Electromagnetic Effects on RFID Performancecradpdf.drdc-rddc.gc.ca/PDFS/unc261/p805066_A1b.pdf · Electromagnetic Effects on Radio Frequency ... an HF antenna, ... à laquelle il

Defence R&D Canada – Atlantic

Copy No. _____

Defence Research andDevelopment Canada

Recherche et développementpour la défense Canada

Electromagnetic Effects on Radio Frequency

Identification (RFID) Performance

DEFENCE DÉFENSE&

Contract Report

DRDC Atlantic CR 2011-070

July 2011

The scientific or technical validity of this Contract Report is entirely the responsibility of the Contractor and the contents do not necessarily have the approval or endorsement of Defence R&D Canada.

Ammar B. KoukiÉcole de technologie supérieure

Prepared by:École de technologie supérieureDepartment of Electrical Engineering1100, Notre-Dame Street West, Office A2954Montréal, QC Canada H3C 1K3 Contract Project Manager: Dr. Ammar Kouki, 514-396-8554PWGSC Contract Number: W7707-754828CSA: Dr. Nezih Mrad, Defence Scientist, 613-993-6443

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Electromagnetic Effects on Radio Frequency Identification (RFID) Performance

Ammar B. Kouki École de technologie supérieure Prepared By: École de technologie supérieure Department of Electrical Engineering 1100, Notre-Dame Street West, Office A2954 Montréal, QC Canada H3C 1K3

Contract Project Manager: Dr. Ammar Kouki, 514-396-8554 PWGSC Contract Number: W7707-754828 CSA: Dr. Nezih Mrad, Defence Scientist, 613-993-6443

The scientific or technical validity of this Contract Report is entirely the responsibility of the Contractor and the contents do not necessarily have the approval or endorsement of Defence R&D Canada.

Defence R&D Canada – Atlantic

Contract Report


July 2011

Signed by

Original signed by Dr. Nezih Mrad

Dr. Nezih Mrad

Contract Scientific Authority (CSA) / Defence Scientist

Approved by

Original signed by Mr. Ken McRae

Mr. Ken McRae

Head/Air Vehicles Research Section

Approved for release by

Original signed by Ron Kuwahara for

Dr. Calvin Hyatt

DRP/Chair

© Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2011

© Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale,

2011

DRDC Atlantic CR 2011-070 i

Abstract …

The goal of this study is to assess the impact of changes in the electromagnetic environment,

particularly changes related to the position and conductivity of surrounding materials, on selected

Radio Frequency Identification (RFID) systems’ performance. To this end, two different RFID

tag antennas have been modeled and their electromagnetic performance was simulated in the

presence of various materials situated at varying distances from the antenna. The first structure is

illustrative of UHF RFID antennas, i.e., an antenna operating around 1.8 GHz, while the second is

an HF antenna, operating in the 13 MHz band. Results show that both type of material used and

distance at which it is placed relative to the RFID tag affect significantly the antenna’s resonance

frequency. Further investigations are needed before a full understanding of the effect of adjacent

(host) material, tag antenna design, frequency and gain on the performance of an RFID system is

developed.

Résumé ….

La présente étude vise à évaluer l’incidence des changements dans un environnement

électromagnétique, notamment ceux associés à la position et à la conductivité des matériaux

environnants, sur les performances des systèmes d’identification par radiofréquence (RFID)

choisis. À cet effet, deux antennes d’étiquettes RFID différentes ont été modélisées, et leurs

performances électromagnétiques ont été simulées en présence de divers matériaux placés à des

distances variables de l’antenne. La première structure représente une antenne RFID UHF, qui

fonctionne à une fréquence d’environ 1,8 GHz, et la seconde structure est une antenne HF, qui

fonctionne dans la bande de 13 MHz. Selon les résultats, le type de matériau utilisé et la distance

à laquelle il est placé par rapport à l’étiquette RFID influent tous deux de façon importante sur la

fréquence de résonance de l’antenne. D’autres recherches sont nécessaires pour bien comprendre

l’effet d’un matériau adjacent (hôte), de la conception d’une antenne d’étiquette, de la fréquence

et du gain sur les performances d’un système RFID.

ii DRDC Atlantic CR 2011-070


DRDC Atlantic CR 2011-070 iii

Executive summary


Ammar B. Kouki; DRDC Atlantic CR 2011-070; Defence R&D Canada – Atlantic; July 2011.

Introduction: Wireless technology has seen significant development in recent years and its

potential for aerospace applications has captivated the interest of several industries. Recently,

particular interest has focused on the investigation of passive Radio Frequency Identification

(RFID) tags for such applications due to their attractive characteristics that includes reduced size

and weight and no power requirements.

To fully exploit RFID technology for SHM applications, several identified issues affecting the

RFID system performance need to be addressed. This work intends to address some of these

issues, particular issues affecting or impacting tag/reader performance when subjected to an

electromagnetic/conducting environment. Being able to predict RFID system performance,

mainly range, acquisition speed and data capacity in a complex environment, provides a cost

effective decision making on the implementation of the technology.

Results: Electromagnetic analyses illustrated the significant impact on an RFID system

performance when placing conducting material in the vicinity of an RFID tag. These observations

were common to both UHF tags as well as HF tags. For UHF tags, the impact on performance

was visible in terms of a change in the radiation pattern, antenna gain and the input impedance.

For HF tags, the impact was observed in terms of a wide range of variation in the resonance

frequency and input impedance. These results seem to corroborate the experience by some

researchers in the field.

Significance: With the increase in the use of Passive RFID tags, this effort provides advanced

understanding of the use of passive UHF and HF RFID tags near conductive and non-conductive

host materials.

Future plans: Conduct further investigations on the optimized material and tag distance to be

used to provide optimum performance of the RFID system. Additionally, further investigation,

including experimental validation, is required in order to establish clear guidelines on the usage of

RFID tags near conducting surfaces.

iv DRDC Atlantic CR 2011-070

Sommaire ...

Electromagnetic Effects on RFID Performance

Ammar B. Kouki. DRDC Atlantic CR 2011-070, R & D pour la défense Canada – Atlantique, juillet 2011.

Introduction : La technologie sans fil a connu un développement important ces dernières années,

et son potentiel pour les applications aérospatiales a capté l’intérêt de plusieurs industries. Depuis

peu, un intérêt particulier a porté sur l’étude des étiquettes d’identification par radiofréquence

(RFID) passives pour de telles applications en raison de leurs caractéristiques attrayantes qui

comprennent une taille et un poids réduits ainsi que des besoins nuls en électricité.

Pour exploiter pleinement la technologie RFID pour les applications SHM, on doit examiner

plusieurs facteurs qui influent sur les performances des systèmes RFID. Le présent document vise

à examiner certains d’entre eux, notamment ceux qui influent sur les performances des étiquettes

ou des lecteurs d’étiquettes placés dans un environnement électromagnétique ou conducteur. La

capacité de prédiction des performances des systèmes RFID, dont la portée, la vitesse

d’acquisition et la capacité des données dans un environnement complexe, permet une prise de

décision économique sur la mise en œuvre de la technologie.

Résultats : Des analyses électromagnétiques montrent que l’insertion d’un matériau conducteur à

proximité d’une étiquette RFID a un impact important sur les performances des systèmes RFID.

Ces observations s’appliquent à la fois aux étiquettes UHF et aux étiquettes HF. Pour les

étiquettes UHF, on a noté un impact sur les performances en raison de la modification du

diagramme de rayonnement, du gain de l’antenne et de l’impédance d’entrée. Pour les

étiquettes HF, on a observé un impact en raison de la grande variation de la fréquence de

résonance et de l’impédance d’entrée. Ces résultats semblent donc confirmer l’expérience réalisée

par quelques chercheurs dans le domaine.

Portée : En raison de la hausse d’utilisation des étiquettes RFID passives, cette étude permet une

compréhension approfondie de l’utilisation des étiquettes RFID UHF et HF passives à proximité

de matériaux hôtes conducteurs et non conducteurs.

Recherches futures : Il faut réaliser d’autres études sur le meilleur matériau et la meilleure

distance de l’étiquette à utiliser pour obtenir des performances optimales du système RFID. De

plus, d’autres études, y compris une validation expérimentale, sont nécessaires pour établir des

directives claires sur l’utilisation des étiquettes RFID à proximité de surfaces conductrices.

DRDC Atlantic CR 2011-070 v

Table of contents

Abstract … ....................................................................................................................................... i

Résumé …. ....................................................................................................................................... i

Executive summary ........................................................................................................................ iii

Sommaire ..... .................................................................................................................................. iv

Table of contents ............................................................................................................................. v

List of figures ................................................................................................................................ vii

List of tables ................................................................................................................................... xi

Acknowledgements ....................................................................................................................... xii

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

2 Methodology ............................................................................................................................. 2

3 Simulations ............................................................................................................................... 5

3.1 UHF antenna – reference structure ................................................................................ 5

3.2 UHF antenna – effect of ground plane .......................................................................... 7

3.3 HF antenna – reference structure ................................................................................... 9

3.4 HF antenna – effect of ground plane ........................................................................... 10

4 Conclusions ............................................................................................................................. 13

References . ................................................................................................................................... 14

Annex A .. UHF Antenna Results .................................................................................................. 15

A.1 Perfect Ground Plane (H = 25 mils) ............................................................................ 15

A.2 Perfect Ground Plane (H = 50 mils) ............................................................................ 16

A.3 Perfect Ground Plane (H = 100 mils) .......................................................................... 17

A.4 Titanium ground plane (H = 25 mils) .......................................................................... 18

A.5 Titanium ground plane (H = 50 mils) .......................................................................... 19

A.6 Titanium ground plane (H = 100 mils) ........................................................................ 20

A.7 Aluminum 2024 – T3 (H = 25 mils) ............................................................................ 21


A.9 Aluminum 2024 – T3 (H = 100 mils) .......................................................................... 23



A.12 Aluminum 7075 – T6 (H = 100 mils) .......................................................................... 26

A.13 Ferromagnetic material (Iron) (H = 25 mils) ............................................................... 27

A.14 Ferromagnetic material (Iron) (H = 50 mils) ............................................................... 28

A.15 Ferromagnetic material (Iron) (H = 100 mils) ............................................................. 29

A.16 Graphite (H = 25 mils) ................................................................................................ 30

A.17 Graphite (H = 50 mils) ................................................................................................ 31

vi DRDC Atlantic CR 2011-070

A.18 Graphite (H = 100 mils) .............................................................................................. 32

A.19 Thermoplastics-PVC (H = 25 mils) ............................................................................. 33

A.20 Thermoplastics-PVC (H = 50 mils) ............................................................................. 34

A.21 Thermoplastics-PVC (H = 100 mils) ........................................................................... 35

A.22 Wood (H = 25 mils) ..................................................................................................... 36

A.23 Wood (H = 50 mils) ..................................................................................................... 37

A.24 Wood (H = 100 mils) ................................................................................................... 38

A.25 Cardboards and paper (H = 25 mils) ........................................................................... 39

A.26 Cardboards and paper (H = 50 mils) ........................................................................... 40

A.27 Cardboards and paper (H = 100 mils) ......................................................................... 41

Annex B ... HF Antenna Results ..................................................................................................... 43

DRDC Atlantic CR 2011-070 vii

List of figures

Figure 1: Layout of a typical RFID antenna. ................................................................................... 2

Figure 2: Typical definition of a substrate in Momentum. .............................................................. 3

Figure 3: Geometry of the UHF dipole antenna. ............................................................................. 5

Figure 4: Input reflection coefficient of the UHF antenna in the absence of a ground plane. ......... 6

Figure 5: 3D model, radiation pattern and antenna parameters of UFH antenna @ 1.85 GHz in

the absence of a ground plane. ...................................................................................... 7

Figure 6: Detailed view of the 3D model of the UHF antenna with a material ground plane. ........ 8

Figure 7: Six turns planar inductor in free space. .......................................................................... 10

Figure 8: Magnitude and imaginary part of the coil’s input impedance. Resonance is

measured at maxim magnitude of the input impedance. ............................................. 10

Figure 9: Detailed view of the 3D model of the HF antenna with a material ground plane. ......... 11

Figure 10: Input reflection coefficient of the UHF antenna with a PEC ground plane at 25

mils. ............................................................................................................................. 15

Figure 11: 3D model, radiation pattern and antenna parameters of UFH antenna @ 1.85 GHz

with a PEC ground plane at 25 mils. ........................................................................... 15


mils. ............................................................................................................................. 16


with a PEC ground plane at 50 mils. ........................................................................... 16


mils. ............................................................................................................................. 17


with a PEC ground plane at 100 mils. ......................................................................... 17

Figure 16: Input reflection coefficient of the UHF antenna with Titanium ground plane at 25

mils. ............................................................................................................................. 18


with a Titanium ground plane at 25 mils. .................................................................... 18


mils. ............................................................................................................................. 19


with a Titanium ground plane at 50 mils. .................................................................... 19


mils. ............................................................................................................................. 20


with a Titanium ground plane at 100 mils. .................................................................. 20

viii DRDC Atlantic CR 2011-070

Figure 22: Input reflection coefficient of the UHF antenna with Aluminum 2024 – T3 ground

plane at 25 mils. .......................................................................................................... 21


with Aluminum 2024 – T3 ground plane at 25 mils. .................................................. 21


plane at 50 mils. .......................................................................................................... 22




plane at 100 mils. ........................................................................................................ 23


with Aluminum 2024 – T3 ground plane at 100 mils. ................................................ 23


plane at 25 mils. .......................................................................................................... 24




plane at 50 mils. .......................................................................................................... 25




plane at 100 mils. ........................................................................................................ 26


with Aluminum 7075 – T6 ground plane at 100 mils. ................................................ 26

Figure 34: Input reflection coefficient of the UHF antenna with Iron ground plane at 25 mils. ... 27


with Iron ground plane at 25 mils. .............................................................................. 27

Figure 36: Input reflection coefficient of the UHF antenna with Iron ground plane at 50 mils. ... 28


with Iron ground plane at 50 mils. .............................................................................. 28

Figure 38: Input reflection coefficient of the UHF antenna with Iron ground plane at 100 mils. . 29


with Iron ground plane at 100 mils. ............................................................................ 29

Figure 40: Input reflection coefficient of the UHF antenna with Graphite ground plane at 25

mils. ............................................................................................................................. 30


with Graphite ground plane at 25 mils. ....................................................................... 30

DRDC Atlantic CR 2011-070 ix


mils. ............................................................................................................................. 31


with Graphite ground plane at 50 mils. ....................................................................... 31


mils. ............................................................................................................................. 32


with Graphite ground plane at 100 mils. ..................................................................... 32

Figure 46: Input reflection coefficient of the UHF antenna with Thermoplastics ground plane

at 25 mils. .................................................................................................................... 33


with Thermoplastics ground plane at 25 mils. ............................................................ 33


at 50 mils. .................................................................................................................... 34


with Thermoplastics ground plane at 50 mils. ............................................................ 34


at 100 mils. .................................................................................................................. 35


with Thermoplastics ground plane at 100 mils. .......................................................... 35

Figure 52: Input reflection coefficient of the UHF antenna with Wood ground plane at 25

mils. ............................................................................................................................. 36


with Wood ground plane at 25 mils. ........................................................................... 36


mils. ............................................................................................................................. 37


with Wood ground plane at 50 mils. ........................................................................... 37


mils. ............................................................................................................................. 38


with Wood ground plane at 100 mils. ......................................................................... 38

Figure 58: Input reflection coefficient of the UHF antenna with paper ground plane at 25

mils. ............................................................................................................................. 39


with paper ground plane at 25 mils. ............................................................................ 39


mils. ............................................................................................................................. 40

x DRDC Atlantic CR 2011-070


with paper ground plane at 50 mils. ............................................................................ 40


mils. ............................................................................................................................. 41


with paper ground plane at 100 mils. .......................................................................... 41

Figure 64: PEC plate at 25 mils. .................................................................................................... 43

Figure 65: PEC plate at 50 mils. .................................................................................................... 43

Figure 66: PEC plate at 100 mils. .................................................................................................. 44

Figure 67: Titanium at 25 mils. ..................................................................................................... 44

Figure 68: Titanium at 50 mils. ..................................................................................................... 45

Figure 69: Titanium at 100 mils. ................................................................................................... 45

Figure 70: Aluminum 2024 – T3 at 25 mils. ................................................................................. 46


Figure 72: Aluminum 2024 – T3 at 100 mils. ............................................................................... 47



Figure 75: Aluminum 7075 – T6 at 100 mils. ............................................................................... 48

Figure 76: Iron at 25 mils. ............................................................................................................. 49

Figure 77: Iron at 50 mils. ............................................................................................................. 49

Figure 78: Iron at 100 mils. ........................................................................................................... 50

Figure 79: Graphite at 25 mils. ...................................................................................................... 50

Figure 80: Graphite at 10 mils. ...................................................................................................... 51

Figure 81: Graphite at 100 mils. .................................................................................................... 51

Figure 82: Thermoplastics at 25 mils. ........................................................................................... 52

Figure 83: Thermoplastics at 50 mils. ........................................................................................... 52

Figure 84: Thermoplastics at 100 mils. ......................................................................................... 53

Figure 85: Wood at 25 mils. .......................................................................................................... 53

Figure 86: Wood at 50 mils. .......................................................................................................... 54

Figure 87: Wood at 100 mils. ........................................................................................................ 54

Figure 88: Paper at 25 mils. ........................................................................................................... 55

Figure 89: Paper at 50 mils. ........................................................................................................... 55

Figure 90: Paper at 100 mils. ......................................................................................................... 56

DRDC Atlantic CR 2011-070 xi

List of tables

Table 1: Characteristics of the tested materials. .............................................................................. 5

Table 2: Resonance frequency and gain of the UHF antenna vs. studied material and position. .... 9

Table 3: Resonance frequency of the HF antenna vs. studied material and position. ................... 12

xii DRDC Atlantic CR 2011-070

Acknowledgements

This work was carried out with the help of Dr. Mehdi Si Moussa and Mr. Mohamed Jemai at

LACIME lab at École de technologie supérieure.

DRDC Atlantic CR 2011-070 1

1 Introduction

Wireless technology has seen significant development in recent years and its potential for

aerospace applications has captivated the interest of several industries. Recently, particular

interest has focused on the investigation of passive Radio Frequency Identification (RFID) tags

for such applications due to their attractive characteristics that include reduced size and weight

and no power requirements.

To fully exploit RFID technology for SHM applications, several identified issues affecting the

RFID system performance need to be addressed. This project intends to address some of these

issues, particular issues affecting or impacting tag/reader performance when subjected to an

electromagnetic/conducting environment. More specifically, we investigate the impact of the

proximity of an RFID tag to a conducting surface having varying conductivity and situated at a

variable distance from the tag itself.

2 DRDC Atlantic CR 2011-070

2 Methodology

To assess the impact of the presence of a large conducting surface in the vicinity of an RFID tag,

electromagnetic simulation of the tag (its antenna) is required. This in turn requires the use of a

tool capable of simulating the antenna structure in the presence of the conducting surface. There

exists a wide variety of electromagnetic simulation tools capable of simulating various

three-dimensional structures with varying degrees of complexity in model construction as well as

varying levels of accuracy and the required computational resources (both in memory and CPU

time usage). A review of these tools and the underlining techniques is beyond the scope of the

present project; however, given that the RFID tag antennas are generally planar, and assuming

that the near-by metallic surface are locally planar, the problem can be accurately modeled and

simulated using the so-called planar electromagnetic field solvers. These solvers offer high

accuracy with low modeling complexity and reasonable simulation times. In this particular study,

we used two such tools:

Sonnet (from Sonnet Software) [1], and

Momentum, part of the Advanced Design System (ADS, from Agilent Inc.) [2].

The first step consists of model construction. The printed antenna models are defined by a layout

drawing of the antenna metallisation surface; see Figure 1, coupled with a definition of the

substrate on which the antenna is made. The substrate can be made of a single or multiple layers

each having its proper electromagnetic characteristics and its own thickness. The electromagnetic

characteristics of interest are the permittivity () and conductivity (), i.e., all materials are

considered non-magnetic, therefore, the permeability is fixed to o = 410-7

Henry/meter for all

materials. The permittivity is expressed as = ro where r is the relative permittivity and o is

the free space permittivity (o = 8.854x10-12

Farad/meter). The conductivity, , is measured in

Siemens/meter (S/m) or Siemens/centimeter (S/cm) and may be sometimes replaced by the

resistivity, = 1/, measured in .m (Ohm meter) or n.m (nano-Ohm meter). Figure 2

illustrates a typical definition of a substrate stack in the Momentum consisting of open air on top,

a commercial dielectric substrate on which the antenna metallization is printed, another air layer

sandwiched between the bottom of the dielectric substrate and a ground plane.

Figure 1: Layout of a typical RFID antenna.


Figure 2: Typical definition of a substrate in Momentum.

Once the model is constructed and the substrate parameters are defined, electromagnetic solvers

are used to solve for the resultant electric current density distribution on the antenna metallisation

over a specified frequency range as well as the input impedance of the antenna. The results take

into account the entire electromagnetic environment in terms of geometry as well as material

properties. The input impedance is important for the matching of the antenna to the RFID chip,

while the current distribution is then used to compute the radiated field, and ultimately the

antenna’s spatial properties, such as radiation pattern and gain. It should be noted that the

simulators compute the antenna’s input reflection coefficient, S11, from which the antenna input

impedance, Zin, can be computed as:

1 11

1 11

SZin Zo

S

(1)

where Zo is the reference impedance, i.e., the impedance of the measurement system, typically

50 .


The present study is limited to the above mentioned simulations and parametric results are

obtained for various configurations, as will be presented in the following sections. The impact on

the performance of the electromagnetic environment and changes in antenna parameters can be

taken into account by using the well-established link budget analysis technique using

Friis’s formula, which, for unobstructed propagation between transmitter and receiver, is

given by [3]:

(2)

where Pt and Pr are the transmitted and received powers, respectively, Gt and Gr are the antenna

gain of the transmitting and receiving antennas, respectively, λ is the wavelength, and R is the

distance separating the transmitter from the receiver. When applied to reading an RFID tag, Pt

would be the power re-radiated by the tag upon interrogation and Pr the power received by the tag

reader. Therefore, the higher the antenna gains, the greater the read range. Since the gains appear

as a product one can compensate for the lack of antenna gain on one side (tag or reader) by

increasing the antenna gain on the other side (reader or tag). Additionally, one can easily see from

this equation that in order to keep the same power levels while doubling the distance, the product

of antenna gains has to increase by a factor of 4 (or 6 dB). Conversely, a 6 dB reduction in

antenna gain will lead to halving the distance between transmitter and receiver for the same

power levels.


3 Simulations

Several simulations were carried out whereby a conducting surface of variable conductivity and

located at a varying distance from an RFID tag antenna was considered. Two distinct antenna

structures were considered. The first operates at ultra-high frequency (UFH dipole antenna at

~1.8 GHz) while the second operates at high frequency (HF coil antenna at ~13 MHz). For both

cases, the properties of the material placed in the vicinity of the tag were varied according to the

list in Table 1. For each material, three geometries were simulated, namely having the material at

25, 50 and 100 mils away from the antenna.

Table 1: Characteristics of the tested materials.

Material Resistivity or Conductivity

Titanium 420 nΩ. m

Aluminum 2024-T3 57.4 nΩ. m

Aluminum 7075-T6 52 nΩ. m

Ferromagnetic material (Iron) 96.1 nΩ. m

Graphite 7.837 µΩ. m

Thermoplastics (εr = 5) 10-8 S/cm

Wood (εr = 4) 3x1017 Ω. m

Cardboard and Paper (εr = 2.5) Negligible conductivity

3.1 UHF antenna – reference structure Figure 3 shows the geometry of a popular RFID tag antenna dimensioned for operation at

1.8 GHz. The geometric parameters of the model are also given. The antenna was simulated on a

standard commercial substrate, Rogers 3006 with a substrate thickness of 25 mils and εr = 6.15. In

the absence of materials around this antenna, its resonant frequency was found to be 1.85 GHz

with a gain of 3.89 dB. Figure 4 shows the simulated S11 where it can be seen that antenna offers

best input matching at 1.85 GHz. Figure 5 shows a 3D view of the model, the obtained radiation

pattern and antenna parameters with a gain of 3.89 dB.

Figure 3: Geometry of the UHF dipole antenna.


Figure 4: Input reflection coefficient of the UHF antenna in the absence of a ground plane.


Figure 5: 3D model, radiation pattern and antenna parameters of UFH antenna

@ 1.85 GHz in the absence of a ground plane.

3.2 UHF antenna – effect of ground plane In the following, we added a ground plane underneath the antenna which will have varying

material properties. The thickness of the ground plane is fixed at 18 µm, which many times the

skin depth at 1.85GHz. The distance, H, between the antenna and the material is varied from 25 to

50 to 100 mils. Figure 6 shows a detailed 3D view of the simulated structure with the antenna on

the top and the tested material underneath the substrate.


Figure 6: Detailed view of the 3D model of the UHF antenna with a material ground plane.

For each combination of ground plane material and distance, an electromagnetic simulation was

carried out and the antenna’s input reflection coefficient, radiation pattern and gain were

computed. The detailed results for each of these scenarios are presented in Annex A. Table 2

presents a summary of these results where the impact on the input matching, seen by a change in

the resonance frequency of the antenna, and on gain is indicated. It is clear that the presence of a

material in the vicinity of the antenna can have significant impact on its performance both by

shifting the resonance frequency and changing the gain. The impact of these variations can be

estimated using equation (2).


Table 2: Resonance frequency and gain of the UHF antenna vs. studied material and position.

Materials Resonance frequency (GHz) Gain (dB)

No Ground plane 1.85 3.89

Ground plane H = 25 mil 1.5 -10.32

H = 50 mil 1.1 -3.79

H = 100 mil 0.75 8.62

Titanium H = 25 mil 1.5 -10.69

H = 50 mil 1.1 9.13

H = 100 mil 0.75 8.53

Aluminum 2024 – T3 H = 25 mil 1.5 -10.42

H = 50 mil 1.1 -0.20

H = 100 mil 0.75 8.53

Aluminum 7075 – T6 H = 25 mil 1.5 -10.41

H = 50 mil 1.1 -0.46

H = 100 mil 0.75 8.53

Iron H = 25 mil 1.5 -10.47

H = 50 mil 1.1 1.82

H = 100 mil 0.75 8.53

Graphite H = 25 mil 1.5 -11.79

H = 50 mil 1.1 9.12

H = 100 mil 0.75 8.52

Thermoplastics (PVC) H = 25 mil 1.6 8.21

H = 50 mil 1.6 8.57

H = 100 mil 1.65 8.60

Wood H = 25 mil 1.6 8.19

H = 50 mil 1.6 8.26

H = 100 mil 2.2 10.57

Paper H = 25 mil 2.15 9.32

H = 50 mil 2.2 9.37

H = 100 mil 2.8 8.24

3.3 HF antenna – reference structure Figure 7 shows the model of the HF RFID tag antenna which consists of an inductor. It is a planar

coil with six turns, each 0.5 mm wide and separated by 0.5 mm. First, we simulated the coil in

free space, i.e., no nearby obstacle or material. The resonance frequency was found to be at

13.6 MHz as shown in Figure 8. This constitutes the baseline or reference configuration to which

other configurations are compared. It should be noted that, unlike the UHF antenna which

operates through radiation, the HF coil works through magnetic coupling. Hence, the concept of

antenna gain is not well defined since the tag and reader will operate in the near field of each

other.


78 mm

41 mm

Six turns

inductor

Figure 7: Six turns planar inductor in free space.

Figure 8: Magnitude and imaginary part of the coil’s input impedance.

Resonance is measured at maxim magnitude of the input impedance.

3.4 HF antenna – effect of ground plane

In the following, we added a ground plane underneath the antenna which will have varying

material properties. The distance, H, between the antenna and the material is varied from 25 to 50

to 100 mils. Figure 9 shows a detailed 3D view of the simulated structure with the antenna on the

top and the tested material underneath the substrate.


Inductor

PEC plate

Substrate

height

Free space

distance H

Figure 9: Detailed view of the 3D model of the HF antenna with a material ground plane.

For each combination of ground plane material and distance, an electromagnetic simulation was

carried out and the antenna’s input impedance and resonance frequency were computed. The

detailed results for each of these scenarios are presented in Annex B. Table 3 presents a summary

of these results where the impact on the resonance frequency of the antenna can be seen. It is

clear that the presence of a material in the vicinity of the antenna can have significant impact on

its performance. The shift in resonance frequency means that less field strength will be radiated

away from the antenna at the desired operation frequency of 13.56MHz. It should be noted that

near field calculation, while possible, are extremely time consuming. Given the large number of

combinations, the results are limited to input impedance and resonance frequency.


Table 3: Resonance frequency of the HF antenna vs. studied material and position.

Materials Distance (mils) Resonance frequency (MHz)

No ground (reference) 13.6

Perfect Conductor (PEC) 25 8.1

50 8.6

100 9.4

Titanium 25 39

50 31

100 24

Aluminum 2024-T3 25 39

50 39

100 24

Aluminum 7075-T6 25 39

50 31

100 24

Iron 25 30

50 23

100 24

Graphite 25 24

50 22.5

100 20

Thermoplastics 25 8

50 9

100 9.5

wood 25 8

50 8.5

100 9.5

Paper 25 9

50 9

100 9


4 Conclusions

The electromagnetic analyses that were carried out clearly demonstrate that there is a significant

impact associated with placing conducting material in the vicinity of an RFID tag. This

observation holds for both UHF tags as well as HF tags. For UHF tags, the impact on

performance was visible in terms of a change in the radiation pattern, antenna gain and the input

impedance. For HF tags, the impact was observed in terms of a wide range of variation in the

resonance frequency and input impedance. These results seem to corroborate the experience by

some researchers in the field. However, further investigation, including experimental validation,

is required in order to establish clear guidelines on the usage of RFID tags near conducting

surfaces. For UHF tags, where antenna gain is well defined and easily computed, the potential

impact on the read range of tags can be estimated using the results of Table 2 and equation (2).

Furthermore, this work has to be considered a preliminary study that can be elaborated further.

Given the time consuming nature of the work, we limited the study to relatively small distances

between the tag and metallic surface. It would be interesting to carry this study further to include

larger separating distances. Additionally, the UHF antenna model used was one that was readily

available in the literature. It would be equally interesting to study the behaviour of an antenna in

the more popular 915 MHz band.


References .

[1] Sonnet Inc., “Sonnet Software”, 2010 Ed. Syracuse, NY, 2010.

[2] Agilent Technologies "Advanced Design System," 2009C Ed. Palo Alto California, 2009.

[3] T. Rappaport, Wireless Communications: Principles and Practice, 2nd Ed., New York:

Prentice Hall, 2001.


Annex A UHF Antenna Results

A.1 Perfect Ground Plane (H = 25 mils)

Perfect ground plane refers to a perfectly conducting material ( ) and Red S11 curves

represent the reference structure – without ground plane

Figure 10: Input reflection coefficient of the UHF antenna with a PEC ground plane at 25 mils.


@ 1.85 GHz with a PEC ground plane at 25 mils.












A.4 Titanium ground plane (H = 25 mils)

Figure 16: Input reflection coefficient of the UHF antenna

with Titanium ground plane at 25 mils.


@ 1.85 GHz with a Titanium ground plane at 25 mils.














A.7 Aluminum 2024 – T3 (H = 25 mils)


with Aluminum 2024 – T3 ground plane at 25 mils.


@ 1.85 GHz with Aluminum 2024 – T3 ground plane at 25 mils.
































A.13 Ferromagnetic material (Iron) (H = 25 mils)

Figure 34: Input reflection coefficient of the UHF antenna with Iron ground plane at 25 mils.


@ 1.85 GHz with Iron ground plane at 25 mils.












A.16 Graphite (H = 25 mils)


with Graphite ground plane at 25 mils.


@ 1.85 GHz with Graphite ground plane at 25 mils.














A.19 Thermoplastics-PVC (H = 25 mils)


with Thermoplastics ground plane at 25 mils.


@ 1.85 GHz with Thermoplastics ground plane at 25 mils.














A.22 Wood (H = 25 mils)

Figure 52: Input reflection coefficient of the UHF antenna with Wood ground plane at 25 mils.


@ 1.85 GHz with Wood ground plane at 25 mils.












A.25 Cardboards and paper (H = 25 mils)

Figure 58: Input reflection coefficient of the UHF antenna with paper ground plane at 25 mils.


@ 1.85 GHz with paper ground plane at 25 mils.














Annex B HF Antenna Results

Figure 64: PEC plate at 25 mils.




Figure 67: Titanium at 25 mils.





Figure 70: Aluminum 2024 – T3 at 25 mils.









Figure 76: Iron at 25 mils.




Figure 79: Graphite at 25 mils.





Figure 82: Thermoplastics at 25 mils.




Figure 85: Wood at 25 mils.





Figure 88: Paper at 25 mils.




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École de technologie supérieure Department of Electrical Engineering 1100, Notre-Dame Street West, Office A2954 Montréal, QC Canada H3C 1K3

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Ammar B. Kouki

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July 2011

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here abstracts in both official languages unless the text is bilingual.)

The goal of this study is to assess the impact of changes in the electromagnetic environment,

particularly changes related to the position and conductivity of surrounding materials, on

selected Radio Frequency Identification (RFID) systems’ performance. To this end, two

different RFID tag antennas have been modeled and their electromagnetic performance was

simulated in the presence of various materials situated at varying distances from the antenna.

The first structure is illustrative of UHF RFID antennas, i.e., an antenna operating around

1.8 GHz, while the second is an HF antenna, operating in the 13 MHz band. Results show that

both type of material used and distance at which it is placed relative to the RFID tag affect

significantly the antenna’s resonance frequency. Further investigations are needed before a full

understanding of the effect of adjacent (host) material, tag antenna design, frequency and gain

on the performance of an RFID system is developed.

La présente étude vise à évaluer l’incidence des changements dans un environnement

électromagnétique, notamment ceux associés à la position et à la conductivité des matériaux

environnants, sur les performances des systèmes d’identification par radiofréquence (RFID)

choisis. À cet effet, deux antennes d’étiquettes RFID différentes ont été modélisées, et leurs

performances électromagnétiques ont été simulées en présence de divers matériaux placés à des

distances variables de l’antenne. La première structure représente une antenne RFID UHF, qui

fonctionne à une fréquence d’environ 1,8 GHz, et la seconde structure est une antenne HF, qui

fonctionne dans la bande de 13 MHz. Selon les résultats, le type de matériau utilisé et la

distance à laquelle il est placé par rapport à l’étiquette RFID influent tous deux de façon

importante sur la fréquence de résonance de l’antenne. D’autres recherches sont nécessaires

pour bien comprendre l’effet d’un matériau adjacent (hôte), de la conception d’une antenne

d’étiquette, de la fréquence et du gain sur les performances d’un système RFID.

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designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a

published thesaurus, e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus identified. If it is not possible to select

indexing terms which are Unclassified, the classification of each should be indicated as with the title.)

RFID; electromagnetic; power; modeling; simulation


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Electromagnetic Effects on RFID Performancecradpdf.drdc-rddc.gc.ca/PDFS/unc261/p805066_A1b.pdf · Electromagnetic Effects on Radio Frequency ... an HF antenna, ... à laquelle il