the way across at the front andrear of the antenna (Figure 31).



Page (32)

Figure 31. Out-of-phase Magnetic Field Pattern

8.4 Rotating Field Antennas

By using different feeder cable lengths, a pair of antennas can be used to produce arotating field. The typical arrangement is a pair of antennas in a crossed arrangement.

The included angle of the antennas should be 90º and one feeder cable after thesplitter, should be a quarter wavelength longer than the other to give a 90º-phase shift.

Figure 32. Rotating Field Antennas

SPLITTER

x

x + 3.65 m



Page (33)

Because the signal travels slower through the coax cable than free air, the cable willactually be shorter than 1/4 wavelength. In this case you multiply a quarter of 22m x0.66 (the velocity factor for RG58 coax cable), so the actual length needs to be 3.66mlonger than the other feeder.

The effect is to 'rotate' the field' and tags will read in any position indicated by thearrow in Figure 32.

8.5 Null Neighboring Antennas

For neighboring antennas (on individual readers) to operate at maximum efficiency,interaction between them should be minimal. The degree of coupling betweenantennas depends on the distance apart and on the angle between them. An antennaat 90º to its neighbor will display minimal coupling, but the exact null point has to befound experimentally. The method to find the null point is carried out by injecting asignal into one antenna (using the MFJ analyzer) and monitoring the voltage inducedinto the other (using a scope), as it is moved relative to the first antenna.

Note:

The minimum point may not be where you expect it, as themagnetic field of an antenna is not uniform. i.e. the voltagein an antenna is greatest on the opposite side from the feedpoint.



Page (34)

Appendix A Return Loss

Return Loss(dB)

Linear VSWR Transmission

40 0.010 1.020 0.00043230 0.032 1.065 0.0043225 0.056 1.119 0.013720 0.100 1.222 0.043218 0.126 1.288 0.068316 0.158 1.377 0.10815 0.178 1.433 0.13514 0.200 1.499 0.16913 0.224 1.577 0.21212 0.251 1.671 0.26611 0.282 1.785 0.33210 0.316 1.925 0.414

9 0.355 2.100 0.5158 0.398 2.323 0.6397 0.447 2.615 0.7906 0.501 3.000 0.9735 0.562 3.570 1.1934 0.631 4.419 1.4553 0.708 5.829 1.7642 0.794 8.724 2.1241 0.891 17.390 2.539

Table 1. Return Loss Figures



Page (35)

20

30

40

5060

80100

200

300

400500600

800

1,000

2,000

3,000

4,0005,0006,000

8,00010,000

20,000

30,000

40,00050,00060,000

80,000100,000

300,000

0.1

0.20.3

0.5

1.0

2335

10

2030

50

100

200300

500

1000

20003000

5000

10,000

20,00030,000

50,000

100,000

13.56 MHz

2

3

44

5

6

8

10

20

30

40

5060

80

100

200

300

400

500

600

800

1,000

2,000

3,000

4,000

5,000

6,000

8,000

10,000

70

c

L

CXCor XL

CAP

ACIT

ANC

E (p

F)

REA

CTA

NC

E (O

hms)

IND

UC

TAN

CE

(µH

)

Appendix B Reactance & Resonance Chart

(Acknowledgements to Radio Communications Handbook)



Page (36)

Appendix C Coax-cable Splitter

50 Ohm

RG58 Cable (50 Ohm)

URM70 Cable (75 Ohm)

URM70 Cable (75 Ohm)

75 Ohm 'T' piece

3.68 m (12')

50 Ohm

50 Ohm



Page (37)

Appendix D Component Suppliers List

CAPACITORS

1. ARCO Electronics http://www.arco-electronics.com/

Mica capacitors - fixed and variable.

2. Tronser http://www.tronser.com

Produce low value multi-turn air-gap trimmers.

SPLITTERS

3. Minicircuits http://www.minicircuits.com/products.htm

Produce splitters for multiple antennas and rotating field antennas

METERS

4. MFJ Enterprises http://www.mfjenterprises.com/index.htm

Make a range of low cost RF equipment including the MFJ-259B

5. RF Parts http://www.rfparts.com/diamond/index.html

Produce a range of VSWR meters including the Diamond 200

http://www.arco-electronics.com/

http://www.tronser.com/

http://www.minicircuits.com/products.htm

http://www.mfjenterprises.com/index.htm

http://www.rfparts.com/diamond/index.html



Page (38)

Appendix E Resistor Capacitance Values

Resistors are used to adjust the Q value of antennas but unfortunately, they also addcapacitance to the circuit. The amount of capacitance is closely related to the value -the higher the value, the lower the capacitance. The type of resistor makes littledifference. E.g. the following table shows the values for 3 different types of 22K (20K)resistors:

Resistor type Value(K Ohms)

Rating(Watts)

Capacitance(pF)

Carbon Film 22 1 72Carbon Film 22 2 72Metal Film 22 1 73Metal Film 22 2 72Thick Film 20 5 81

The table below gives some capacitance 'adjustment' values for some commonCarbon Film (Power Oxide) 2w resistors.

Resistor type Value(K Ohms)

Rating(Watts)

Capacitance(pF)

Carbon Film 10 2 200Carbon Film 15 2 168Carbon Film 22 2 65Carbon Film 33 2 48Carbon Film 47 2 32Carbon Film 68 2 27

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