UPTEC E11008

Examensarbete 30 hpMaj 2012

Design of circular polarized dual band patch antenna

Thomas Edling

Påbyggnadsprogrammet till civilingenjörsexamen i elektroteknikMaster Programme in Electrical Engineering

Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student

Abstract

Design of circular polarized dual band patch antenna

Thomas Edling

At the moment Swedish Transport Administration uses a monitor system that candetect urgent errors as warm ball-bearings and flat wheels etc. with stationarydetectors. To avoid these errors Swedish Transport Administration, UPWIS AB andUppsala University work with a system that will continuously monitoring the train todetect the errors as fast as possible. This will save money in the future for SwedishTransport Administration and all other partners that use the rails. Swedish Transport Administration has already RFID readers beside the rail to detecttrains position. The new monitoring system will use these readers and send data fromthe monitoring system via these readers to a database.The aim of this thesis work is to design and build a RFID antenna to send data fromthe monitoring system to the RFID readers. The antenna should be a circularpolarized and it needs to manage the harsh environment on the train. This thesis work started with a theoretical study which investigated four commonantenna types (dipole, loop, PIFA and patch/microstrip) to evaluate which antennatype that is the best solution for this application. It was decided to design a patchantenna from the theoretical study since it fulfils all the requirements for the antenna. Simulations and tests shows that the antenna is circular polarized and have amaximum reading distance of 5 m for 868 MHz. For 2.45 GHz it is linear polarizedand has a reading distance of at least 10 m. With other hardware settings the antennawill have longer reading distance at 2.45 GHz. When all parts of the test bed was finished the test bed was mounted on themeasurement wagon. The final test shows that the antenna fulfils the task. Theantenna transmitted the data from the sensor boxes to the RFID readers. The report suggests future work to minimize the reading distance and size for theantenna. These are: transfer sensor data to RFID tag by “multi hop”, hardwareimprovement for instance antenna diversity and using another substrate (higherdielectric constant).

ISSN: 1654-7616, UPTEC E11008Examinator: Nora MassziÄmnesgranskare: Anders RydbergHandledare: Mathias Grudèn, Magnus Jobs

Sammanfattning

För tillfället använder Trafikverket ett övervakningssystem som kan detektera akuta fel som

varma kullager och platta hjul m.m. med hjälp av stationära detektorer. För att kunna

förhindra dessa fel arbetar Trafikverket, UPWIS AB och Uppsala universitet med att ta fram

ett system som kontinuerligt ska övervaka tågen för att så fort som möjligt Detta ska

förhoppningsvis också spara en hel del pengar i framtiden för trafikverket och alla partner

som använder rälsen.

Trafikverket har redan RFID läsare vid sidan av rälsen för att kunna detektera tågens

position. Det nya övervakningssystemet använder dessa befintliga läsare och skickar data

från övervakningssystemet via dessa läsare till en databas.

Målet med detta examensarbete är att designa och bygga en RFID antenn för att skicka data

från övervakningssystemet till RFID läsarna. Antennen ska vara cirkulär polariserad antenn

och kunna utsättas för den extrema miljö som den kommer befinna sig i.

Examensarbetet började med en teoretisk litteraturstudie som undersökte fyra vanliga

antenntyper (dipol, loop, PIFA, patch/microstrip) för att utvärdera vilken antenntyp som

skulle vara bäst lämpad för applikationen. Efter att litteratur studien gjorts bestämdes att en

patchantenn var ett bra alternativ eftersom denna antenn typ kunde uppfylla alla krav som

ställdes på antennen.

Simuleringarna och tester visar att antennen är cirkulär polariserad med ett läsaravstånd på 5

m for 868 MHz. För 2.45 GHz är den linjärpolariserad med ett läsaravstånd på minst 10 m.

Med andra hårdvaruinställningar kommer antennen ha ett längre läsaravstånd för 2.45 GHz.

När testbädden var klar och monterad testades systemet på tåget. Testet visade att RFID

antennen uppfyllde uppgiften d.v.s. överförde data från sensornoden till RFID läsaren.

Rapporten tar även upp förslag på framtida arbete för att minska läsaravståndet för antennen

och storleken på antennen. Dessa är användandet av ett annat substat

(högre r

ε ), överföra sensordata till RFID taggen via ”multi hop” och hårdvaruförbättringar

(antenndiversitet).

Acknowledgements

I would like to thanks all that have been participated with this project, summer and autumn

2011 for their good work so the test bed could be mounted on the train before the snow

came.

I am grateful for all help by my subject inspector Prof. Anders Rydberg and my supervisors

Mathias Grudén and Magnus Jobs at the Department of Solid State Electronics at Uppsala

University. I am also grateful for all work Kjell Brunberg and Erik Jansson at UPWIS AB

have done with this project these 5 month.

I would also thanks Ulf Hellström at Swedish Transport Administration and Infranord for all

help.

Finally I would thanks my team mates (thesis worker) Malkom Hinnemo, Filip Zherdev and

Nils Edvinsson with all help and support that have been needed during the thesis work with

measurement etc.

I wish also all good luck with the project in the future. I think in the end the final version of

the system will be very good and useful for Swedish Transport Administration.

Contents

Nomenclature

List of figures………………………………………………………………………..…..i List of tables……………………………………………………………………...….…iii

Chapter 1 ............................................................................................................................. 1

1 Introduction ......................................................................................................................... 1

1.1 Aim of Thesis Work ......................................................................................................................... 1

1.1.1 Antenna specification ................................................................................................................ 1

1.2 Radio frequency identification .......................................................................................................... 2

1.3 Structure of the project ..................................................................................................................... 3

1.4 Outlines for the thesis ....................................................................................................................... 3

Chapter 2 ............................................................................................................................. 4

2 Theory and concepts for patch antennas ............................................................................ 4

2.1 Patch antenna design ......................................................................................................................... 4

2.1.1 Structure .................................................................................................................................... 4

2.1.2 Calculations for the patch antenna dimension ........................................................................... 5

2.1.2.1 Length of antenna ............................................................................................................... 5

2.1.2.2 Width of the patch antenna ................................................................................................ 7

2.1.3 Feed techniques ......................................................................................................................... 7

2.1.3.1 Microstrip Line Feeding ..................................................................................................... 8

2.1.3.2 Coaxial Feeding ................................................................................................................. 9

2.1.3.3 Coaxial probe with capacitive feed ...................................................................................10

2.1.3.4 The aperture-coupled patch ...............................................................................................11

2.2 Dual band techniques .......................................................................................................................12

2.2.1 Using higher modes ..................................................................................................................12

2.3 Short-circuited patch ........................................................................................................................14

2.4 Substrate ..........................................................................................................................................15

2.4.1 Dielectric substrates .................................................................................................................16

2.5 Polarization ......................................................................................................................................17

2.5.1 Linear polarization ...................................................................................................................17

2.5.2 Elliptical polarization ...............................................................................................................19

2.5.3 Circular polarization .................................................................................................................20

2.5.3.1 Techniques for circular polarization .................................................................................21

2.5.4 Axial ratio .................................................................................................................................22

2.6 Quality factor (Q-factor) ..................................................................................................................22

2.7 Bandwidth ........................................................................................................................................23

2.7.1 Techniques for wider bandwidth ..............................................................................................24

2.8 Return Loss, S11 parameter ..............................................................................................................25

2.9 Ground plane size effects .................................................................................................................26

2.10 Eddy current ...................................................................................................................................27

2.11 Metallic environments effect on antennas......................................................................................28

2.12 Ferrite shielding .............................................................................................................................28

Chapter 3 ........................................................................................................................... 30

3 Design ideas for antenna ................................................................................................... 30

3.1 Microstrip and Patch antenna...........................................................................................................30

3.1.1 Advantages ...............................................................................................................................30

3.1.2 Disadvantages ...........................................................................................................................30

3.1.3 Tradeoffs ..................................................................................................................................31

3.2 Planar inverted f antenna .................................................................................................................31

3.2.1 Advantages ...............................................................................................................................33

3.2.2 Disadvantages ...........................................................................................................................33

3.2.3 Techniques for wider bandwidth ..............................................................................................33

3.2.4 Techniques for reducing the physical size ................................................................................33

3.3 Loop antenna ...................................................................................................................................34

3.3.1 Advantages ...............................................................................................................................34

3.3.2 Disadvantages ...........................................................................................................................34

3.4 Dipole antenna .................................................................................................................................35

3.4.1 Advantages and disadvantages .................................................................................................35

3.5 Decision for antenna type ................................................................................................................36

Chapter 4 ........................................................................................................................... 37

4 Design of dual band patch antenna ................................................................................... 37

4.1 Design procedure .............................................................................................................................37

4.2 Antenna design ................................................................................................................................38

4.2.1 Starting and simulation stage ....................................................................................................38

4.2.2 Minimizing stage ......................................................................................................................39

4.2.3 Matching stage to final stage ....................................................................................................40

4.2.3.1 Design errors .....................................................................................................................40

4.2.4 Layout.......................................................................................................................................42

Chapter 5 ........................................................................................................................... 46

5 Simulation results and tests ............................................................................................... 46

5.1 General simulations settings in CST ................................................................................................46

5.2 General about simulations results ....................................................................................................46

5.3 Simulations result ............................................................................................................................48

5.3.1 S11 parameter ............................................................................................................................48

5.3.2 Smith chart ...............................................................................................................................51

5.3.3 Far field pattern ........................................................................................................................52

5.3.4 Axial ratio .................................................................................................................................54

5.4 Measure antenna with Network Analyzer ........................................................................................56

5.5 Outside test ......................................................................................................................................57

5.6 Final set up on train .........................................................................................................................60

Chapter 6 ........................................................................................................................... 63

6 Final words ...................................................................................................................... 633

6.1 Future work ......................................................................................................................................63

6.1.1 Update hardware .....................................................................................................................633

6.1.2 Smaller RFID antenna ............................................................................................................633

6.1.3 Multi-hop system ....................................................................................................................655

6.2 Conclusions......................................................................................................................................66

References ......................................................................................................................... 68

i

List of figures

Figure 1.1: IEEE 802.15.4 channel selection ..................................................................... 2

Figure 2.1: Structure of a rectangular patch antenna ........................................................ 4

Figure 2.2: Common forms of patch layer .......................................................................... 4

Figure 2.3: Fringing field .................................................................................................... 5

Figure 2.4: Microstrip line feed .......................................................................................... 8

Figure 2.5: Coaxial feeding method .................................................................................... 9

Figure 2.6: Coaxial probe with capacitive feed method .................................................... 10

Figure 2.7: The aperture-coupled patch ........................................................................... 11

Figure 2.8: Propagation of TEM, TE, TM waves ............................................................... 12

Figure 2.9: Geometry for field configuration .................................................................... 13

Figure 2.10: Electric field ...................................................................................... 14

Figure 2.11: Short-circuited techniques ............................................................................ 15

Figure 2.12: Linear polarization ........................................................................................ 17

Figure 2.13: Horizontal, vertical linear polarization ......................................................... 18

Figure 2.14: Elliptically polarized wave ........................................................................... 19

Figure 2.15: Circular polarization ................................................................................... 20

Figure 2.16: Circular polarization (RHCP, LHCP) .......................................................... 20

Figure 2.17: Dual feed ...................................................................................................... 21

Figure 2.18: Single feed .................................................................................................... 21

Figure 2.19: Axial ratio ..................................................................................................... 22

Figure 2.20: Q factor.......................................................................................................... 23

Figure 2.21: 2 port network. .............................................................................................. 25

ii

Figure 2.22: Eddy current. ................................................................................................. 25

Figure 3.1: PIFA ............................................................................................................... 32

Figure 3.2: Patch layer for PIFA ...................................................................................... 32

Figure 3.3: Loop antenna .................................................................................................. 34

Figure 3.4: Dipole structure .............................................................................................. 35

Figure 4.1: Design procedure ............................................................................................ 37

Figure 4.2: Simulation error .............................................................................................. 41

Figure 4.3: Top view on feed .............................................................................................. 42

Figure 4.4: Top view on ground ......................................................................................... 42

Figure 4.5: Top view on patch............................................................................................ 43

Figure 4.6: Side view (long side) ........................................................................................ 43

Figure 4.7: Side view (Short side) ...................................................................................... 43

Figure 4.8: Built antenna, top view .................................................................................... 43

Figure 5.1: 11S parameter for 868 MHz ............................................................................. 48

Figure 5.2: parameter for 2.45 GHz ............................................................................ 49

Figure 5.3: parameter for 868 MHz and 2.45 GHz .................................................... 50

Figure 5.4: Smith chart for 868 MHz ................................................................................ 51

Figure 5.5: Smith chart for 2.45 GHz ............................................................................... 51

Figure 5.6: Far field pattern for 868 MHz ......................................................................... 52

Figure 5.7: Far field pattern (left side) for 2.45 GHz ........................................................ 53

Figure 5.8: Far field pattern (right side) for 2.45 GHz ..................................................... 53

Figure 5.9 a-b: Axial ratio for 836 and 868 MHz. ............................................................. 54

Figure 5.10: Axial ratio 2.45 GHz (left side) ..................................................................... 54

11S

11S

iii

Figure 5.11: Axial ratio 2.45 GHz (right side)................................................................... 55

Figure 5.12 a-c: S11 parameter from Network analyzer for 868 MHz and 2.45 GHz. ..... 56

Figure 5.13: Set up for the outside test .............................................................................. 57

Figure 5.14: RFID reader .................................................................................................. 59

Figure 5.15: Set up for final test......................................................................................... 60

Figure 5.16: Sensor node box on train ............................................................................... 61

Figure 5.17: Measure wagon for the final test ................................................................... 62

Figure 6.1: Small RFID antenna ........................................................................................ 64

Figure 6.2: Set up for multi hop-system ............................................................................. 64

List of tables

Table 1: Tradeoffs patch antenna………………………….……………………….……..............31

Table 2: Parameter value for built antenna……………………………………………….............44

Table 3: Antenna position, outside test………………………………………...………..…...........58

1

Chapter 1

1 Introduction

Todays monitoring of railways wagons are used to discover warm ball-bearings, locked brakes, flat wheels and other different problems that can occurs on the train. At the moment this is done by stationary detectors. It means that it is only detecting urgent errors which can cause unplanned delays in the traffic. Therefore it is very desirable to continuously monitoring the trains and store the information from the sensors on the trains in a database.

Together with Swedish Transport Administration, UPWIS AB and Uppsala University the goal with the project during summer and autumn 2011 is to build and install a test bed based on wireless sensors on the railway wagons. Swedish Transport Administration, UPWIS AB and Uppsala University are together a part of the research center Wireless Uppsala VINN Excellence Center for Wireless Sensor Networks (WISENET)

1.1 Aim of Thesis Work

During autumn 2011 a test bed will be finished if all goes by the schedule. The aim with this thesis work is to; build an antenna for this test bed that will transfer data as temperature and vibration from different types of wireless sensors to readers beside the rail.

1.1.1 Antenna specification

Environmental conditions: In a metallic environment on railway wagons.

Polarization at 868 MHz: Circular polarization

Read range at 868 MHz: The RFID antenna has a minimum vertical and horizontal1 read range. The minimum vertical read range is 3 m and the minimum horizontal read range is between 1.6-3.8 m depending on which settings are used for the hardware. [1]

1 Vertical and horizontal direction, see figure 5.14.

2

Polarization 2.45 GHz: Depending on transmitting antenna

Read range 2.45 GHz: Minimum range 25 m

Physical size: As small as possible, no maximum size

Frequency band: The 868 MHz antenna uses the ISO 18000-6 type C standard, 860-960 MHz. The main goal is not to cover the whole frequency band, only the part of the band that is used in Europe, i.e. 865.7-867.6 MHz.

The 2.45 GHz antenna operates with the IEEE 802.15.4 standard, 2.4-2.4835 GHz.

Figure 1.1: IEEE 802.15.4 channel selection [2]

The IEEE 802.15.4 frequency band is divided into 16 channels. Figure 1.1 shows how the channels are divided. Each channels center starts at2.400 0.005 ,

16,...,3,2,1=n and are 2 MHz wide. [2]

1.2 Radio frequency identification

Radio frequency identification (RFID) belongs to the technology automatic identification and data capture (AIDC). This technology collects data automatically from tags so it can be used [2]. RFID uses radio waves to transfer data from the electrical tags to the reader. The technology is used in many applications as asset tracking, manufacturing, supply chain management, payment systems, security and access control. The tags can be divided into two groups, active and passive. The active tags have their own power supply and the passive activates by induction from the reader’s antenna. [3]

3

1.3 Structure of the project

Phase 1: The project begins with to investigate the advantages and disadvantages of few different common types of antennas. After that, determine which antenna type that could be a good choice for the project.

Phase 2: Find solutions how to build the antenna so it meets the antenna specification.

Phase 3: Build the antenna.

Phase 4: Test the antenna.

Phase 5: Implement the antenna on the train.

1.4 Outlines for the thesis

Chapter 2 presents useful theory for the design of the patch antenna. In chapter 3 different types of antennas is investigated to decide which antenna type that is a good solution for this project. Chapter 4 shows the design procedure of the antenna. Chapter 5 presents test results from different tests of the built antenna. Chapter 6 summarizes the thesis work and gives recommendation of future work

4

Chapter 2

2 Theory and concepts for patch antennas

2.1 Patch antenna design

2.1.1 Structure

The patch antenna belongs to the class resonant antennas. For a rectangular patch antenna see figure 2.1. It is resonant when the length, L is around half multiples of the resonant frequency. The patch antenna consists in general of three major layer, ground plane, substrate and patch. [4]

Figure 2.1: Structure of a rectangular patch antenna [4]

Figure 2.2: Common forms of patch layer [4]

5

Figure 2.1 shows that the patch layer is rectangular but figure 2.2 shows also other common forms of patch layer. Circular and triangular is used often instead of rectangular because it is easier to derive the mathematical expression for the model (antenna). [4]

2.1.2 Calculations for the patch antenna dimension

To design one simple patch antenna following parameters needs to be calculated: length, width and eventual feed line for microstrip antenna.

2.1.2.1 Length of antenna

Figure 2.3: Fringing field [4]

To calculate the length of the patch antenna the fringing fields that occurs needs to take into account. The fringing field occurs at the ends of the patch. The electric field does not end abruptly at the edges and therefore create the “fringing fields”. These fields can be represented as two radiation slots which means that the patch looks electrically larger than the physical size. Because of that the calculated length need to be extended with the fringing factor ∆ so the antenna design is for patch with /2 and no fringing. [4]

6

For a half - and quarter wave patch the resonate frequency is given by [4]

( )( )1.2

22

1

00 eff

rLL

fεµε ∆+

=

where 0ε is the permittivity in vacuum, 0µ is the permeability in vacuum, Δis the

fringing factor and effe is the effective electric constant which take the fringing field

outside the patch into account [5].

Effective electric constant give by formula [4]

)2.2(1/,1212

1

2

1>+

−+

+= hW

W

hrreff

εεε

Fringing factor gives by formula [4]

)3.2(8.0/

264.0/258.0

3.0412.0

+

+

−

+=∆

hW

hWhL

eff

eff

ε

ε

To optimize the length and resonant frequency with formula (2.1), a praxis value for L is used. [6]

)5.2(

)4.2(49.0~48.0

rr

g

gg

f

c

L

ελ

λλ

=

=

where c is the velocity of light in vacuum.

7

2.1.2.2 Width of the patch antenna

The width of the patch gives by formula [4]

)6.2(1

2

2

1

00+

=rrf

Wεεµ

It is recommended that the width of the patch is in following interval [7]

)7.2(2LWL <<

2.1.3 Feed techniques

There are many methods to feed the patch and all have their advantages and disadvantages. These feeding methods can be classified into two groups, contacting and non-contacting. For the contacting methods, the patch antenna feeds directly to the patch and for the non-conducting method electromagnetic field coupling is used to transfer the power to the patch.

8

2.1.3.1 Microstrip Line Feeding

The microstrip line consists of a conducting strip connected to the patch. The microstrip line has often the same thickness as the patch but the width is smaller. To obtain good

impedance matching an inset cut ( 0x ) can be made. The length of the inset controls the impedance matching [4]. Figure 2.4 shows how to use microstrip as feed technique.

Figure 2.4: Microstrip line feed [4]

Advantages with microstrip feed are [4]:

• One of the easiest methods to fabricate.

• Easy to match by controlling inset length.

Disadvantages with microstrip feed are [4]:

• Give undesirable cross polarization effects.

• Make the patch larger.

• Bandwidth decreases when the thickness of the substrate increases.

9

2.1.3.2 Coaxial Feeding

The coaxial feed method is one of the most common feed techniques. The inner conductor of the coaxial goes through the substrate from ground to the patch and the outer conductor are connected to the ground plane [4]. Figure 2.5 shows how to use coaxial probe as feed technique.

Figure 2.5: Coaxial feeding method [4]

Advantages with coaxial probe feed are [4]:

• Easy to fabricate.

• Easy to match because the feed position can be placed anywhere to the patch to get impedance matching.

Disadvantages with coaxial probe feed are [4]:

• Narrow bandwidth.

• For thicker substrates, the increased probe length makes the input impedance more inductive which lead to matching problem.

• Difficult to model specially for thick substrate.

10

2.1.3.3 Coaxial probe with capacitive feed

The difference between the usual coaxial feed and this method is that the inner conductor of the coaxial goes not the whole way up to the patch and the end of the inner conductor is connected to a circular plate. If a regular probe were used, a larger inductance would be introduced, which results in impedance mismatch. To cancel the inductance a reactance need to be added. This feeding method with the capacitive disk does that [8]. Figure 2.6 shows how to use coaxial probe with capacitive feed as feed technique.

Figure 2.6: Coaxial probe with capacitive feed method [8]

Advantages with coaxial probe feed are [9]:

• Wide bandwidth.

• The capacitive disk cancels the inductive impedance of the probe.

Disadvantages with coaxial probe feed are [10]:

• May reduce efficiency.

• Can be difficult to design, depending on substrate. Not difficult with air.

11

2.1.3.4 The aperture-coupled patch

Below the patch are there two layers of substrate. The substrate layers are separated with a ground plane. A microstrip line is placed below the lower substrate layer. The energy is coupled to the patch from the micrstrip line by a slot in the patch. Figure 2.7 shows how to use the aperture-coupled patch as feeding technique. [4]

Figure 2.7: The aperture-coupled patch [4]

Advantages with the aperture-coupled patch feed are [4]:

• Many parameters to choose between to match the antenna as height of substrate, width, length and position of the slot.

• Purifier polarization since the feed is isolated by the ground plane between the substrates.

Disadvantages with the aperture-coupled patch feed are [4]:

• Difficult to fabricate.

• Narrow bandwidth.

12

2.2 Dual band techniques

2.2.1 Using higher modes

The region between the patch and the ground plane for a rectangular patch (see figure 2.1) can be viewed as a rectangular box with electric walls above and below it and magnetic walls at the sides. The whole system can be treated as a rectangular waveguide [4]. There are different types of waveguides and in them can three different types of electromagnetic waves propagate, TEM, TE and TM [11], [12].

Transverse Electro Magnetic (TEM) wave: The electric and magnetic field are perpendicular to each other and both are transverse to the direction of propagation ( y ).

There is no component for the electric and magnetic field in direction of propagation

( )0== yy HE . TEM wave exist in waveguide that consist of two conductors, as

coaxial.

Transverse electric (TE) wave: The electric and magnetic field are perpendicular to each other and both are transverse to the direction of propagation ( y ) and in the

propagation direction 0≠yH and 0=yE . TE wave exist in waveguides that consist of

one conductor.

Transverse magnetic (TM) wave: The electric and magnetic field are perpendicular to each other and both are transverse to the direction of propagation ( y ) and in the

direction of propagation 0≠yE and 0=yH .

Figure 2.8: Propagation of TEM, TE, TM waves

13

Figure 2.8 shows the direction of electric and magnetic field for TE, TM, TEM waves when z-axis is the direction of propagation for the TEM, TE and TM waves in the waveguide. For patch antennas only TM waves can propagate in the dielectric rectangular waveguide.

Figure 2.9: Geometry for field configuration [4]. TM wave propagates with E-field in y and z, y-direction

and H-field in x-direction.

From the expression for the field configuration, the resonant frequencies for the waveguide can be calculated. Figure 2.9 shows the geometry for the cavity that is used for the field configuration. The resonant frequencies for the cavity are given by [4]

( ) )8.2(2

1222

+

+

=

W

p

L

n

h

mf

mnpr

πππ

µεπ

In the x-axis the electric field varies negligibly [13], [4] and formula (2.8) can be approximated to following resonant frequency

( ) )9.2(2

122

+

=

W

p

L

nf

npr

ππ

µεπ

14

2.3 Short-circuited patch

It is common that microstrip and patch antennas are short-circuited. The techniques that are used are shorting pin in the patch or a shorting wall along a line at the end of the patch. In applications there a small size antenna are needed a shorting wall is a good alternative. For a half-wave rectangular patch the electric field distribution is given by

)/cos(0 LxE π .

Figure 2.10: Electric field xTM 010 [4]

Figure 2.10 shows that the electric field ( xTM 010 ) has a maximum at the both edges of the

patch and zero in the middle. There is o180 phase between the fields maximum. Since the electric field is zero in the middle (plane 2/Lx = ) an electric wall can be placed there. The shorting wall will not change the designed resonant frequency for the half-wave rectangular patch [10]. This geometry of the patch antenna is called quarter wave patch because the distance between the radiation edge and the shorting wall is 4/λ . The largest difference between a half wave patch and a quarter wave patch is that the quarter wave patch has one radiation edge and half wave patch has two. Since the structure is different between a half wave patch and a quarter wave patch it gives changes for the antenna characteristics. This is few of them [5].

1. The quarter wave patch has a broader E plane pattern since the half-patch has two radiation edges.

2. The quarter wave patch has lower gain since it only has one radiation edge, half wave patch has two.

3. The ration between half wave patch and quarter wave patch is two for the radiation conductance, one half for the radiation resistance, 1≠rε and two for the stored

energy.

15

4. The quarter wave patch and the half wave patch have nearly the same bandwidth.

Figure 2.11: Short-circuited techniques [5], [10]

Figure 2.11 show different ways to add shorting pins and shorting wall. It can be done by one or few shorting pins. The shorting wall can also consist of shorting pins.

2.4 Substrate

Substrate materials play an essential role for the patch antenna design. The substrate have many properties that should be considered: the dielectric constant, loss tangent, their variation with temperature and frequency, homogeneity, isotropic, thermal coefficient and temperature range, dimensional stability with processing and temperature, humidity and aging, and thickness uniformity of the substrate [3]. One properties that substrate has is the permittivity. The permittivity is associated with how much electrical charge a material (substrate) can store in a given volume. The

permittivity )(ε is complex and has one real part )( 'ε and one imaginary part )( ''ε [14].

)10.2(''' εεε j−=

The loss tangent ( δtan ) measure the amount of electrical energy converted to heat in the dielectric and accounts for the power losses in passive device such the transmission line or patch antenna and defined as the ratio between the real part and imaginary part of the complex permittivity [10], [14].

)11.2()tan(''

'

ε

εδ =

16

The relative permittivity or dielectric constant )( rε is the ratio between the real part of

the complex permittivity and the permittivity of vacuum )/10854.8( 120 mF

−⋅=ε [14],

[15].

)12.2(0

'

ε

εε =r

Since the speed of propagation in given medium is [15].

)13.2(]/[11 0

00

smc

c

rrrr µεµµεεεµ===

The dielectric constant affects the speed it will also affect the wavelength and frequency [15].

)14.2(][0 mf

c

f

c

rελ ==

2.4.1 Dielectric substrates

One common type of substrate is dielectric substrates. The substrate is used to fulfill two different factors, mechanical support for the structure and determines the electrical characteristics of the circuit or antenna. [10]

Mechanical properties

• Mechanical strength, for example vibration resistance and shape stability.

• Small dilatation factor.

Electrical properties

• Relative permittivity rε , which determines the miniaturization factor. If all other

parameters are fix the size of the circuit is proportional to rε/1 .

• Small dielectric losses. Should have 001.0tan <δ .

17

2.5 Polarization

The polarization of an antenna in a given direction is defined as “the polarization of the wave transmitted (radiated) by the antenna” [4].

2.5.1 Linear polarization

An antenna has linear polarization if the field vector (electric or magnetic) for the transmitted wave of the antenna only has one component or two orthogonal linear

component that are in time phase or o180 (or multiples of o180 ) out of phase [4].

Figure 2.12: Linear polarization [16]

18

Figure 2.13: Linear polarization [17]

Figure 2.12 shows that the electric field propagates in one plane and the magnetic field propagates in the plane orthogonal to that. Figure 2.13 shows that the linear polarized wave can propagate in two different planes. When it propagates in the x-y plane it is horizontal linear polarization and in the x-z plane it is vertical linear polarization.

19

2.5.2 Elliptical polarization

An antenna has elliptical polarization if the field vector (electric or magnetic) for the transmitted wave of the antenna has two orthogonal linear components that can have the same or different magnitude. When these components have different magnitude the

time-phase difference between the components can not be o0 or multiples of o180 because it will then be linear polarization. When the two components have the same magnitude the time-phase difference between the two components must not be odd

multiples of o90 because it will be circular polarization. [4]

Figure 2.14: Elliptically polarized wave [16]

Figure 2.14 shows how the elliptically polarized wave propagates. The electric field propagates in two planes at the same time with different amplitude.

20

2.5.3 Circular polarization

An antenna has circular polarization if the field vector (electric or magnetic) for the transmitted wave of the antenna has two orthogonal linear components with the same

magnitude and the time-phase difference between this is odd multiples of o90 .

There are two types of circular polarization. If the field rotation is clockwise then right-hand circular polarized (RHCP) or if the field rotation is counterclockwise then left hand circular polarized (LHCP) [18].

Figure 2.15: Circular polarization [18]

Figure 2.16: Circular polarization [18]

Figure 2.15 shows how the circular polarized wave propagates. The electric field propagates in two planes at the same time with equal amplitude. Figure 2.16 shows that if the electric field rotates clockwise (RHCP) and counterclockwise (LHCP).

21

2.5.3.1 Techniques for circular polarization

There are many ways to get circular polarization for the patch antennas. Figure 2.17-2.18 shows some common ways to get left-and right handed circular polarization. They can be divided into two groups, dual feed and single feed patches.

Figure 2.17: Dual feed [10]

Figure 2.18: Single feed [10]

The dual feed patch antennas feeds with equal amplitude but with o90 phase difference. The single feed patch antennas using truncated corner, truncated circle, corner feed or slots in the patch. [8], [10].

22

2.5.4 Axial ratio

The axial ratio )(B

AAR

Ο

Ο= is the ratio between the length of the major axis )( AΟ and

the minor axis )( BΟ of the E-field, )( BA Ο≥Ο . The E-field consists of an x, y

component ),( yx EE . [4]

Figure 2.19 Axial ration: A electric field created by the two electric field component Ex and Ey. [4]

For a linear polarized wave the )0( >>∞= dBAR since the electric field has one

component and then 0=ΟB . For a circular polarized wave the major and minor axes have equal length )( BA Ο=Ο which gives )0(1 dBAR = . For an elliptical polarized

wave the major and minor axes have different length which gives ∞<< AR1 . [4]

2.6 Quality factor (Q-factor)

Q factor is a parameter that describes how much power that transform as losses in the system. A high Q factor indicates a lower rate of energy loss relative to the stored energy. [19]

)15.2(2 0

12

00

Bandwidth

f

ff

fQor

LossPower

storedEnergyfQ =

−== π

23

Figure 2.20: Figure shows how 0f , 1f and 2f

are defined for Q factor calculations. [19]

Figure 2.20 shows the amplitude in function of the frequency for the antenna. For a

given center frequency ( )0f , 1f and 2f is the frequency when the center frequency has

dropped 3 dB from the maximum value. A low Q factor gives a broad band (wide) bandwidth or a high Q factor gives a narrow band (small) bandwidth.

The quality factor can also be expressed as a function depending of the substrate

thickness h and the dielectric constant rε [8].

)16.2(4 0hf

cQ

rε=

Q decrease when rε decrease or h increase.

2.7 Bandwidth

The bandwidth is defined as “The range of usable frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard” [20].

There are many ways to calculate the bandwidth, but for narrow band antennas the

fractional bandwidth )( fracB is common [4].

24

( )

)18.2(100*

)17.2(%

0

12

0

12

f

ffB

f

ffB

frac

frac

−=

−=

where =2f upper frequency, =1f lower frequency and =0f center frequency.

Formula 2.19 shows how the bandwidth depends on different parameters for a rectangular patch [5]

)19.2(,1

77.32

λλε

ε<<

−= h

h

L

WB

r

r

where rε is the permittivity, W is the width of the patch, L is the length of the patch, h

is height of substrate, λ is the wavelength

The bandwidth can also be calculated from formula

)20.2(4 2

00

rc

hf

Q

fB

ε=≈

2.7.1 Techniques for wider bandwidth

There are many techniques for higher bandwidth. This is few of the most common [10].

• Thicker substrate

• Bigger antenna, in general. When the antenna is electrically small the bandwidth decrease [21].

• Lower permittivity

• Different feeding techniques gives different bandwidth. Choose feeding technique that gives wider bandwidth.

25

2.8 Return Loss, S11 parameter

If the antenna has two ports, input and output (two ports network), the incident power to the antenna can be reflected, radiated or absorb as losses.

As figure 2.21 shows,

)21.2(lossradri PPPP ++=

Figure 2.21: 2 port networks

Many antennas are designed with low loss materials which mean that most of the power that is not reflected is radiated. [22].

When the characteristic impedance of the transmission lines are different from the impedance at the input port at the antenna, this lead to losses of the incident power from the 2 port network system and part of the incident power will be reflected back through the transmission line.

The return loss or reflection loss measures the effectiveness of the power delivery from the transmission line to the antenna [23]. The return loss is identified by the S11 parameter from two port network theory. The return loss )(RL is defined as [24]

)22.2(log20log20log10)( 1010SL

SL

i

r

r

i

ZZ

ZZ

E

E

P

PdBRL

+

−−=

−=

=

where iP is the Incident power [watt], rP is the reflected power [watt], iE is the incident

electric field, rE is the reflected electric field, LZ is the load impedance, SZ is the

transmissions line characteristic impedance [23], [24].

From formula 2.22 the reflected power can be calculated for a two port network.

26

)23.2(1010log10)(RL(dB) 10101011

1111 S

ir

S

r

i

r

i PPP

P

P

PdBS =⇔=⇔==

2.9 Ground plane size effects

It has been shown that the size of the ground plane affects the excitation efficiency of non-resonant modes. Therefore the thickness of the ground plane can be used to modify the mode excitation. For the patch antenna design with small ground plane it is not only to optimize the patch size but also the size and thickness of the ground plane. [10]

In many cases for antenna design one goal is to reduce the antenna size inclusive the ground plane. In many designs the ground plane is larger than the patch. There is a problem with finite ground plane. It can raise the diffraction of radiation from the edges of the ground plane which can lead to changes in the radiation pattern, radiation conductance and resonant frequency. Studies have shown when the antenna size is equal to the ground plane size it result in higher resonant frequency which not an infinitely ground plane gives. If the ground plane width and length increase with the length

EXTS , the fractional change in resonant frequency is given by [5]

( ) )24.2(1/,/ln11

lim0

00

>>+

≈∆

→hWforhW

h

f

f

r

r

rSk EXT λε

ε

π

where f∆ is the changes in resonant frequency with the extensional ground plane.

When formula (2.24) is not satisfied for theEXTS , formula (2.25) is used instead.

)25.2(240

BW

h

f

f

rer ε−=

∆

where

( ) ( )[ ] )26.2(8 0

200

20

0

0EXTEXT SkYSkJ

WkB −=

η

where 0J , 0Y is the first and second order of the Bessel function.

Another common recommendation for ground plane size is [25]:

27

)28.2(6

)27.2(6

patchantennaground

patchantennaground

WhW

LhL

+=

+=

When the height of the antenna increase the size of the ground plane increase which gives a noticeable effect on the total size of the antenna for small high antennas, i.e. the

ratio between antennapatch hL / or antennapatch hW / decrease.

2.10 Eddy current

“Eddy current” is current induced in conductors. If a large conductive metal plate is moved through a magnetic field which intersects perpendicularly to the sheet, the magnetic field will induce small "rings" of current which will actually create internal magnetic fields opposing the change. Figure 2.22 shows were the eddy current can be created for a coil over a conducting material. [26]

Figure 2.22: Eddy current [26]

28

2.11 Metallic environments effect on antennas

Metallic environment can have huge consequences for the antenna characteristics (antennas without or with ground plane). The eddy currents and image theory are two explanations why antennas do not work so well in metallic environment.

Eddy currents: When a metal plate or similarly is placed near an antenna the magnetic field (from coil in figure 2.22) creates eddy current which occurs in the metal plate. These current gives undesirable effects as absorption of power which leads to detuning of the antenna. This detuning will decrease the operating distance and quality factor [27].

Image theory: When a dipole is placed near a metal surface it will produce a image. The image has current with opposite sign. The image produces an electric field which will cancel the electric field caused by the original source (dipole). The effect can be negligible by move the dipole away from the metal surface. This means that you can not place an antenna on a metal surface and expect it to radiate. Consequences of the image theory are that the antenna can get detuned (shift in different resonant frequency). The metal will also affect the impedance at the antenna since a capacitance will be introduced and this will reflect a part of the energy back to the terminals because the antenna is miss matched [27], [28].

2.12 Ferrite shielding

To reduce the generated eddy currents ferrite material can be used as a shield between the antenna and the metal behind the antenna [27]. Ferrite materials have high electrical

resistance, up to Ω610 which decrease the effect of the eddy currents [29]. Electrical resistance is the resistance that materials prevent electrical current flow through for example a conductor. The electrical resistance increase when the conductor is longer or the area is smaller [30]. The eddy current reduce operating distance but a ferrite shielding will not increase the operating distance above values achievable in non metallic environment. [27]

The ferrite is a substance that consists between a mix of iron oxide and oxides (or carbonates) with other related material, such as nickel, magnesium and zinc. Ferrites are used in many applications that involve magnet induction. [27]

29

Advantages with ferrite materials

By choosing ferrite materials there is a large section of materials depending which combination of iron oxide and carbonate which are used. Ferrite materials have a high Q value and resistivity. The ferrites can be shaped in many different forms and it is a good choice for low cost applications. [31]

Disadvantages with ferrite materials

In many applications ferrite materials is not a good choice since it is a tenuous material, especially pure ferrite. This lead also to problem when it will be used in machines. To solve this problem the pure ferrite is mixed with other materials that will change the materials properties. Mix ferrite with other materials can be both expensive and take time. Ferrite is also weak at thermal shocks. [32]

30

Chapter 3

3 Design ideas for antenna

The first phase in the thesis work was to investigate what type of antenna that meets the requirements for the antenna. The investigated antenna types are microstrip, patch, PIFA, loop and dipole. These are all very common antenna types and are used in many applications.

3.1 Microstrip and Patch antenna

3.1.1 Advantages

Patch antennas can be made small and flat. One layer of 1.5 mm FR-4 is used in many applications which give them a low profile. The patch antennas can also be very light weighted. Since the patch antennas only consist of two metal surfaces and a substrate layer the patch antennas can be made at a low cost depending on the choice of substrate and feeding technique. FR-4 is relative a cheap substrate material. In many designs patch antennas is easy to manufacture. The structure of a patch antenna makes it possible to integrate the antenna with circuits, the ground plane is a useful property and it simple to create array antennas. [10]

3.1.2 Disadvantages

Since the patch antennas are often small it affect many of the antenna properties negatively. The patch antenna has often low efficiency and narrow bandwidth. The different types of feeding techniques can give surface waves. The antenna is known for their tolerance problem and it requires good quality of the substrate to got high temperature tolerance. It can also be difficult to make arrays with high performance since it requires complex feed system. Design an antenna with good purity of the polarization can also be difficult to achieve. [10]

31

3.1.3 Tradeoffs

Table 1 shows approximate tradeoffs for a rectangular patch. [4]

Table 1: Tradeoffs patch antenna

Antenna property Height of substrat

Substrate relative

permittivityWidth for the Patch

High radiation efficiency thick low wide

Low dielectric loss thin low ---

Low conductor loss thick --- ---

Wide (impedance) bandwidth thick low wide

For many antenna designs the requirements in table 1 are common. For example, design of a small patch with wide bandwidth can be hard because parameters that decrease the size of the antenna as low width and thin substrate give the antenna a small bandwidth.

3.2 Planar inverted f antenna

Planar Inverted F Antenna, PIFA, has been popular for portable wireless devices. The structure consists of a ground plane and a top element that have a DC shorting plane that’s connecting them. Often a feed wire is used from the ground to the radiation top element [33]. PIFA’s can be designed with or without ground plane under the top plate element but then it is closer to a monopole [34].

32

Figure 3.1: PIFA [34]

Figure 3.1 shows PIFA with and without ground plane under radiation part.

Figure 3.2: Patch layer for PIFA [35

Figure 3.1 shows PIFA with and without ground plane under radiation part. Figure 3.2 shows different patch layer. They can have slots as L-shape, U-shape, spiral-shape and

33

others. The purpose with all this different structure for the patch layer is that the current needs to travel a longer way on the patch surface which increase the electrical length. Increasing the electrical length ( )λ leads to a decreased frequency, fc /=λ [22], [36].

3.2.1 Advantages

The PIFA is designed after a simple concept. It has low manufacturing cost and can be made small. The PIFA has good electrical characteristics and low losses in the design. The design reduces the backward radiation and can be built with high gain in vertical and horizontal polarization. The structure makes it easy to tune by adjusting the length of the arm and matching problems can be solved by changing the position of the feed. [35]

3.2.2 Disadvantages

The PIFA has narrow bandwidth. To reduce the size of the antenna the shorting pin is placed near the feeding probe but this result in narrow impedance bandwidth. The design can be difficult with air as substrate, special with thin patch layer and ground plane.[35] In applications there small size is a requirement the size of the ground plane can be a limiting factor [37].

3.2.3 Techniques for wider bandwidth

One of the easiest ways to increase the bandwidth is using a thick substrate layer with air. This can easily be done when the increased size of the antenna is not a problem [35]. Other alternative is to use parasitic resonators with resonant lengths close to the resonance frequency [38] or stack more than one radiation patch [39]. The size of the ground plane can also be used to increase the bandwidth [35].

3.2.4 Techniques for reducing the physical size The physical size of the PIFA can be reduced by adding more than one shorting pin. These do not need to be placed near each other [40]. Another easy method is to increase the permittivity of the dielectric material. The Effect of this is lower bandwidth [41]. Figure 3.2 shows that the patch layer include different types of slots which use to reduce the physical size of the antenna [42]

34

3.3 Loop antenna

Loop antenna is a closed loop antenna and can be divided into two groups, small loop antennas and large loop antennas [43]. They can have different forms as a rectangle, square, triangle, ellipse, circle and many others configurations [4]. A small loop antenna is like a coil since they have the same current distribution as the ordinary circuit coil and having the same phase and amplitude thru the whole coil. For this condition the perimeter of the loop antenna must be more than λ1.0 .

Figure 3.3: Loop antenna [44]

3.3.1 Advantages

Loop antennas have good bandwidth and they can broadcast and receive over a wide range of radio frequencies. Since they can be made small they are used in many wireless devises. The loop antennas can also be made at a low cost. [44]

3.3.2 Disadvantages

It can be difficult to match the impedance for small loop antennas since they have high reactance. In result of this the loop antennas are often used as receiver antenna when the mismatch of the impedance can be tolerated. The small loop has also high current flowing in the antenna which is caused by the low radiation resistance. The flowing currents result also in power losses by the heat. [44]

The size of the loop antenna is a limiting factor. Small loop antennas have poor efficiency and the efficiency can be increased by increasing the size. Therefore often big loop antennas are used. As the dipole antenna, the loop antenna is not good for application that needs a ground plane. [44]

35

3.4 Half-wave d

A half wave dipole antenna is a straight electrical conductor measuring 1/2 wavelength from end to end and connected at the center to a radioFigure 3.4 shows a basic design of a

3.4.1 AdvantagesDipole antennas are very easmonopole antenna can be used since itlow and high frequencies (UHF and long wave) the

wave dipole antenna

dipole antenna is a straight electrical conductor measuring 1/2 wavelength from end to end and connected at the center to a radio-frequency (RF) Figure 3.4 shows a basic design of a half-wave dipole antenna.

Figure 3.4: Half-.wave dipole structure [49-46]

3.4.1 Advantages and disadvantages

Dipole antennas are very easy to design [47]. Instead of using a dipole antenna, a can be used since it has a lighter weight and be smaller in

low and high frequencies (UHF and long wave) the antenna performs adequately. [

dipole antenna is a straight electrical conductor measuring 1/2 wavelength frequency (RF) feed line [45].

using a dipole antenna, a be smaller in size. For

antenna performs adequately. [48]

36

3.5 Decision for antenna type

The designed antenna will be in a metallic environment and therefore an antenna type that will not be affected so much by metal behind is necessary. The antenna will probably be 1-5 cm from the metal but even from this distance the metal can affect the antenna. Therefore a dipole antenna and loop antennas will probably not be a good choice. Even PIFA’s without ground plane below the patch is probably not a good solution. The image theory and eddy current can be two reasons why this will not work.

The choice were then between microstrip or patch antenna and PIFA with a ground plane under radiation patch. These two antenna types are similarly to each other in construction as feed techniques and minimizing techniques. It decides to design a microstrip or patch antenna since it can fulfil the antenna specification. This is few reason why:

• It is easy to calculate the resonant frequency (2.1) and it gives the opportunity to use higher modes.

• There are many ways to minimize the antenna.

• There are many feeding techniques to choose between.

• It is a more stable construction than PIFA. PIFA with air as substrate make the construction unstable (not compact) and with other substrate the bandwidth decrease.

• Can design it with circular polarization.

37

Chapter 4

4 Design of dual band patch antenna

4.1 Design procedure

To design the circular polarized dual band antenna this 6 stage procedure has been used, see figure 4.1.

Figure 4.1: Design procedure

Starting stage: The design procedure starts with to find an appropriate structure for the antenna.

Simulation stage: After a proper structure has been found it will be simulated in CST microwave studio to investigate their properties (frequency, bandwidth, polarization etc).

Minimizing stage: Minimize the physical size of the antenna and still have the same properties.

Designs stage: Build the simulated antenna and test the antenna.

Matching stage: Match the antenna with the actual simulation design.

Final stage: Verification of the antenna design so it will fulfill the antenna specification.

38

4.2 Antenna design

This chapter goes through the design procedure from starting stage to final stage.

4.2.1 Starting and simulation stage First it needs to choose which feeding technique that will be used. A small size and wide bandwidth are two properties with high priority. A microstrip line is not the best choice since the microstrip line increase the size of the antenna and the bandwidth decrease if a thick substrate is used. An aperture-coupled patch would not either be the best solution since it gives narrow bandwidth and difficult to fabricate.

Therefore a feeding technique that feed the antenna below the patch will be the best alternative since it decrease the size (width or length) of the antenna. Example of these feeding techniques is coaxial probe or coaxial probe with capacitive feed. The coaxial probe gives narrow bandwidth and coaxial probe with capacitive feed gives wider bandwidth. Than the choice will be coaxial probe with capacitive feed. The disadvantage with this feeding technique is the manufacturing process. It can be difficult to match the antenna.

Since the reader antenna that is used in tests is right handed circular polarized the designed antenna also need to be right handed circular polarized for low polarization loss factor as possible. If the designed antenna is LHCP and reader antenna is RHCP the polarization loss factor will be maximal, which means they can not communicate with each other. To simplify the manufacturing process of the antenna truncated corner are used instead of the dual feed patch.

When feed and polarization technique have been chosen the design continuous with a already existing large patch antenna for 868 MHz. It has a ground plane with the dimension of 2552552.5 mm and with patch dimension 1531532.5 mm. The used feed technique is coaxial probe with capacitive feed with air substrate of height 13 mm. The patch has also truncated corner. Simulations result for the antenna shows that it has a bandwidth of 90 MHz (810-890 MHz) with 10 . For the 2.45 GHz frequency band the antenna is detuned and not matched. Between 1.9 GHz and 2.287 GHz the 6 . The simulated antenna was detuned in frequency so it operates best for 828 MHz with circular polarization straight out from the patch and linear polarization at the short sides (circular close to antenna). For 2.45 GHz the antenna should radiate from the short sides of the antenna. The simulated antenna radiates with around 5 dBi at 2.15 GHz at the sides of the antenna.

39

4.2.2 Minimizing stage

To reduce the size of the antenna three techniques been used: shorting wall, change of substrate and use of available antenna parameters. First a shorting wall is used to minimize the antenna. That is an effective method and reduced the size of the patch with 50%. By using this method to reduce the size of the antenna the bandwidth will decrease (formula 2.19) since the bandwidth is dependent of the ratio between the width and length of the patch, " . By reducing W the bandwidth decreases. Air is an excellent choice as substrate if high bandwidth is a priority. The minimum bandwidth that is needed is at least 2 MHz bandwidth for the antenna. The goal was to get higher than 2 MHz bandwidth to compensate detuning changes of frequency. A substrate with higher dielectric constant lowers the resonance frequency, by formula 2.1 but it will also decrease the bandwidth (formula 2.19 and 2.20). A substrate with high dielectric constant is Rogers Corporation TMM 10i [49], #$ 9.8

and'( ) 0.002. The advantage with this substrate is that Rogers Corporation has many different heights of the substrate available. A 12.7 mm thick substrate is needed. The antenna was simulated with TMM 10i which gave a resonance frequency lower than 868 MHz. To increase the frequency the antennas size can then be minimized (formula 2.1 and 2.6). The antenna dimension after minimizing it with TMM 10i is 108523.5 mm. The minimized antenna is matched for 868 MHz, *+*,-. 42 . The bandwidth for 868 MHz is 24 MHz (856-880 MHz) for 10 . For the 2.45 GHz frequency band the antenna is detuned and not matched. At 2.138 GHz the 10.7 and for 2.75 GHz the 4.8 . There occurs one big problem with the minimized antenna. The substrate (TMM 10i) has a delivery time of 7-8 weeks which is not acceptable for the current project. The whole project time is 20 weeks and when the materials are delivered the project would nearly be ended. Therefore it was necessary to change substrate or design. Therefore the substrate is changed to FR-4. FR-4 is a very common material that is cheaper2 than TMM 10i and with #$ 4.3and'( ) 0.02. FR-4 is only available in few thicknesses (0.8-1.5mm). To get a height of 12-13 mm several layers need to be stacked. The consequence with FR-4 instead of TM10i is that FR-4 has lower dielectric constant which increases the size of the patch antenna (formulas 2.1, 2.2, 2.6). The antenna will also get higher bandwidth with FR4 instead of TMM 10i since it has lower dielectric constant (formula 2.19-2.20) Formula 2.7 gives also a recommendation for the width of the patch. In this design the ratio between the length and width of the patch is exactly

2 FR-4: 1.5mm, 207 €/01 TMM 10i: 1.5 mm, 3105 €/01 TMM 10i: 12.7 mm, 7549 €/01

40

two. The ground plane affects the size of the antenna. To minimize the total physical size of the antenna the ground plane is kept as small as possible. By formula 2.27-2.28 the ground plane should be extended with the factor 62345332 (81.21 mm) for the antenna. This will increase the total physical size of the antenna significantly. The reason why the extension part (62345332) is so long depends on the large thickness of the antenna. The recommended size of the ground plane will not be used. To decide the size of the ground plane, simulation result and the manufacturing process has been taking into account. Each FR-4 plate that is bought is 160 mm long. To simplify the manufacturing process, the choosing length of the antenna is 160 mm. Then the ground plane is larger than the patch and will not increase the resonate frequency as much as possible. The width is determined by the simulation results so the antenna is match for the right frequency.

4.2.3 Matching stage to final stage

4.2.3.1 Design errors

From the beginning the simulated model of the antenna for 868 MHz has a bandwidth about 25 MHz for RL < -10 dB. When the antenna was built the network analyzer3 showed that the antenna has a resonance frequency around 650 MHz and 1.3 GHz. By change the dimensions of the patch, radius of feed circle (from 3 mm to 10 mm) and radius of the hole in the patch (from 0.5 mm to 1.5 mm) the design get more stable and easier to build.

After these changes the antenna works much better and increase the frequency closer to 868 MHz but the antenna did not work as the simulations. The simulation software (CST) showed that the antenna is matched for the resonant frequency at 838 MHz (about 10 mm from metal), 16 MHz bandwidth for RL < -10dB but the network analyzer showed that the antenna is matched for the resonance frequency 880 MHz (around 10 mm from metal), that is 42 MHz difference.

There are few differences between the simulated version and the built version. For example there is a small air gap between the inner conductor of the coax and the substrate for the built antenna. There is no air gap in the design and the substrate in the simulated design is one thick layer. In the built antenna there are nine thinner layer of

3 Model: Agilent Technologies: E8364B (10 MHz-50 GHz) PNA Series Network Analyzer

41

substrate that should correspondlayers solid to each other.

The final test bed needederrors was found. In CST the patch circle was not centered over the feed circle which it were drilled and for CST the radius

Figure 4.2: Shows how the S

curve shows the S11 parameter for the antenna with no errors in the desi

Simulation shows that the built antenna is better match then the simulation for the design with errors but there areS11 parameter, see figure 5.12

hould correspond to the thick layer. The nine layers were glued to get al layers solid to each other.

bed needed two RFID antennas. When the second antenna was builterrors was found. In CST the patch circle was not centered over the feed circle which it were drilled and for CST the radius was 3 mm, not diameter of 3 mm.

Shows how the S11 parameter changes when the errors in the design are

parameter for the antenna with no errors in the design and green curve shows the S

parameter with errors.

Simulation shows that the built antenna is better match then the simulation for the design with errors but there are still 44 MHz difference between the built antennas

parameter, see figure 5.12a and the simulated version, red curve in figure 4.2

layers were glued to get al

hen the second antenna was built few errors was found. In CST the patch circle was not centered over the feed circle which it

was 3 mm, not diameter of 3 mm.

errors in the design are corrected. The red

gn and green curve shows the S11

Simulation shows that the built antenna is better match then the simulation for the difference between the built antennas

version, red curve in figure 4.2.

42

4.2.4 Layout

Figure 4.3-4.8 shows the structure from five different views of the antenna. The antenna is also built with the given parameters in figure 4.3-4-8.

Figure 4.3: Top view on feed

Figure 4.4: Top view on ground

43

Figure 4.5: Top view on patch

Figure 4.6: Side view (long side)

Figure 4.7: Side view (Short side)

44

Figure 4.8: Built antenna, top view

Figure 4.3 shows the top view on feed with substrate behind. Figure 4.4 shows the top view from ground. From this view only the ground is shown and the coaxial feed in the middle of the ground. Figure 4.5 shows the patch layers form with the substrate behind. Figure 4.6 shows the antenna form the long side. The antenna is built with nine layers FR-4, 1.5 mm height each. Figure 4.7 shows the antenna from the short side. The substrate is divided into nine sub layers (SL) as figure 4.6. Figure 4.8 shows the built antenna from the top view.

Table 2 shows all value for the parameters in figure 4.3-4.8

Table 2 Parameter Description Value (mm)

r2 Feed circle radius 10 r3 Radius of circle in patch 1.5

h1 Distance between top of ground and bottom of feed circle 12

h2 Distance between top of feed circle and bottom of patch 1.5

L1 Length of ground plane 160

L2 Distance between ground plane edge and centre of circle in patch 27.5

45

L3 Distance between ground plane edge and centre of circle in patch 80

L4 Length of truncated corner 14

L5 Distance between shorting wall and truncated corner at the shorter short side. 35.1

L7 Distance between truncated corner at the patch long side at the longer short side. 52

L8 Distance between patch edge and ground plane edge 14 L9 Length of shorting wall 118 W1 Width of ground plane 63.5 Other values

Ground plane thickness 0.035 Feed circle thickness 0.035 Patch thickness 0.035

46

Chapter 5

5 Simulation results and tests

5.1 General simulations settings in CST

CST microwave studio is used as simulation software. The simulation settings in CST (mesh shells properties, frequency range, accuracy etc.) affect the simulations result a little, special the S11 parameter. Therefore the S11 parameter looks a bit different depending on which settings are used.

Following simulation settings have been used:

Frequency range: 800-870 MHz, 2.22-2.5 GHz, 0.7-2.5 GHz

Solver settings (Accuracy): -30dB

Global Mesh Properties

• Lines per wavelength: 15

• Lower mesh limit: 5

• Mesh line ratio limit: 20

5.2 General about simulations results

The report presents four different simulation results (S11 parameter, smith chart, far field pattern, axial ratio). Simulation results for antenna are simulated with an infinite ground plane, positioned 11.5 mm away from the antenna. On the train the antenna will be 10-50 mm away from the train which corresponds to the infinite ground plane in simulations.

S11 parameter (return loss) describes the relationship between the input power and reflected power to the system for a two ports network. The transmission line and load (antenna) are better matched if the S11 is lower. A lower S11 means also that less power is reflected.

Smith chart is a visualizing tool for the impedance at the transmission lines as a function of frequency. For the impedance matching the smith chart is very useful. The smith

47

chart has two axes. The horizontal line is the resistance axis and the circular boundary is the reactance axis. By study the smith chart, it shows how good the antenna is matched. It is good match if red curve in smith chart goes through the point 1 at the horizontal line. In that point the reflections coefficient ( )Γ is zero and no power is reflected by the

load [22]. If 1=Γ , the circular boundary of smith chart, all power is reflected. When

the S11 parameter is lower the curve in smith chart will be closer to 1. For the simulation

results the the smith chart shows a normalized value for the impedance ( SZ ) there 1=SZ in

the smith chart represent a impedance of the antenna SA ZZ 50= .

The far field pattern shows how good the antenna radiates in the far field region for different angles from the antenna. The antenna radiates with higher power for that angels in the far field pattern there is red, radiates with lower power at that angels in the far field pattern there is yellow and radiates with no power at the angels there the far field pattern is blue. For the simulation results the far field pattern is given with the absolute value. Figure 5.6-5.8 shows also the power in dBi which means that the power is calculated for a point source [8].

Axial ratio shows the polarization for the antenna. It is circular polarized if the pattern is green (0 dB) and linear polarized where the pattern is red. The radiation efficiency

)( REe is the ratio between the radiated power and input power [10], [50].

)1.5(input

radiated

REP

Pe =

The optimum value for )0(1 dBeRE = since then all the input power is radiated. A low

value of REe , i.e. )0(1 dBeRE << , indicate a system (for example a two port network)

with losses and where not much power is radiated. The losses can arise from conduction losses or dielectric losses. The total efficiency is the radiation efficiency multiply with a loss factor. When there are no losses the total efficiency is equal to the radiation efficiency or the ratio is 1 [67-51]. The directivity measure how directional the radiation pattern is for the antenna. An antenna with directivity dB0 radiates at all directions

equally. When the directivity increase the antenna radiates better in any direction.

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5.3 Simulations result Following chapter presents simulation resultchart, far field pattern and axial ratio for both frequency band, 868 MHz and 2.45 GHz. The simulation results aredifferent between the simulated result and the analyzer, see chapter 5.4.

5.3.1 S11 parameter

Figure 5.1: 11S parameter for

Figure 5.1 shows that the antenndBRL MHz 22836 −=

dBRL MHz 06.2868 −=

5.3 Simulations result Following chapter presents simulation results from CST for the Schart, far field pattern and axial ratio for both frequency band, 868 MHz and 2.45 GHz.

ion results are for the built antenna dimensions, see chapter different between the simulated result and the measured result from th

chapter 5.4.

parameter

parameter for 868 MHz. Frequency range between 800 MHz and 870 MHz.

shows that the antenna is not matched for 868 MHz. dB gives that 0.6 % of the incident power is reflected.

dB gives that 62.2 % of the incident power is reflected.

S11 parameter, smith chart, far field pattern and axial ratio for both frequency band, 868 MHz and 2.45 GHz.

for the built antenna dimensions, see chapter 4.2.4. There is a result from the network

between 800 MHz and 870 MHz.

that 0.6 % of the incident power is reflected.

% of the incident power is reflected.

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Figure 5.2: S11 parameter for 2.45 GHz. Frequency range between 2.22 GHz and 2.45 GHz.

Figure 5.2 shows that the antenna is not match for 2.45 GHz. dBRL GHz 5.22284.2 −= gives that 0.3 % of the incident power is reflected.

dBRL GHz 06.2402.2 −= gives that 62.2 % of the incident power is reflected

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Figure 5.3: S11 parameter for 868 MHz and 2.45 GHz. Frequency range between 700 MHz and 2.45

GHz.

Figure 5.3 shows the S11 parameter with both frequency bands (868 MHz and 2.45 GHz) in the same figure.

dBRL MHz 3.11836 −= gives that 7.4 % of the incident power is reflected.

dBRL MHz 5.3868 −= gives that 44.7 % of the incident power is reflected

dBRL GHz 3.25284.2 −= gives that 0.3 % of the incident power is reflected.

dBRL GHz 96.1402.2 −= gives that 63.7 % of the incident power is reflected..

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5.3.2 Smith chart

Figure 5.4: Smith chart for 868 MHz. Frequency range between 800 MHz and 870 MHz. The blue point

shows the impedance ( sZ ) for 836 MHz and black point for 868 MHz.

Figure 5.5: Smith chart for 2.45 GHz. Frequency range between 2.22 GHz and 2.45 GHz. The blue point

shows the impedance ( SZ ) at 2.284 GHz and black point at 2.402 GHz.

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Figure 5.4 shows that the antenna is good matched at 836 MHz since the blue point is near 1=SZ and worse matched at 868 MHz since the black point is far from 1=SZ .

Figure 5.4 shows even that Ω+= iZ A 25.646 at 836 MHz and for 868 MHz,

Ω+= iZ A 66.1874.6 Figure 5.5 shows that the antenna is good matched at 2.284 GHz

since the blue point is near 1=SZ and worse matched at 2.402 GHz since the black

point is far from 1=SZ . Figure 5.5 shows even that Ω−= iZ A 5.256 at 2.284 GHz and

for 2.402 GHz, Ω+= iZ A 1.2039.165

5.3.3 Far field pattern

Figure 5.6: Far field pattern for 868 MHz

Figure 5.6 shows the far field pattern for 868 MHz. The antenna radiates with 5.92 dBi in front of the antenna and worse at the sides60 78. The radiation efficiency is -0.4844 dB, the total efficiency is 4.708 dB and the directivity is 5.924 dBi. For 868 MHz there are much losses in the antenna since the ratio between the radiation efficiency and total efficiency is not near 1.

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Figure 5.7: Far field pattern (left side) for 2.45 GHz, or more exactly 2.402 GHz.

Figure 5.8: Far field pattern (right side) for 2.45 GHz, or more exactly 2.402 GHz.

Figure 5.7-5.8 shows that the antenna radiates with 7.63 dBi at best from the both short sides of the antenna for 2.402 GHz. The radiation efficiency is -2.287 dB, the total efficiency is -6.605 dB and the directivity is 7.631 dBi. For 2.402 GHz there are much losses in the antenna since the ratio between the radiation efficiency and total efficiency is not near 1.

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5.3.4 Axial ratio

Figure 5.9a-b: Axial ratio for 836 and 868 MHz. Figure 5.9a shows axial ratio (in the far field pattern)

for 836 MHz and 5.9b shows axial ratio for 868 MHz.

The simulation results show that the antenna is much better matched for 836 MHz than 868 MHz. A result of this is shown in figure 5.9 a-b. Figure 5.9a and 5.9b.shows that the antenna has much better circular polarization for 836 MHz than 868 MHz since the axial ratio value (dB) is closer to zero and it has also lower value for axial ratio at more angels in the far field pattern.

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Figure 5.10: Axial ratio 2.45 GHz (left side), or more exactly 2.402 GHz.

Figure 5.11: Axial ratio 2.45 GHz (right side), or more exactly 2.402 GHz.

Figure 5.10 that the antenna has circular polarization at the left side close to the antenna and linear polarization far away from the antenna. Figure 5.11 show that the antenna has not so good circular polarization at the right side close to the antenna and linear polarization far away from the antenna.

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5.4 Measure antenna with Network Analyzer

When the antenna was built, the antenna was tested with the network analyzer4 to verify the resonance frequency of the antenna. Figure 5.12a-c shows the measured result with the network analyzer for the S11 parameter

Figure 5.12 a-c: S11 parameter from Network analyzer for 868 MHz and 2.45 GHz. The red curves show

the antenna without a metal wall behind the antenna and the blue curves shows the antenna with a metal

wall around 1 cm behind the antenna.

4 Model: Agilent Technologies: E8364B (10 MHz-50 GHz) PNA Series Network Analyzer

57

Figure 5.12a shows S11 parameter for 868 MHz. For 868 MHz the dBRL wallmetal 5.6−=

which means that 22.4 % of the incident power is reflected and dBRL wallmetalno 2.8−=

gives 15 % of the incident power is reflected.

Figure 5.12b shows S11 parameter for 2.45 GHz. For 2.402 GHz (channel 1) the

dBRL wallmetal 9.11−= which means that 6.4 % of the incident power is reflected and

dBRL wallmetalno 12−= gives 6.3 % of the incident power is reflected.

Figure 5.12c shows S11 parameter for both 868 MHz and 2.45 GHz. The difference between RL in figure 5.12a-b and figure 5.12c is very small.

5.5 Outside test

The outside test for the antenna owns place at Ångströms goods reception, see figure 5.13. The built antenna (antenna 1 in figure 5.13) was placed on the short side of the green container. Another receiver antenna, RFID 868 MHz (antenna 2 in figure 5.13) was placed different distances (d) away from antenna 1 to measure the reading distance for antenna 1. The container corresponds to the train wagon in the real application. This test is to verify that the antenna fulfills the requirements of the reading distance.

Figure 5.13: Set up for the outside test.

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In the set up for the outside test Antenna 1 is placed with the antennas shorting wall down to the ground. Table 3 shows where in figure 5.13 the antenna 2 where placed and the power of the received signals for antenna 1. The x,y,z-coordinates defines the distance antenna 2 is moved from the position (x,y,z)=(0,0,1) which is Antenna 1 position.

Table 3 Antenna 2 position (m) Signal strength (dBm) x coordinate y coordinate z coordinate

0 4 1 -42

1 4 1 -44

2 4 1 -50

3 4 1 -63

x>3 4 1 no signal

-1 4 1 -45

-2 4 1 -49

-3 4 1 -59

x<-3 4 1 no signal

0 3 1 -44

1 4 1 -48

2 4 1 -57

3 4 1 -63

-1 4 1 -48

-2 4 1 -44

-3 4 1 -54

0 2.7 1 -46

1 2.7 1 -46

2 2.7 1 -55

3 4 1 -63

-1 4 1 -51

-2 4 1 -49

-3 4 1 -63

0 2 1 -34

1 2 1 -40

2 2 1 -53

-1 2 1 -40

-2 2 1 -55

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Table 3 shows that the antenna has a maximum read range of 5 m. This result is enough for the application. When the signal power is lower than -70 dBm the reader concider the signal as background noise and ignore it [1].

Figure 5.14: RFID reader

Figure 5.14 shows how the RFID reader can be placed beside the rail. It is placed about 3 m away from the rail. The distance between the RFID reader and the RFID antenna is defined as the vertical direction or as in figure 14 the y direction. The train goes in the horizontal direction (x direction). Result from the outside test shows that if the distance between the RFID reader and RFID antenna is 4 m the RFID reader can receive data from the RFID antenna 3± m in the x-direction, totally 6 m in horizontal direction.

There was also made a test in free space, no metal behind the antenna for the other frequency, 2.45 GHz to check if the antenna works for that frequency. The antennas short side was directed to a 2.45 GHz reader antenna. It is interesting to check the reading distance when the short side is directed to the reader antenna and not the patch since on the train the short side will be directed to the antenna in the sensor boxes.

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5.6 Final set up on train

Figure 5.15: Set up for final test

Figure 5.15 shows the experimental set up on the train. There are two RFID boxes and four sensor boxes (two sensor box 1 and two sensor box 2) that are mounted on the train. The sensor boxes collect temperature and vibration from the ball bearings and transmit it to the RFID box. The RFID box transmits the data from sensor boxes to the RFID reader that are mounted beside the train. The sensor box 1 on the left side of the train can send their data to the sensor box 2 on the right side and vice versa but sensor boxes can not save the data from other sensor boxes. Therefore the sensor boxes need to transmit the data direct to the gateway. The RFID boxes is placed on both sides of the train so the RFID antenna can send the data to the reader antenna regardless which side the train goes at or if the readers only is placed at one side of the train. It could also be good to have RFID boxes on both sides if the sensor boxes on the right side can not send their data so the RFID box on the left side.

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For the final test the hardware can be programmed with different advanced communicate settings between the RFID reader and RFID antenna. For the most advanced settings the RFID antenna need a horizontal read range of 3.8 m and with the most basic settings a horizontal read range of 1.6 m. For the more advanced settings a verifying test is done for the received message. For this final test the most basic settings is used even if the antenna should handle a horizontal read range of 3.8 m. [1] The RFID box and sensor boxes contain following:

• RFID box: 868 MHz antenna, 2.45 GHz antenna, solar panel (energy source) , lithium polymer battery, circuit board and sensors.

• Sensor box 1: 2.45 GHz antenna, thermoelectric element (energy source), lithium polymer battery, circuit board and sensors.

• Sensor box 2: 2.45 GHz, piezoelectric strip (energy source), lithium polymer battery, circuit board and sensors.

Figure 5.16: Sensor node box on train

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Figure 5.17: Measure wagon for the final test

Figure 5.16 shows how a sensor node can be placed at the bogie on the train. Figure 5.17 shows the measure wagon which is used for the final test.

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Chapter 6

6 Final words

6.1 Future work

This chapter will present future work for the RFID antenna.

6.1.1 Update hardware

The aim is to make the RFID boxes as small as possible. The RFID boxes contain antennas, for the moment two single band antennas, 868 MHz and 2.45 GHz antenna. For this test bed the size of the RFID box is 5.55.145.22 xxLxWxh = cm. If the RFID antenna is used for both frequencies the total volume of the RFID box decrease with 15 %. Also if the RFID antenna is used as a dual band with the solar cell (energy harvesting element) and battery placed beside the antenna instead of below it which could reduce total volume of the RFID box with around 50 %. It is not sure how the solar cell and battery will affect the 2.45 GHz antenna which radiates from the short side of the RFID antenna. Using the RFID antenna as a dual band the hardware needs antenna diversity which it has not for the moment.

6.1.2 Smaller RFID antenna

The current RFID antenna is not the smallest one and it can be smaller. By doing it smaller, it can be worth to test ferrite material as protection from the metal environment [51]. Another thing that can be worth to test is using high dielectric materials at substrate. It reduces the size of the antenna but also decrease the bandwidth. The RFID antenna needs on the other side only a bandwidth of 2 MHz but during this thesis work the affect by higher dielectric materials for the bandwidth at higher frequency as

64

2.45 GHz has not been tested.

Figure 6.1: Small RFID antenna

It is possible to build much smaller patch antennas for 868 MHz than the fabricated for this thesis work. Figure 6.1 shows example of a ceramic patch antenna hWL ×× =25x25x3 mm for 900 MHz, 48=rε .

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6.1.3 Multi-hop system

For the test bed that has been installed autumn of 2011 the system works as figure 5.4 shows. The ambition is that the system should work as shown in figure 6.2. The propose system use “Multi-hop”, i.e. the nodes relay the information from each other to the gateway. This means that the antenna in the gateway box for 2.45 GHz only need a reading range of 10 m instead of 25 m and if the information can be send from one side of the train to the other, only one gateway box is needed.

Figure 6.2: Set up for multi hop-system.

Figure 6.2 show how the new multi-hop system will work. All sensors transmit data to all other sensors and receivers and all sensors transmit the incoming data further to all sensor. By this system the data travels from the sensor furthest away from the RFID box to the closest one. In figure 6.2 the orange arrow indicate that the box transmit the data to the other box and the box which receive the signals also transmit it further, not just receive the signals.

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6.2 Conclusions

In this project a circular polarized dual band patch antenna for 868 MHz and 2.45 GHz has been designed and built. From the beginning of the project, the meaning was that the antenna would be used as a dual band but during the project the plans change and the antenna will only be used as a single band antenna for 868 MHz. The 2.45 GHz frequency will be used by another antenna. The antenna design is not optimized as a dual band but it can be used as a dual band.

The antenna will be in harsh environment with a lot of metal. This has affected the design of the antenna. It was chosen to build a patch antenna since it has a ground plane which reduces the affect of the environment. The antenna is also designed to have much higher bandwidth than the 2 MHz (865.7-867.6 MHz). This is done by FR-4 as substrate and coaxial probe with capacitive feed. To get circular polarization, truncated corner has been used.

The built antenna is not the smallest one and there are many ways to do it smaller. One interesting method is to use high dielectric materials, ceramic materials with 50≈rε .

The report present simulation result with TMM 10i ( 8.9=rε ) instead of FR-4 (

)4.4=rε and the TMM 10i reduced the size with 44 %.

The RFID box contains two antennas and if one small RFID antenna is one important priority there is a lot of work to minimize these 2 antennas together. The question is: One or two antennas? It might be much easier to built two smaller antennas that can correspond to one larger antenna.

When the structure of the antenna was decided, it was focused to build an antenna with wide bandwidth. The feed technique, coaxial probe with capacitive feed is a good solution for wider bandwidth but with the current design of the antenna with several thin layers of FR-4 which gives matching problem for the antenna since it occurs small changes between the simulation and the built antenna.

There is much easier to build a patch antenna with coaxial probe with capacitive coupling if air is used as substrate but when another substrate is used, two different layer of substrate, one over and one below the feed circle are needed which complicate the design. Since it was not able to use two layers, it was not possible to buy the high thickness of the lower layer; the solution was to stack many layers.

67

In this thesis work different types of techniques for circular polarization have not been tested. Truncated corner is used for circular polarization but other techniques as dual feed may give better results. Since the antenna is only used for 868 MHz the polarization is not match for 2.45 GHz. This will probably result in polarization miss match between the transmitted antenna (antenna in sensor boxes) and receiver antenna (built antenna) if the built antenna would be used as a dual band antenna in the RFID box. The polarization miss match effect will probably be negligible of the transmitted antenna is near the reader antenna.

The built antenna was tested on the train and it fulfilled all the tasks during the final test. The test outside shows that the antenna can communicate with the RFID reader from a distance of 5 m and in the real situation the reader will be placed around 3 m away from the antenna. When hardware is programmed with the most advanced communication settings the antenna needs a horizontal read range of 3.8 m. The outside test shows that the antenna has a horizontal read range at 6 m. Therefore a more advanced communication setting can be used in the future. The horizontal read range of 3.8 m is also when the train has a velocity of 250 km/h.

Since the antenna in the final test only will be used for 868 MHz, test for reading distance at 2.45 GHz was not a priority. The outside test shows that the antenna has a reading distance at 10 m for 2.45 GHz but this was done when the hardware was programmed for channel 16 in the 2.45 GHz band (2.4775-2.4825 GHz). Since my antenna is matched best for channel 1, 2.4025--2.4075 GHz and worst for channel 16 the result is misleading. If the antenna will be used for 2.45 GHz it will have better reading distance than 10 m since channel 1 than will be used instead of channel 16. Figure 5.12b (red curve) show that there is a big different in RL for the both channels.

For channel 1 dBRL wallmetalno 7.13−= and for channel 16 dBRL wallmetalno 7.5−=

In the future there is a lot of works with the gateway: A good choice is to first decide if a dual band or two single band antennas are the best choice for the final version of the system. If a dual band antenna seems to be the choice the antenna diversity must be added. After that add the multi hop function and figure out which energy harvesting element that is the optimum choice for the gateway. At last, optimize the size of the gateway box and make it streamlined.

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