NORTHEASTERN UNIVERSITY

Graduate School of Engineering

Thesis Title: Miniaturized Antennas on Novel Magneto(di)electric Substrates

Author: Andrew Daigle

Department: Electrical and Computer Engineering

Approved for Thesis Requirement for Master of Science Degree

________________________________ ________________ Thesis Advisor: Date: ________________________________ ________________ Thesis Reader: Date: ________________________________ ________________ Thesis Reader: Date:

________________________________ ________________ Department Chair: Date:

Graduate School Notified of Acceptance:

________________________________ ________________ Department Chair: Date:

NORTHEASTERN UNIVERSITY

Graduate School of Engineering

Thesis Title: Miniaturized Antennas on Novel Magneto(di)electric Substrates

Author: Andrew Daigle

Department: Electrical and Computer Engineering

Approved for Thesis Requirement for Master of Science Degree

________________________________ ________________ Thesis Advisor: Date: ________________________________ ________________ Thesis Reader: Date: ________________________________ ________________ Thesis Reader: Date:

________________________________ ________________ Department Chair: Date:

Graduate School Notified of Acceptance:

________________________________ ________________ Department Chair: Date:

MINIATURIZED ANTENNAS ON NOVEL MAGNETO(DI)ELECTRIC SUBSTRATES

A Thesis Presented

by

Andrew Daigle

to

The Department of Electrical and Computer Engineering

in partial fulfillment of the requirements

for the degree of

Master of Science

in

Electrical Engineering

Northeastern University

Boston, MA

Fall 2008

1

Acknowledgments

I would like to take my advising Professor, Professor Nian Sun for his constant

help and support during the time spent working on this project. His novel approach to the

problems I encountered was invaluable to my experience. Also, I would like to thank him

for allowing me to do my research in his group here at Northeastern University.

I would also like to acknowledge and thank the other members of the Center for

Microwaves, Magnetic Materials and Integrated Circuits. In particular I would like to

thank Guomin Yang, for his invaluable help with HFSS, and the theory behind these

antennas. Without the friendly help from the lab personnel and the scholarly and

constructive work environment within the center this process would have taken much

longer.

Many thanks go out to the friends that I have made over the years, here at

Northeastern, and in Boston. Thank you for listening to me talk about antennas and

simulations all the time, and for putting up for me while I studied for exams and wrote

my thesis.

Finally a special thanks goes out to my family, who have always been behind me

whatever my goal at the time happened to be. Their love and support has helped me to

achieve my goals, and I will forever appreciate it. I love you all.

2

Abstract

In this paper we have examined the properties of high permittivity

magneto(di)electric materials, and their uses in antenna applications. In particular, we

have used these materials to counteract the narrowing of bandwidth, and the impedance

mismatch due to the use of high permittivity ceramics as antenna substrates by

themselves [1-6]. We have shown on a variety of different antennae how utilizing

magnetic films can help to improve the antennas performance be it through bandwidth,

directivity, or gain. These results include HFSS simulations, as well as fully

experimentally tested and fabricated antennae.

To achieve these substrates, we have also examined the properties and fabrication

process of a wide range of materials. These materials include metallic thin films such as

(Fe60Co40)85B15 and (Fe1-xGax)85B15 [7-10] as well as and spin spray (Ni1-xCox)Fe2O4 and

(Ni1-xZnx)Fe2O4 ferrite materials [11]. In putting together these results we performed

many different measurements on these films in regards to their thermal stability,

magnetization and optimal thicknesses. We also examine the high permittivity ceramics

themselves in detail to determine the conditions for their fabrication, and their

optimization [12-14].

Together all of these materials have been used to observe miniaturization and

significant enhancement of our antenna designs [15-18].

3

Contents

Acknowledgments________________________________________________________1

Abstract________________________________________________________________2

Contents _______________________________________________________________3

List of Figures___________________________________________________________5

List of Tables ___________________________________________________________8

1. Relevant Background Information _________________________________________9

1. Introduction to Thesis_______________________________________________9

1.1. Reason for Study_______________________________________9

1.2. Outline of Thesis _____________________________________10

2. Patch Antenna Design______________________________________________14

2.1. Previous Design Models________________________________14

2.2. Investigation Into Changing Permittivity ___________________18

2.3. High K Material Investigation ___________________________25

2. (Fe60Co40)85B15 Films and Their Uses in Antennas ___________________________32

1. Introduction to (Fe1-xCox)85B15 Films _________________________________32

1.1. Background__________________________________________32

1.2. Fabrication and Testing ________________________________34

3. Antennas with Integrated Ferrite Films ____________________________________40

1. Introduction to Spin Spray Ferrites ___________________________________40

1.1. Background__________________________________________40

1.2. The Fabrication Process ________________________________42

1.3. Results Achieved with the Northeastern Spin Spray System____44

4

2. Theory behind Spin Spray Ferrites ___________________________________46

2.1. Theory: Increased Permeability leading to Higher FMR _______46

2.2. Theory: Permeability and Better Antenna Performance________51

3. Incorporating Films into Antenna Designs______________________________57

3.1. Single Films in Antennas _______________________________57

3.2. Multiple Films in Antennas _____________________________62

3.3. Conclusions__________________________________________67

4. Novel Antenna Designs ________________________________________________69

1. Loop Antennas ___________________________________________________69

1.1. Introduction to Loop Antennas___________________________69

1.2. Results______________________________________________70

1.3. Conclusions__________________________________________71

5. High Permittivity Miniaturized GPS Antennae using Ferrite Films ______________72

1. Thin Linear GPS Antenna___________________________________________73

1.1. Introduction__________________________________________73

1.2. Results______________________________________________75

1.3. Conclusions__________________________________________77

2. Thick Linear GPS Antenna__________________________________________78

2.1. Introduction__________________________________________78

2.2. Results______________________________________________79

2.3. Conclusions__________________________________________87

6: Thesis Conclusions____________________________________________________88

7: References __________________________________________________________90

5

List of Figures

Fig. 1 Overview of Patch Antenna [19] _____________________________________15

Fig. 2 Equation for Effective Epsilon of Patch Antenna [19] ____________________15

Fig. 3 Relation between the Permittivity and Effective Permittivity [19] ___________16

Fig. 4 Equations for the L and W of Patch Antennas based on the TLM [19] ________17

Fig. 5 Effect of Miniaturization of Resonant Frequency [19] ____________________18

Fig. 6 Geometry of the Rectangular Patch Antenna ____________________________20

Fig. 7 Radiation Pattern of Far Field for Rectangular Patch Antenna ______________21

Fig. 8 E-Plane and H-Plane Equations [19] __________________________________22

Fig. 9 Effect of Changing Permittivity on the Return Loss of Different Antennas ____23

Fig. 10 Definition of Fringe Factor [19] _____________________________________24

Fig. 11 Permittivity of (Ba1-xSrx)TiO3 Ceramics with Varying Ba Content [12-13] ___27

Fig. 12 Permittivity of BaSrTiO Ceramics with varying MgO content _____________28

Fig. 13 XRD of (Ba1-xSrx)TiO3 Ceramics with varying MgO content _______________29

Fig. 14 FTU results of (Ba1-xSrx)TiO3 Ceramics with MgO content of 60%__________30

Fig. 15 Quality Factor of Ca Doped Titanates [14] _____________________________31

Fig. 16 Saturation Magnetization vs. Temp ___________________________________33

Fig. 17 Saturation Magnetization vs. Hc (G) _________________________________33

Fig. 18 Saturation Magnetization vs. Hk (G) __________________________________34

Fig. 19 (Fe60Co40)85B15 Antenna #1 [15] ____________________________________36

Fig. 20 (Fe60Co40)85B15 Antenna #1 [15] ____________________________________37

Fig. 21 (Fe60Co40)85B15 Antenna Radiation Pattern [15]_________________________38

Fig. 22 Northeastern Anechoic Chamber Measurement System ___________________39

6

Fig. 23 Permittivity and Permeability of (Ni1-xZnx)Fe2O4 Ferrite [11] ______________41

Fig. 24 XRD Data of (Ni1-xZnx)Fe2O4 Ferrite [11] _____________________________41

Fig. 25 Depiction of the Spin Spray Plating Process [26] ________________________43

Fig. 26 Depiction of the Spin Spray Plating Process [26] ________________________43

Fig. 27 VSM data of (Ni1-xCox)Fe2O4 Ferrite _________________________________44

Fig. 28 AFM Data for (Ni1-xCox)Fe2O4 Ferrite ________________________________45

Fig. 29 SEM Data for (Ni1-xCox)Fe2O4 Ferrite ________________________________45

Fig. 30 Depiction of 2.1 GHz Patch Antenna [16] _____________________________52

Fig. 31 Alumina to Free Space Boundary Conditions [16] _______________________54

Fig. 32 Boundary Conditions of Ferrite and Alumina [16] _______________________55

Fig. 33 Antenna No. 1 [16] _______________________________________________58

Fig. 34 Antenna No. 2 [16] _______________________________________________58

Fig. 35 Antenna No. 3 [16] _______________________________________________58

Fig. 36 Antenna No. 4 [16] _______________________________________________58

Fig. 37 Measured Return Loss of the Four Antennas [16] _______________________59

Fig. 38 E plane Radiation Patterns [16] ______________________________________60

Fig. 39 Gains of Antennae at Different Elevation Angles [16] ____________________61

Fig. 40 Antenna #1 with one Ferrite Layer on top [16] __________________________63

Fig. 41 Antenna #1 with two Ferrite Layers on top [16] _________________________63

Fig. 42 Antenna #1 with three Ferrite Layers on top [16] ________________________63

Fig. 43 Return Loss of Loaded Non-magnetic Antenna [16] _____________________64

Fig. 44 H-Plane of Loaded Non-magnetic Antenna [16] _________________________65

Fig. 45 E E-Plane of Loaded Non-magnetic Antenna [16] _______________________66

7

Fig. 46 Antenna Gain at different Elevation Angles [16] ________________________66

Fig. 47 Loop Antenna Design [18] _________________________________________69

Fig. 48 Ferrite Placement on Loop Antenna [18] ______________________________71

Fig. 49 Return Loss of Loop Antenna [18] ___________________________________72

Fig. 50 Geometry of the Rectangular Patch Antenna [17] ________________________74

Fig. 51 Antenna with Magnetic Film above the Rectangular Patch [17] _____________75

Fig. 52 Simulated Return Loss against Frequency for the Five Different Cases [17] ___76

Fig. 53 Simulated radiation patterns of X-Z plane [17] __________________________76

Fig. 54 Simulated radiation patterns of Y-Z plane [17] __________________________77

Fig. 55 Design of Thick GPS Antenna ______________________________________79

Fig. 56 Return Loss of Thick GPS Antenna Design ____________________________80

Fig. 57 Impedance Measurement of Thick GPS Antenna ________________________80

Fig. 58 Electric Field on the Patch for the Thick GPS Case ______________________81

Fig. 59 Radiation Pattern of Thick GPS Antenna ______________________________82

Fig. 60 Miniaturization effect of Ferrite Placement on Thick GPS Antenna__________83

Fig. 61 Return Loss of Antenna vs. Ferrite Thickness __________________________83

Fig. 62 Gain and Directivity of Thick Antenna with Ferrite Films _________________85

Fig. 63 Return Loss of Thick GPS Antenna Design ____________________________86

Fig. 64 Impedance Matching of Thick GPS Antenna Design _____________________86

8

List of Tables

Table 1: Single Later Ferrite Antenna Parameters ______________________________62

Table 2: Loaded Non-magnetic Antenna Parameters____________________________65

9

Chapter 1: Relevant Background Information

1. Introduction to Thesis

1.1 Reason for Study

Recent technological achievements in the communications industry have led to an

increased demand on novel types of antenna fabrication and miniaturization. Among

these antennas, patch antennas in particular have been critical in many forms of

communication ranging from radar, to wireless communication systems and GPS [1-6].

Clearly, it has become critically important to produce these patch antennas, not only with

large bandwidths, and tunable ranges, but also to reduce them in size. To achieve the goal

of miniaturization in antennas many groups have turned to high dielectric constant

materials, reactive impedance substrates, dielectric resonators, and magneto-dielectric to

use for their substrates [1-6].

These groups have shown that this high dielectric constant material, while

reducing the size of the antenna, has severe impacts on the radiation and bandwidth of

these devices; in particular there is a large problem with the impedance bandwidth on

high permittivity ceramic substrates [2-3]. This behavior; however, has been investigated

recently by different groups and it has been shown that this bandwidth reduction can in

fact be improved on patch antennas on high-permittivity substrates [3-5]. This is because

the high dielectric material introduces a capacitance between the antenna and its ground

plane and can be affected by loading the patch itself with another high permittivity

material [4]. However, this loading material increases the physical dimensions of the

10

patch antenna, which is undesirable for many applications. These thicknesses are often on

the order of the substrates themselves, almost doubling the overall size of the patch

antenna.

Since our goal is using high permittivity materials to develop miniaturization in

our patch antennas this is not a practical approach for us. We will instead use other

technologies such as metal metallic films and new spin spray ferrite technology to

improve the bandwidths of these devices. We hope that by incorporating these novel

materials into our substrates, and thus creating magneto(di)electric substrates we will be

able to harness the miniaturization given by the high permittivity materials, while at the

same time not falling victim to the inherent problems of the substrates.

1.2 Outline of Thesis

The first goal of this paper will be to empirically show how using higher

permittivity materials lead to antenna miniaturization, and why these materials would be

desired in the communications industry. To do this we will first introduce some theory on

the design of patch antennas, and then show how this theory enables us to utilize high

permittivity materials. Once we have proven this behavior we will simulate it by utilizing

HFSS simulation software. While doing this we will introduce previous works on

utilizing high permittivity materials.

11

In our simulations our goal was to determine the effect of changing the

permittivity of a patch substrate by measuring the resulting change in the return loss. A

shift of the resonant frequency downward indicates that the overall antenna can be made

smaller. This is due to the fact that reducing size of the antenna shifts the resonance

frequency upward. Therefore, a smaller antenna with a high permittivity substrate will

resonate at the same frequency as a larger antenna with a lower permittivity substrate. As

previously mentioned to visualize this behavior we simulated a patch antenna utilizing

HFSS simulation techniques.

Our first simulation was on a reasonable dielectric constant material (K~2.2).

Once this was completed we will determined how changing the permittivity of the

substrate affected the overall performance of the antenna. Again, our goal was to show

how changing the impedance of the substrate directly affects the impedance bandwidth of

the antenna. We also wanted to mirror the background information given in the prior

section of this thesis. Once we have proven that increasing the permittivity of the

substrate leads to antenna miniaturization we will continue on to using much higher

permittivity materials, which is our final design goal.

The second step of this process; after the proof of permittivity miniaturizing the

size of the antennae, will be to determine the correct ceramic to use. Since the material

used as the substrate for our antenna is vitally important a good deal of time will be spent

to determine an effective high k material to be used as in our antenna designs. To this end

we have investigated many different types of ceramics, and have fabricated them in our

12

lab, paying careful attention to the thermal stability of the permittivity of the material

over our desired temperature range which was -30 to 80°C. We also studied the loss

tangent involved with each of the substrates. In doing this we have developed a process

of ceramic fabrication which results in high quality ceramics. While many different

ceramics will be introduced, special care will be given to those in the titanate family,

such as (Ba1-xSrx)TiO3 and SrTiO3. In particular we have looked into doping these

ceramics with varying amounts of MgO, for stability in the loss tangent, and CaO for

thermal stability. We will introduce the process of making these ceramics, as well as give

some experimental data on ceramics which we have produced. In this section of the

thesis, we will also introduce the inherent problems associated with the use of high

permittivity materials for antenna substrates. The next two sections of this thesis will

offer in-depth solutions to solving these inherent problems.

The third goal of this study will be to look at the different methods of improving

the parameters of common patch antennas, paying special attention to those parameters

which are of concern when using high permittivity substrates. In particular we will look

at different methods of improving the antennas bandwidth and gain. Things we will

examine include the shape of the patch, and the type of feeding to use for our particular

design. In addition to the physical dimensions of the antenna we will examine the use of

loading of different materials to increase these parameters. In particular we will discuss

the work done here at Northeastern involving the use of biased metal magnetic films in

patch antenna design. This work is very promising in that it shows how the incorporation

of magnetic materials into patch antennas can significantly aid in the antennas bandwidth

13

and directivity, two problems very closely associated with high permittivity ceramics

[15].

Once the use of metal magnetic thin films has been shown to improve the

relationship between the antennas bandwidth, and gain we will show how new ferrite

materials are being used in antenna designs as well. These new ferrite films which have a

much higher FMR resonance than other ferrites and can therefore be used in higher

frequency microwave devices. In particular spin spray ferrite technology can be used

towards this end. In this section we will examine how these films have been developed

and the theory behind their use as a component in microwave devices [11]. This work has

also been done at Northeastern University. This is an important step into the realization

of high permittivity antennas as these films are for the first time self biased, and can lead

to vastly improved antenna [16-18].

Finally all of these novel materials will be put together to create a miniature GPS

antenna utilizing high permittivity ceramics, and spin spray thin film technology. This

discussion will be over a paper was recently accepted at the 2008 IEEE/APS conference

in San Diego, California [17]. This work has recently been continued upon with thick

substrates, these simulations have also been included.

Therefore; in this paper we will examine novel materials used in antenna design

and their inherent problems. We will approach these problems, such as the problem of

impedance matching associated with patch antennas fabricated on different permittivity

14

substrates. After completion of this effort, we will see how the antennas are affected by

the use of high K (permittivity ~91.7) materials, and further affected by a ferrite or

metallic magnetic layers in the antenna itself. This work will enable us to gain a better

understanding of miniaturization associated with high permittivity ceramics, and will get

us heading in the right direction towards actually fabricating these miniature antennas in

a laboratory environment.

2. Patch Antenna Design

2.1 Previous Design Models

Patch antennas are very commonly used in the communications industry due to a

variety of factors. First, they exhibit omnidirectional performance. Secondly, the

directivity and gain parameters make it optimal for communications. Also, patch

antennas are useful as they are very easily analyzed with a variety of different models

such as the transmission line model, and the cavity model [19-20]. In this section we will

give a brief introduction to the patch antenna, as it will be the main design that we will

use to fabricate our high permittivity antenna. Specifically we will use the transmission

line model in order to determine the relative size of our patch.

15

Fig. 1 Overview of Patch Antenna [19]

In the transmission line model it is necessary to first determine the effective

permittivity of the substrate and the air above the patch, this is done using a simple

equation. In the next section of this thesis we will examine how changing this

permittivity value affects the resonance peak of the antenna, and in the future we will see

how adding layers of ferrite films between the substrate and the patch furthers this

relationship. For now; though, let us examine the simplest case of patch antenna design.

In this case, the effective epsilon is given by the following formula [19-20]. We have also

included a small graph showing the relationship between this value and varying other

substrate permittivities.

Fig. 2 Equation for Effective Epsilon of Patch Antenna [19]

16

Fig. 3 Relation between the Permittivity and Effective Permittivity [19]

As previously stated the goal of the transmission line model is to determine the

patch size of the antenna based of off a few set parameters, the first being the permittivity

of the substrate you are using. The other necessary parameters for this calculation are the

thickness of the substrate which you will use and the desired resonant frequency of your

antenna. Using these values, and the well known equations relating the radiation of the

antenna to its physical dimensions we can easily solve for the values of L, ∆L and W.

The value of ∆L is due to the fringing fields of the antenna, making the antenna appear

larger than it actually is. Using this value of ∆L we can give the effective length as

L+2∆L. This value is equal to one half of the wavelength to excite the TM010 mode

[19-20]. This procedure is outlined in the following equations.

17

Fig. 4 Equations for the L and W of Patch Antennas based on the TLM [19]

These equations are very important, as they show one important parameter which

is outlined on the figures in the next page. As we decrease the size of W and L, we see a

shift in the resonance frequency upward. This is exactly the problem of miniaturizing

antennas, while keeping low frequencies. Our goal is to utilize the permittivity value in

the equation to make up for this. If we can increase the permittivity of the substrate, we

should see the resonant frequency decrease. Then, we can decrease the size of our patch

so that the resonant frequency shifts back up to its original position. This behavior will be

studied briefly in the next section of this thesis utilizing HFSS simulation software.

18

Fig. 5 Effect of Miniaturization of Resonant Frequency [19]

2.2 Investigation into Changing Permittivity

As demonstrated in the previous section of this thesis, it is well known that

changing the permittivity of the antennas substrate has an effect on the resonant

frequency of that microwave device [19-20]. As previously discussed we want to further

19

understand this result, by replicating it in a simulation environment. To do this, we will

fist introduce the project designed to show the effect of changing permittivity on a

standardized patch antenna. This antenna, simulated with HFSS software, is a basic

coaxial back fed antenna (inner pin diameter of .3mm outer dielectric diameter 1.2mm,

and intrinsic impedance = 57.2ohms). It was intended to be a 2.4 GHz antenna, utilizing a

2.2~K substrate.

For simulation purposes, we have assumed that there is an infinite ground plane,

and the patch material itself is a PEC. Since we will be using PVD deposited copper films

which have a conductivity of 59.6×106 S/m, which is very close to that of silver, the

metal with best conductivity of 63.01 × 106 S/m. Our goal here is simple, we want to

introduce to the reader how small changes in the permittivity of the substrate can shift the

resonant frequency of the antenna downward. This, in essence will lead to a

miniaturization factor due to the increased permittivity.

20

Fig. 6 Geometry of the rectangular patch antenna. (a) Top view. W1=4cm, W2=3cm,

height=.32 cm, offset of feed point: .5cm in the X direction.

Once we had successfully modeled that patch and run the verification check to

ensure that that wave port was connected correctly, we tried to determine the far field

radiation associated with the simple patch antenna. This plot can be seen on the

following page, and it is exactly what one would expect for an antenna of this shape. It is

isotropic and uniform due to the patch's geometry. The reason that there is only a single

lobe due to the assumption we have made that the material was a PEC, and that there was

an infinite ground plane. For practical purposes, this antenna would have small back

lobes as well, but those are not what we are interested in for this investigation. Also, for

practical fabrication we would expect some sort of side lobes due to the fringing fields

around the ground.

21

Fig. 7 Radiation Pattern of Far Field, for Rectangular Patch Antenna

(Assumptions: The Patch itself is a perfect electrical conductor, and the ground plane acts

as an infinite ground). This field was solved for via HFSS, but the equations below are

those given in theory to produce this result.

22

Fig. 8 E-Plane and H-Plane Equations [19]

Once, we had finalized the antenna design all that was left was to determine the

effect of increasing the permittivity of the substrate by small amounts. In doing this we

can see the actual affect that the substrate plays in the antennas performance. Each

successive substrate maintained all of the characteristics of the previous substrate

including the loss tangent, and the conductivity. The only change was in the dielectric

constant which varied from a value of 2.2 up to 3. It is clear to see from the simulation

data plotted in Fig. 9 that the operating frequency of the antenna shifts to lower values as

the permittivity increases.

23

Fig. 9 Effect of Changing Permittivity on the Return Loss of Different Antennas

This is exactly the behavior that we were expecting to see, and exactly the

behavior which has been shown due to others research [1-6]. In essence, this shift is due

to the permittivity of the material which shortens the wavelength inside the material, and

allows for the miniaturization of this antenna by reducing the physical size dependence

involved with the resonance of the patch given by the following formulas. These

formulas show how the fringe factor g is related to the permittivity. In the case of our

study the original value of W, was decided to give the resonant frequency of 2.4 GHz.

24

Fig. 10 Definition of Fringe Factor [19]

Using the formula above it is easy to see the length reduction faction g,

responsible for the change in size of the patch antenna being studied. In essence this shift

of the resonance peak downward enables us to shorten the lengths of W and L, and still

get a resonance peak at 2.4 GHz. This result will be the main reason for utilizing high

permittivity materials for the substrates in our GPS antenna designs.

These results are not all positive; however, they also show the main reason why

high permittivity ceramics are not normally used for antenna miniaturization purposes. It

is clear to see from the return loss patterns, that as the permittivity increase, the return

loss decreases. This is due to the higher permittivity creating an impedance mismatch

between the feeding system and the antenna. Also, you can see that as the permittivity

increases the peaks become much narrower, indicating that the permittivity is negatively

25

affecting the bandwidths of the antennas. This is also a known issue regarding utilizing

high permittivity materials for antenna substrates [1-6].

However, before we can address these problems associated with high permittivity

ceramics, we first need to determine a ceramic that is suitable for antenna applications.

This means that we need a ceramic, with a suitably high permittivity. Too high, and the

negative impacts of the permittivity with make our antenna impossible to fabricate or use

in a practical setting. Too low, and we will not see the desired effects of the permittivity

in regards to antenna miniaturization. Since we are looking to make a practical and usable

antenna we will also need to pay attention to a few other parameters as well. These are,

temperature stability, and loss tangent. We will need the permittivity to be stable over a

large temperature range -40 to 80°C; the loss tangent should be on the order of 1*10-4. In

the next section of this report we will discuss materials that meet these specifications, and

decide upon one which will best suit our needs. We will then use this material, along with

some other design ideas to create a working miniaturized GPS antenna in the later

sections of this report.

2.3 High K Material Investigation

Now that we have shown how the permittivity is closely related to the

miniaturization of our antenna designs it is time to look more closely at the material

parameters that we desire in our substrates. When choosing a high permittivity material

for an antenna substrate particular care must be spent on choosing one which will meet

26

the demands of your design. For our design we are looking for a material with a

permittivity on the order of 100, a low loss tangent, and good thermal stability of its

parameters (<10%) over the temperature range of -40 to 80°C.

In this regard we have spent a good deal of time researching different ceramics,

and trying to determine which one would be optimal for our design. Some of the most

promising and most widely used ceramics are those of the Titanate variety. In particular

(Ba1-xSrx)TiO3 ceramics have been widely used on projects ranging from antennas, to

capacitor design. They are widely used because of the tune ability in their permittivity

(100-2000k) as shown in Fig. 11. This range is achieved by interchanging the quantities

of Barium and Strontium within the ceramic. Special care needs to be shown to these

values, as small changes in Barium content can significantly change the permittivity of

the sample [12]. In order to determine if this material was correct for our antenna

application we have spent a great deal of time fabricating and studying different

properties of this material.

27

Fig. 11 Permittivity of (Ba1-xSrx)TiO3 Ceramics with varying Ba content [12-13]

It is clear from Fig. 11 that this material itself is not great for antenna applications

for a few different reasons. First the permittivity is much too high, making it impossible

to fabricate a patch which resonates in the frequency range desired. The bandwidth of the

antenna fabricated on a substrate of that permittivity would not be wide enough for any

practical applications. This data, as previously mentioned has been widely documented

among antenna researchers [1-6]. We are looking for an antenna that has a bandwidth of

3-20 MHz for that reason the permittivity cannot be on the order of 1000.

Secondly, the loss tangent is much too high. This would make the antenna almost

impossible to fabricate, and would further deteriorate the antennas performance. In order

to make this material useful for our application we need to first combat these problems.

28

Fortunately researchers have been able to avoid these problems by doping the material

with different elements. Oxides in particular have been shown to significantly reduce the

permittivity, to our desired order of 100, while at the same time reducing the loss tangent

of the material. In particular, MgO has been widely used [13]. Fig. 12 shows how

incorporating MgO into the (Ba1-xSrx)TiO3, positively affects the loss tangent, while at

the same time reducing the permittivity to a more acceptable range K~100. Other

materials that have been used include Ta, and Ca [14].

Fig. 12 Permittivity of (Ba1-xSrx)TiO3 Ceramics with varying MgO content [12-13]

Clearly, this material exhibits the properties that make an antenna radiate

effectively. However; since I am also looking for a material that is thermally stable over a

large temperature range it still needs to be tested in a laboratory environment. To this end

I have fabricated samples of differing MgO quantities in order to test their performance at

different temperatures.

29

The process that was used to create these materials, was one common to ceramics.

It consisted of measuring the constituent powders, according to molecular weight, and

then mixing them together in a high power ball mill for three hours. These powders were

then pressed at 1500psi, and sintered at 1100°C for 8 hours. After sintering, the pucks

were broken down using a mortar and pestle. Differing quantities of MgO were then

added, and the process was repeated. The only difference was that in the second and

subsequent firings, the temperature was increased to 1450°C. The pucks were annealed in

an alumina cylinder, surrounded by alumina powder. This was to promote even heating,

and to reduce the surface tension that resulted in shape discrepancies of the finished

product. These finished pucks then underwent a series of measurements to ensure they

were correct for our application.

Fig. 13 XRD of (Ba1-xSrx)TiO3 Ceramics with varying MgO content

30

Fig. 14 FTU results of (Ba1-xSrx)TiO3 Ceramics with MgO content of 60%

It is clear to see from the XRD data that we were able to achieve a very consistent

(Ba1-xSrx)TiO3 ceramic doped with MgO. It is also clear to see that the permittivity of the

material was exactly within the bounds which we were looking for. However, from the

temperature measurement given from the ferroelectric test unit Fig. 14 it is clear to see

that the dielectric change over the temperature range are not within the bounds that we

are looking for. This means that the thermal stability is not sufficient (>10% over the

temperature range). However; recent work done with Ca doping addresses this problem

[14].

31

Fig. 15 Quality Factor of Ca doped Titanates. [14]

Based off of these results we have decided upon a doped ceramic titanate.

Unfortunately our ceramic processing facility, while allowing us insight into the process

and the materials, was not consistent enough to produce repeatable substrates. Therefore

we have worked in conjunction with Trans Tech Inc to get these materials. Even though

these materials were provided by Trans Tech Inc our research into their properties proved

invaluable in suggesting what doping should be done on their ceramics. The material

provided, has great thermal stability, low loss, and a very stable permittivity. This

material will be used in the simulations presented in the later sections of this report.

However, even though this material has amazing dielectric properties. It still has

many of the inherent problems associated with other high permittivity materials.

Therefore the next two chapters of this report will be dedicated to dealing with these

problems. In particular we will look into loading these materials with other novel

materials.

32

Chapter 2: (Fe60Co40)85B15 Films and Their Uses in Antennas

1. Introduction to (Fe1-xCox)85B15 Films

1.1 Background

Clearly, used alone these high permittivity materials would not be sufficient to

meet all of the goals of our desired antenna. To this end we have examined a variety of

different approaches to achieving the miniaturization that we require, without the extreme

loss of bandwidth that is prevalent with the use of these materials. In particular we have

spent a great deal of time looking into incorporating thin magnetic films into the design

of our antennas.

Normally, magnetic films would not be used in antennas due to their large loss

tangents and were restricted to low application frequencies of 500 MHz or less [15]. Due

to this reason, the first films that we looked at were not Ferrites which have very low

FMR resonance; but metal metallic films. These films are ideal for this application due to

their high saturation magnetization of up to 24 kG, as well as their self-biased

ferromagnetic resonance of several GHz. In addition to these characteristics, these films

can be easily fabricated in a room temperature processing facility, adding to the ease of

use for these antennae.

Another important thing to note is the stability of the films properties over a wide

range of temperatures, this is very important as we would like our antenna to be able to

operate in a variety of different environments. As you can see in the chart below, our film

33

(measured by vibrating sample magnetometer, in extremely low to high temperatures)

only changed in its saturation magnetization ratio to RT by about 3% at the temperature

extremes. This tells us that using this film in our antenna will not degrade our antennas

performance as the temperature of the environment fluctuates.

Fig. 16 Saturation Magnetization vs. Temp

Fig. 17 Saturation Magnetization vs. Hc (G)

-40 -20 0 20 40 60 80 1000.9500

0.9700

0.9900

1.0100

1.0300

Saturation Magnetization Ratio to RT (20 C)for 1um FeCoB Film

Temperature (C)

Rat

io

-40 -20 0 20 40 60 80 1000.3

0.4

0.5

0.6

Relationship between Hc(G) and Temp(C) for 1um FeCoB Film

Temperature (C)

Hc

(G)

34

Fig. 18 Saturation Magnetization vs. Hk (G)

1.2 Fabrication and Testing

In the first study that we performed using these metal metallic films to enhance

antenna performance we fabricated antennas which operated in the 2.1GHz range. We

developed three antennae, two antennas with magnetic films 1um thick underneath the

patch, and one without. The antenna without the magnetic film under the patch would act

as the control for this experiment. These antennas were first simulated using HFSS

software, to determine the correct positioning of the metal metallic layer, as well as to

ensure sizing and correct impedance matching between the feed system and the antenna.

We determined that we would need to shape the size of the metal metallic film to be the

same size as the patch due to the impedance matching problems the metal metallic film

posed underneath the patch's feed line. Once the HFSS designs had been optimized for

omni-directional performance, we moved on to fabrication and testing of these antenna.

-40 -20 0 20 40 60 80 10019.75

19.8519.9520.05

20.1520.25

Relationship between Hk(G) and Temp(C) for 1um FeCoB Film

Temperature (C)

Hk

(G)

35

For fabrication, these antenna were deposited on Alumina substrates

(permittivity=10.1) of a thickness of 2mm. The metal metallic films were deposited under

a magnetic field of approx. 45 Gauss, to achieve an in plane anisotropic permeability.

One antenna had the magnetization along the H plane of the antenna, and the other had

the magnetic field along the E plane. We did this in order to determine which would have

more of an effect on the antennas performance. These films were deposited utilizing a

physical vapor deposition system (PVD), and their composition was (Fe60Co40)85B15. This

composition was selected as per the discussion on thin films above. Its saturation

magnetization was 16kG; this, along with its low coercivity and high resistivity made it

perfect for our application. As previously mentioned this film was then shaped via

photolithography to the shape of the patch antenna, taking careful precautions to remove

the film underneath the feed line. This photolithography process consisted of a 3um

photoresist, and a combination of nitric, and acetic acids with water of a ratio of 1:1:6.

The copper films were then deposited on top of the existing metal metallic films

also by utilizing a physical vapor deposition system. Each of the films was then measured

with a profilometer to ensure a constant 3um in thickness in the feed line, and 4um for the

antenna body. Photolithography was used to create the individual antenna shapes. The

process involved utilizing a 7um photoresist, and the correct masks we were able to

create very accurate antenna designs. Our etching acid solution was a combination of

water and nitric, acetic, and sulfuric acids of a ratio of 60:5:5:2ml.

36

The return losses for each of these antennas were then measured using a network

analyzer. For all three cases, we used a bias field to see if it would further affect the

antennas performance. Obviously we were more interested in the H and E antenna field

Fig. 19 (Fe60Co40)85B15 Antenna #1 [15]

measurements with magnetic films. For the non magnetic case, the return loss almost

exactly matched that which we expected from the HFSS simulations. For this case we

received a 10dB bandwidth of approximately 24 MHz with an error of approximately

3MHz. This was right in line with the simulation results of 28 MHz which secures this

antennas position as the control in this experiment.

The above figure shows the case where the applied bias feed was parallel to the

feed line direction. The first thing to note here is the extreme bandwidth enhancement

37

shown by this antenna. Its bandwidth is given as 37 MHz, with an error of approximately

3MHz. This is over a 50% enhancement over the case of the nonmagnetic antenna. The

reasoning behind this will be discussed shortly, but quickly it is due to the improved

impedance matching between the copper patch layer, and the alumina substrate. This

performance is exactly what we are looking for in our high permittivity patch, a way to

enhance the bandwidth and enable the use of high permittivity materials!

Fig. 20 (Fe60Co40)85B15 Antenna #1 [15]

Another important thing to note is the tune-ability of the resonant frequency

offered by the bias of the magnetic patch. On the case where the magnetic fields are

applied perpendicular to the feedline, we see a upwards shift of the resonance frequency

of about 7MHz. In the parallel case we see the opposite shift. Therefore, utilizing a small

38

bias field, of less than 50Oe we can achieve tune-ability of approximately 50% of the

antenna bandwidth!

Fig. 21 (Fe60Co40)85B15 Antenna Radiation Pattern [15]

Once we had determined the effect of the bias fields on the perpendicular and

parallel cases of our antenna we wanted to see the radiation patterns of each of the

designs. This would enable us to get the directivities of the antennas as well as their gains

as comprised to a standardized horn antenna. To do this we first had to gain access to an

anechoic chamber (special thanks goes out to the ECE lab department for letting us use

their chamber for our measurements). Once we had secured access to the chamber we

then created a setup through which we could measure the gain, and directivities of the

antennas. We did many tests involving using stepper tools for antenna rotation in

conjunction with Matlab code, and national instruments recording software. The outline

for these antenna measurements can be seen in the figure below

39

Fig. 22 Northeastern Anechoic Chamber Measurement System

40

Chapter 3: Antennas with Integrated Ferrite Films

1. Introduction to Spin Spray Ferrites

1.1 Background

As previously introduced with the incorporation of (Fe60Co40)85B15 metal

magnetic films into our antenna designs we have spent a great deal of time on

incorporating novel magnetic materials into our research. In particular, another material

which we have studied in detail was self biased ferrite films. Previously these films had

been limited to the 500-600 MHz range. However, recent technological advances in spin

spray technology producing these ferrites has allowed for their use up to almost 3 GHz

[11].

These (Ni1-xZnx)Fe2O4 ferrites are very promising materials for applications in

realized devices because not only of their resonance above our selected frequency, but in

particular they are important as they enable miniaturization of antennas as well as

enhancement of bandwidth and directivity. This is due to the high permeability that these

materials have. This behavior is outlined in the figure below.

Our hope was that due to these parameters these films would act much the same

way as the metal magnetic films would, but at the same time their permeability would

allow for better antenna bandwidth and matching impedance.

41

Fig. 23 Permittivity and Permeability of (Ni1-xZnx)Fe2O4 Ferrite [11]

Fig. 24 XRD Data of (Ni1-xZnx)Fe2O4 Ferrite [11]

Two films that we were looking into in particular were spin spray (Ni1-xCox)Fe2O4

ferrite and (Ni1-xZnx)Fe2O4 ferrites. These films have already been deposited in a room

42

temperature process here at Northeastern University, and the process/ results will be

outlined in the next section. The most important thing to note here is that this film has

many advantages over the (Fe60Co40)85B15 film discussed earlier. Firstly this is due to the

self-biased nature of the film. Secondly, the resonance frequency of the film is much

higher than that of other bulk ferrites, allowing for us to use this film at much higher

frequencies.

1.2 The Fabrication Process

As previously mentioned these films were deposited using a spin spray system

developed here on campus. This system, outlined in the following two figures, essentially

is designed to produce layer by layer ferrites by spraying their respective components on

to a prepared substrate. This is done by using two different solutions, one oxidizing (pH

buffer and oxidant) and one reaction solution (precursor/reaction). The sample rotates

under each of the nozzles, and the films are layered on top on one another. Take note, that

there are many design characteristics such as temperature, rotation speed, and pH that

need to be carefully monitored to produce high quality films. Many thanks go out to the

team of graduates and undergraduates at Northeastern who made this system possible.

43

Fig. 25 Depiction of the Spin Spray Plating Process [26]

Fig. 26 Depiction of the Spin Spray Plating Process [26]

44

1.3 Results Achieved with the Northeastern Spin Spray System

Since these films were used in conjunction with the new antenna designs, and

high permittivity substrates, it was important to understand their material characteristics.

To that end the films deposited have undergone extensive testing to determine their

material parameters. In particular their magnetic properties such have been determined

via VSM (vibrating sample magnetometer). And their physical properties have been

examined by TEM and SEM. Unfortunately; the resonance frequencies of the (Ni1-

xZnx)Fe2O4 ferrites produced appear to be below those produced in literature, around

1GHz which makes them impractical to our GPS antenna designs. Therefore we have

decided to go with the (Ni1-xCox)Fe2O4 Ferrite instead. The material parameters of this

material can be seen in the figures that follow.

Fig. 27 VSM data of (Ni1-xCox)Fe2O4 Ferrite

NFCO on Alumina

-1.5

-1

-0.5

0

0.5

1

1.5

-15000 -10000 -5000 0 5000 10000 15000

Field(Oe)

No

rmal

ized

Mo

men

t

Inplane

Outplane

45

Fig. 28 AFM Data for (Ni1-xCox)Fe2O4 Ferrite

Fig. 29 SEM Data for (Ni1-xCox)Fe2O4 Ferrite

It is clear to see from the background information provided, as well as the

preliminary results from our own setup that these films will be very useful to us as we

design our high permittivity antennas. The next step of our design process was to then

utilize these films in a simulation environment to determine how they should be

46

integrated into our antenna designs, and how thick the films should be; also we will show

the theoretical basis behind their application.

2. Theory behind Spin Spray Ferrites

2.1 Theory: Increased Permeability leading to Higher FMR

As previously mentioned ferrites have not been widely used in antenna fabrication

for a variety of different reasons; however, they have been used quite widely in lower

frequency applications including RF devices. The reason for this is their high

permeability and relatively high permittivity k~15 allow for miniaturization [16].

One of the most important reasons that ferrite films are not used at higher

frequencies would be the inherent loss at those respective frequencies; this loss is due to

two separate things. One is the ferro/ferromagnetic resonance (FMR) of the material, and

the other is due to domain wall motion within the material itself. For most microwave

ferrite ceramics, these two issues force the FMR to be around 600 MHz, almost

eliminating the materials parameters at high frequencies. This means that in essence the

FMR frequency is the upper limit of the material for antenna substrates which wish to

obtain permeability greater than 1. We will now show theoretically why this is the case,

and how using a permeability greater than one can lead to enhancement of antenna

parameters. Using this information we will be able to simulate and design antennas which

utilize this behavior.

First we define the permeability tensor of a uniformly magnetized sphere with its

47

magnetization in the Z-direction as given by the following formulas [21]:

=

o

jk

jk

µµ

µµ

00

0

0

][)

(eqn. 1)

In this case the permeability µ is given by the following formula

−+=

221

ωωωω

µµo

moo (eqn. 2)

22 ωωωω

µ−

=o

mok

(eqn. 3)

netoo Hγµω = (eqn. 4)

som Mγµω = (eqn. 5)

Here γ is the gyromagnetic ratio, Ho is the net magnetic field along Z direction

and soMµ is the saturation magnetization. The angular frequency ω0 is the FMR frequency,

which leads to large magnetic loss tangent.

For a uniformly magnetized sphere, the FMR frequency is linearly proportional

to the net magnetic field Hnet, where γ is the gyromagnetic constant close to 2.8 MHz/Oe.

Large bias fields in the order of 1000 Oe are needed to reach GHz FMR frequency, and

allow operation frequencies in the GHz range.

48

The relative permeability of the magnetic sphere can be described by:

net

sr H

Mπµ

4=

(eqn. 6)

This value is clearly inversely proportional to the net magnetic field Hnet. We can

therefore readily reach the Snoek limit:

srFMR Mf πγµ 4⋅=⋅ (eqn. 7)

This means that the product of the FMR frequency and the relative permeability is

a constant that is determined by the saturation magnetization of the magnetic media. This

is very important to us as it shows the importance of increasing the permeability of our

material, to get a better understanding of this we will need to first determine the net field

on our sample.

When determining the net field it is important to include demagnetization field,

which can be effectively used to boost the FMR frequencies of a magnetic body, and

therefore enhance the operation frequency range. The demagnetization field of a

magnetic body can be expressed using the following equations [22]:

(eqn. 8) rdrr

rr

rdrr

rrrH

ms

mvd

′′−

′−+

′−

′−=

∫∫

∫∫∫

2

3

3

3

||4

1

||4

1)(

ρπ

ρπ

49

In this formula ρmv is defined as the volume magnetic charge density, and ρms is defined

as the surface magnetic charge density. The demagnetization field can also be written in

matrix form where the magnetic field can be related to the demagnetizing factor tensor,

and the magnetization. This relationship is given below

⋅

=

=

z

y

x

zzzyzx

yzyyyx

xzxyxx

z

y

x

d

M

M

M

NNN

NNN

NNN

H

H

H

Hv

(eqn. 9)

In this case we define N as the demagnetizing factor tensor, and use the assumption that

we are dealing with magnetic spheres, i/e Nij=0 when i≠j and Nxx=Nyy=Nzz=4π/3 (cgs unit

system), therefore there is no net field contribution from demagnetization field.

=

zzzyzx

yzyyyx

xzxyxx

NNN

NNN

NNN

Nt

For the case of thin magnetic films in the X-Y plane, the demagnetizing tensor

has the values of Nxx=Nyy=0, Nzz=4π, and Nij=0 when i≠j. These values lead to a strong

demagnetization field that is equal to Hd=4πMs. The permeability of the film in the film

plane is still given by:

50

net

sr H

Mπµ

4=

Here Hnet is the net in-plane field which includes the demagnetization field of the

sample. When you now look at the FMR frequency you see that it has increased to be:

1)4( +=+⋅= rnetnetsnetFMR HHMHf µγπγ (eqn. 10)

The FMR frequency of magnetic films is therefore boosted by a factor of 1+rµ times of

that of magnetic spheres, allowing self-biased FMR frequency as well as the operation

frequency of magnetic films at GHZ range. Similarly, the product:

)1(4 +⋅⋅=⋅ rsFMRr Mf µπγµ

(eqn. 11)

is also boosted to 1+rµ times of that of the magnetic spheres, indicating a significantly

boosted Snoek Limit for magnetic films.

Our recent work on microwave magnetic thin films, including metallic magnetic

films and ferrite films, indicate that these magnetic thin films can readily operate at GHz

frequency range under self-bias condition [7-10, 23], and have been widely used in

RF/microwave devices, including antennas.

Now that it has been determined how increasing the permeability of the films can

lead to utilizing these ferrite films at higher frequencies, and to utilize them in GPS and

51

higher frequency bands we are now going to look at how the material parameters of the

films lead to enhanced antenna parameters. In particular we will look at self-biased spinel

NiCo-ferrite (Ni1-xCox)Fe2O4 films fabricated by a low-cost spin-spray deposition

processing. The thin film geometry and the large in-plane anisotropy field of the NiCo-

ferrite film enable a high magnetic permeability up to several GHz beyond the Snoek's

limit of cubic spinel-type ferrites. These microwave magnetic thin films provide a great

opportunity for achieving self-biased magnetic patch antenna substrates with µr >1 at high

frequency.

2.2 Theory: Permeability and Better Antenna Performance

Our first study on the film utilized the same geometry of patch antenna as our

(Fe60Co40)85B15 film study. This antenna was designed for operation at 2.1 GHz is shown

in the figures below. As stated previously, it is a conventional microstrip patch on an

alumina substrate with a thickness of 2 mm. The relative permittivity of the alumina

substrate is εr=9.9 and the relative permeability is 1. The copper patch for this non-

magnetic antenna has a length L3 = 22.2 mm, width W3 = 30 mm and thickness of 3 µm.

The width of the feed-line is 2.0mm and the length is 22.3mm. (All the dimensions are

noted in the caption of Fig. 1).

52

3L

3W

1W

1L

2L 2W

(a)

(b)

Fig. 30 Depiction of 2.1 GHz Patch Antenna [16]

The dominant mode of the designed patch antenna is the ZTM010 mode, the resonant

frequency for this mode is a well known and given by the following equation. [19]

rr

rL

cf

εµ2)( 010 =

(eqn. 12)

In this equation, rµ is defined as the relative permeability of the alumna substrate

and rε is the relative permittivity, c is the speed of light in free space, and L is the

effective length of the patch. The bandwidth of VSWR=2 for patch antennas with a

magneto-dielectric substrate is also well known and given by the following equation. [24]

53

]174[2

/96BW 0

rr

rr H

εµ

λεµ

+=

(eqn. 13)

Where H is the thickness of the substrate, and 0λ is the wavelength of the resonant

frequency. It is clear to see from the above equation that increasing the permeability of

the substrates will lead to an enhancement of the antennas bandwidth, which is exactly

the behavior we wish to get. This will help to counteract the bandwidth narrowing effect

that is given by dramatically increasing the permittivity due to the division by εr in the top

half of the equation.

In addition to the benefits given the radiation frequency and bandwidth, there are

other positive aspects to using the ferrite film in the construction of our antennas. In

particular the high permeability of the films leads to a better wave impedance match

between our substrates and the air. The theory for this matching will be outlined shortly,

but in essence this behavior makes matching of the antenna to the feed networks much

easier.

To prove this concept we will use boundary conditions at the leading edge of the

media between the Alumina and Free Space. It is clear that if the impedances at the

interfaces of the substrates are very similar then transmission will be greatest. Also it is

clear to see that if the impedances are widely mismatched it will be very difficult to

match these two items. Granted, this is a general case and the case of patch antennas are a

54

little more complicated due to near/far field issues the general theory stands. This general

behavior is shown in the following figure.

iE tErE

rH iH tH

),( 00 εεµ a ),( 00 εµ

Fig. 31 Alumina to Free Space Boundary Conditions [16]

Now if we were to insert a material between the two elements that had properties

that were beneficial to the matching process, such as our ferrite material, we can examine

how the reflection coefficients at the interfaces change, and how this will affect the

power radiated from one section to another. This behavior is shown in the theory to

follow where we will examine the power densities on each of the interfaces using the

material parameters that have been discussed earlier. In doing this we will be able to

show that the radiated power will improve by incorporating the ferrite film, and thusly

the matching will be improved.

55

iE tErE

rH iH tH

tE ′

tH ′

rE ′rH ′

),( 00 εεµ a ),( 00 εµ),( 00 εεµµ ff

Fig. 32 Boundary Conditions of Ferrite and Alumina [16]

The first step of this proof will be to determine the transmission across the

alumina interface which can be expressed as [19]:

i

t

a E

ET =

+=

0

02

ηηη

(eqn. 14)

Next, we write the average incident and transmitted power densities. Using the

relationship between these values we can determine the ratio of the incident to

transmitted power for the case with and without the ferrite layer. First we examine the

properties without the ferrite layer [20].

a

ii

av

EP

η2|| 2

= (eqn. 15)

0

2

2

||

η

tt

av

EP =

(eqn. 16)

56

20

02

0

2

)(

4

||

2

2||

ηηηηη

η +=⋅=

a

aia

t

iav

tav

E

E

P

P (eqn. 17)

iav

tav PP 73.0= (eqn. 18)

Now we will add a third layer as the medium between the first and the second

layers. This will give us a revised copy of eqn. (17), and is shown below. Solving for the

ratio of the input to transmitted power it is clear to see a difference between the two

cases.

20

0

2 )(

4

)(

4

f

f

fa

fa

iav

tav

P

P

ηη

ηη

ηη

ηη

+⋅

+=

(eqn. 19)

iav

tav PP 78.0= (eqn. 20)

Therefore, we have proven that these films not only increase the FMR resonance

of the patch, but they also positively impact the bandwidth and matching between the

substrates and the patch. Now that the theory has been well defined we will attempt to

simulate and fabricate these antennas starting with antennas with one ferrite film and

working our way to more films, and novel antenna shapes and design ideas.

57

3. Incorporating Films into Antenna Designs

3.1 Single Films in Antenna Design

The first study conducted with the use of these ferrite films was done to determine

the correct placement of the film in regards to the antenna, and it was done utilizing the

same 2.1 GHz patch design as the (Fe60Co40)85B15 film study. The project was headed by

Guomin Yang at Northeastern University [16]. I was fortunate enough to work with him

on this project on the fabrication and testing of these antennas.

To be more specific four different antennas were designed and studied. The first

antenna named Antenna #1 was our control antenna and had no ferrite films placed on it.

The other antennas preceded as follows Antenna #2 had a ferrite film placed underneath

the radiating patch only, Antenna #3 had a ferrite film placed over the entire area of the

substrates top, and Antenna #4 had the ferrite film placed between the ground and the

substrate. As previously mentioned photolithography was used to create these antennas

on alumina substrates, permittivity of ~9.9.

The ferrite film used in this study was a (Ni1-xCox)Fe2O4 ferrite with the following

material parameters; thickness of 2µm, relative permittivity of 13, relative permeability

of 10, and a low loss tangent. Aside from differences in ferrite positioning all antennas

were fabricated to be identical.

58

Fig. 33 Antenna No. 1 [16]

Fig. 34 Antenna No. 2 [16]

Fig. 35 Antenna No. 3 [16]

Fig. 36 Antenna No. 4 [16]

The first testing that we did on these antennas after they had been fabricated was

to measure their return loss and resonant frequencies using our Network Analyzer. As

mentioned previously we expected the resonant frequency of the non-magnetic patch to

be approximately 2.1GHz. This expectation was due to the size of the patch in correlation

with the theory, and with our HFSS (high frequency simulation software) simulations.

Our control patch (Antenna No.1) showed a resonant frequency of 2.147 GHz, and a

59

bandwidth of 18 MHz These measurements are outlined in the figure below, and are very

consistent with what we were expecting. Antenna No.2 shows a downward shift in the

resonance peak down to 2.136 GHz. This indicates the miniaturization effect of the ferrite

film in the antenna. This is also what is called the tunable range offered by the film, and

in this case is approx 11MHz. This value is more than 60% of the overall bandwidth of

the non-magnetic case. In addition to that enhancement, the second antenna also offered a

larger bandwidth with about a 16% increase.

Fig. 37 Measured Return Loss of the Four Antennas [16]

Antenna #3 had even better performance than that of Antenna #2. The resonant

frequency was shifted down even further to 2.124 GHz that is a shift of 23 MHz from the

non-magnetic antenna, or 127% of the bandwidth of the non-magnetic antenna. This

antenna also experienced a significant bandwidth enhancement. The -10dB bandwidth is

37MHz in this case, which is more than 200% of that of the non-magnetic antenna.

60

Finally placing a ferrite film above the ground plane of the antenna led to significant

enhancement of the impedance matching. As shown by the antennas -38 dB return loss

indicated in the previous figure. This antenna also experienced the downward shift in

resonant frequencies albeit not as large of a shift as our other cases.

After we had recorded the return loss measurements we went to the anechoic

chamber to measure the normalized E and H plane radiation patters for each of our

antennae. These measurements are given in the figure below. As you can see from the

figure, the radiation patterns are quite similar for each of the antenna, with the exception

of the back lobes and high angle gain. The ferrite films seem to positively affect both of

these parameters. In fact antennas with ferrite films experienced a 1-2 dB reduction in

the back lobes.

Fig. 38 E plane Radiation Patterns [16]

61

Fig. 39 Gains of Antennae at Different Elevation Angles [16]

In Table 1 we have listed all of the relevant data collected for this antenna. What

we have shown is that the ferrite films not only minimize the size of the antenna by

shifting the central frequency of the antenna from 2.147GHz, to 2.14GHz, they also help

with many other important antenna parameters. For example placing ferrite films also

increases the efficiency as well as the bandwidth. One of the most interesting findings

from this study; however, was the increase in the -5dB bandwidth of the antenna. This is

important because it shows that in addition to the gain and efficiency we are seeing

improved omnidirectional performance as well. We will further examine parameters

when we look at the case of high permittivity antennas in the later chapters of this thesis.

62

Table 1: Single Layer Ferrite [16]

3.2 Multiple Films in Antenna Design

Once we had finished our test on antennae that had one ferrite film, we were very

interested to see how antennae with multilayer NiCo-ferrite films would function.

Therefore we designed a systematic approach to measure the multilayer effect. We

started by loading the non-magnetic antenna, Antenna #1, by placing ferrite films on top

of it. These films were deposited on transparencies and attached over the patch as

depicted in the figures below. Of course, after each layer was added, we studied the

results using the same methods outlined in the previous section of this thesis.

Antenna

No.1

Antenna

No.2

Antenna

No.3

Antenna

No.4

Central Freq.

(GHz)

2.147 2.136 2.124 2.140

Bandwidth (MHz) 18 21 37 30

Gain (dBi) 1.3 1.1 1.3 1.5

Efficiency 41% 37% 45% 54%

-5dB Beamwidth

(H-plane)

140o 134o 147o 158o

63

Fig. 40 Antenna #1 with one Ferrite Layer on top [16]

Fig. 41 Antenna #1 with two Ferrite Layers on top [16]

Fig. 42 Antenna #1 with three Ferrite Layers on top [16]

When these antennas were measured using a Network Analyzer we saw a huge

enhancement in the impedance matching due to adding more ferrite films. The antenna

with three layers of films had a return loss of -30dB compared to about -10dB for the

nonmagnetic antenna. That’s an almost 300% improvement in matching. More

importantly when adding three layers of ferrite film we see a central frequency shift of

227% the antenna bandwidth. This means that the ferrite is having a significant

miniaturization effect on our antenna. In addition to those figures, we also see significant

64

bandwidth enhancement as we did in the single film study. All of these parameters have

been outline in Table 2 below. Upon further study as outlined in chapter 5 we see that this

bandwidth enhancement is due to the initial mismatch of the antenna design. For antennas

which are well designed to start out we do not see this enhancement. We do however; see

the downward shift in the resonant frequency. This is important as it allows us to

miniaturize our antennas.

Fig. 43 Return Loss of Loaded Non-magnetic Antenna [16]

65

Table 2: Loaded Non-magnetic Antenna [16]

Fig. 44 H-Plane of Loaded Non-magnetic Antenna [16]

Antenna No.1 1 layer of film 2 layers of film 3 layers of film

Central Freq. (GHz) 2.147 2.134 2.117 2.106

Bandwidth (MHz) 18 21 28 29

Gain (dBi) 1.3 1.6 2.1 2.4

Efficiency 41% 56% 65% 74%

-5dB Beamwidth (H-plane) 140o 155o 156o 160o

66

Fig. 45 E-Plane of Loaded Non-magnetic Antenna [16]

Fig. 46 Antenna Gain at different Elevation Angles [16]

67

To reiterate the importance of the ferrite film as a loading factor for this antenna

we have seen significant increases in many important antenna parameters. The efficiency

has increased with each ferrite film added from 41% to 56%, 65% and 74% respectively.

The antenna gain has increased for each film as well going from .3dBi to .8dBi and 1.1

dBi over the non-magnetic antenna. As we mentioned in the theory section these

increases are due to the improved wave impedance between the ferrite covered antennas

and free space.

We can also see from the figure depicting the elevation gain at different angles

that there is a significant enhancement of the gain due to the ferrite film loading. This

again was to be expected, and is another reason why these films are great for antenna

applications. Of course, this process was followed for each of the other antennas as well.

However, as we are only interested in the principal applications of the loading process as

it applies to our high permittivity substrates we will not introduce that data here.

3.3 Conclusions

We were able to show that self-biased films could be successfully introduced into

patch antennas. In particular, incorporating these films into our designs of the 2.1GHz

patch antenna led to significantly enhanced antenna performance. The ferrite films have

shown to increase the bandwidths of the antenna when placed in single films as outlined

in section 3.2. In addition to this enhancement we also see enhancement of other antenna

parameters including its wide angle gain, and its return loss. These are very important

68

parameters, as they will help the antenna operate. The return loss will allow for a better

radiation efficiency, and the increase to the wide angle gain will help with the antennas

omnidirectional performance. These are really good results, because the process used to

make these antennas was quite simple and done in a low temperature environment. This

makes them optimal for use in industrial applications.

Also, we have seen that loading the antenna with films also has a dramatic effect

on the antennas performance much in the same way that placing the films underneath the

patch does. In particular it has a large impact on the bandwidth of the antenna. This again

is significant due to the ease of construction of these loading layers, and the relative

physical dimensions imposed.

Remember, however; that these results are for low permittivity antennas.

Therefore we need to do a separate study on how these films will affect antennas on

higher permittivity substrates. We would expect that their behavior will not be exactly the

same in that case due to a variety of different things. First the fringing fields of the higher

permittivity substrate in regards to antenna radiation. Secondly, the large mismatch of the

permittivities will make impedance matching very difficult. Finally, the equations used

for the sizing of the antennas as outlined previously in this report are for antennas on

lower permittivity substrates. All of these things will be addressed in chapter 5

69

Chapter 4: Novel Antenna Designs

1. Loop Antennas

1.1 Introduction to Loop Antennas

We also used this film in conjunction with a planar circular loop antenna designed

by Draper Laboratories in Cambridge, Ma [18]. We wanted to investigate the

miniaturization factor of the film in regards to novel antenna designs. In particular we

wanted to see if it was comparable to the metallic film case. In examining this behavior,

we could also determine the tune-ability of the antenna's bandwidth.

The design of this antenna can be seen in the following figure. It was selected for

several reasons; first the shape enables almost complete linear polarization, which can be

switched to circular based off of simple geometry shaping. Secondly, loop antennas are

smaller in radius than circular patch antennas of the same frequency which means we will

be able to make the antenna smaller, one of the goals of this study.

Fig. 47 Loop Antenna Design [18]

70

The antenna consisted of a circular microstrip loop, and a tuning stub. Both

deposited by patterned copper at Draper Laboratories. The feed point was at the direct

center of the stub and ring connection, with a distance of .5mm to the outer edge. The

radius of the outer ring was 12.4 mm and the inner ring was 11.4 mm. The length of the

tuning stub was 6.22 mm with a width of 1mm. The substrate for this antenna was

provided from Rodgers Materials, and had a thickness of 1.27 mm and a permittivity of

10.2. Before testing in our anechoic chamber extensive testing was done using HFSS

software.

1.2 Results of Loop Antenna

The antenna was measured in our anechoic chamber, and with a network analyzer

to determine the return loss and radiation patterns under a variety of different

circumstances. Ferrite films were placed over the antenna (a), underneath the ground

plane (b), and in both positions (c) as shown in the following figures. We wanted to see

the effect of the ferrite film in this case, but did not expect as much of a shift due to our

inability to place the ferrite directly underneath the patch due to the fact that it had

already been fabricated.

71

Fig. 48 Ferrite Placement on Loop Antenna [18]

1.3 Conclusions for Loop Antenna

Interestingly enough we did see a very significant shift due to the ferrite film. The

central frequency of the patch without the films was about 1.72 GHz, and the -10 dB

bandwidth was 5MHz. Case (a), with a ferrite film above the patch showed a central

frequency shift of 20 MHz about 4 times its bandwidth. Placing a film underneath the

ground resulted in a smaller shift of only about 50% of the antenna without films.

However, it also resulted in an enhancement of the S11 return loss to -18.2dB. Placing

both films on the antenna resulted in an even larger shift. These results are clearly

72

indicative of the ferrite films ability to shift the overall size of the antenna downward,

which is the overall goal of this study.

When the radiation patterns of this antenna were looked into in our anechoic

chamber we saw exactly what we would expect. The Y-Z plane is symmetrical, and the

X-Z plane is asymmetrical due to the location of the tuning stub. These radiation patters,

as well as the detailed return loss measurements have been outlined in the following Fig.

Fig. 49 Return Loss of Loop Antenna [18]

What we can conclude from this study is that there is a miniaturization effect tied

to the use of the ferrite films in the antennas fabrication. We observed a 400% bandwidth

enhancement in the testing of the antenna, and it is certainly a valid approach to the

concerns raised from using ultra high permittivity ceramics in the fabrication of

73

miniaturized antennas. In future simulations we will also be using a ferrite layer

underneath the patch to see if that enhances this effect even more.

Chapter 5: High Permittivity Miniaturized GPS Antennas

1. Thin Linear GPS Antenna

1.1 Introduction

Now that we have thoroughly demonstrated the ability dielectric substrates to

reduce the physical dimensions of general patch antennas, and the ability of properly

placed thin films to counteract these substrates. We can now proceed to using ultra high

permittivity materials (K~91.7) to try and make a miniaturized GPS antenna.

To this end, an antenna has been designed to operate in the range of normal GPS

operation on an ultra high permittivity substrate Fig. 62. The goal of this design is to

enhance the matching impedance, and the bandwidth that are negatively altered by the

ultra high capacitance given by the substrate.

Fig. 62 (a) and (b) show the schematic top view and side view of the patch

antenna. This antenna consists of a rectangular patch with a slot on the right side, and the

width of the slot is 0.254mm and the length is 4.38mm. A metallic side-wall is adopted to

improve the antenna's directivity, with the same height as the dielectric substrate. A

circular ground plane with the radius of 101.6mm is added at the back of the dielectric

substrate. The feed point is located on the 45 degree diagonal, with a distance of 0.38mm

74

along the x-axis. The substrate has relative permittivity of 91.7 and a thickness of 1.0

mm. All the other parameters are listed in the caption of Fig. 4.

L1 L2

W2

R

Metallic wall

Substrate

L3

W3

Feed point

W1

(a) (b)

Fig. 50 Geometry of the Rectangular Patch Antenna. (a) Top view. W1=4.38mm,

W2=9.77mm, W3=1.2mm, L1=0.254mm, L2=9.70mm, L3=15.1mm and R=101.6mm. (b)

Side view, H=1.0mm. [17]

Utilizing new spin spray technology we were able to coat these substrates with a

2um (Ni1-xZnx)Fe2O4 Ferrite. Once this had been completed we deposited a 2um copper

layer with the use of our PVD (physical vapor deposition system). Photolithography was

then used to develop this copper into our desired patch antenna shape. Finally more

copper was added for each of the antennas ground plane. Based on this process we have

designed four different antennas, one with the ferrite layer above the patch, the second

with the ferrite layer below the patch, the third with the ferrite layer above the ground

plane and finally with two layers of ferrite film below the patch and above the ground

75

plane. Also the rectangular patch antenna didn’t contain a ferrite layer is acting as our

control for this experiment.

(a (b)

(c) (d)

Fig. 51 (a) Antenna with Magnetic Film above the Rectangular Patch (b) Magnetic film

under the patch. (c) Magnetic film above the ground plane. (d) Both above the ground

plane and under the patch. The thickness of the film is h=0.002mm [17]

1.2 Results

The results for this study were done via HFSS simulations. This was due to the

impracticality of the fabrication of the backed antenna design. We are currently

optimizing a way to drill the holes for the feedline utilizing an ultrasonic drilling

machine. However, at the time of this report the process has not been optimized. Our

results via HFSS do seem very promising however. The fist result would be that of the

return loss for the antenna under a variety of different film placements. As you can see

the ferrite film has a dramatic effect on the impedance matching on this antenna.

76

Fig. 52 Simulated Return Loss against Frequency for the Five Different Cases [17]

Fig. 53 Simulated radiation patterns of X-Z plane [17]

77

(c.)

Fig. 54 Simulated radiation patterns of Y-Z plane. [17]

1.3 Conclusion

In order to prove the validity of this project we first ran detailed HFSS

simulations on our material. Fig. 52 shows that the central resonant frequency of the

antenna without films is about 1.573 GHz, and the S11 peak magnitude of -14.5dB. When

a ferrite film is added above the patch, the resonant frequency shifts down to 1.572 GHz.

When a ferrite film is added under the patch, the resonant frequency is 1.573 GHz with a

peak magnitude of -19.4dB, indicating a better impedance matching. In the third case, a

ferrite film is added above the ground plane, the central resonant frequency is 1.568GHz

with a magnitude of 20.7dB, a shift of 5MHz relative to the non-magnetic antenna.

Finally two films are added above the ground plane and under the patch at the same time,

which moves the resonant frequency further down to 1.569 GHz. Therefore the patch

antenna combined with ferrite films can improve the impedance matching and shift down

78

the resonant frequency. Also the radiation patterns of these antennas are calculated with

the help of HFSS, as shown in Fig. 52 and 53. We can see that the radiations are

unaffected by the ferrite films, thus proving that the miniaturization does not compromise

the antenna gain.

However, as promising as these results are this antenna is impractical for

industrialization based on the fact that the bandwidth is much too low. Even though it

meets our design criteria of antenna miniaturization in the GPS frequency band, its

operating frequency is not wide enough for practical applications. In the next section of

this thesis we will outline how changing the thickness of our substrate can alleviate these

problems.

2. Thick Linear GPS Antenna

2.1 Introduction

In addition to our study of linear GPS antennas on high permittivity substrates, we

also wanted to look into the effect of using thicker substrates on our antenna designs. In

essence our belief was that using a thicker substrate should enable us to get our antennas

operating bandwidth wider, making a more practical antenna. Our design in this case is

rectangular and back fed, and shown in the figure below. This is a change from the

previous design, as when we looked at the electric field on the patch we saw problems

with the radiation in the previous case. This will be expanded upon later, though; in

79

essence these changes from the thin patch correspond to our intensive studies on the way

the antenna radiates. Square patches tend to radiate from the corners, while when the

patch is rectangular in shape we see better radiation from the leading edges, which in turn

give us better return loss and better overall antenna performance. This also results in a

much more stable antenna. The dimensions of this new antenna design are as follows.

L=.5089 in, W=.3587, and thickness of 5mm. Again, the antenna was designed to operate

over GPS frequencies.

Fig. 55 Design of Thick GPS Antenna

2.2 Results

As was to be expected the thick substrate resulted in a very good return loss. The

figure below indicates a return of over -30dB. We also looked into the impedance

matching for the antenna, in order to get a better understanding of the role that the ferrite

plays when it is incorporated into our designs. We can see in the case of the non-

80

magnetic antenna, that the matching is actually quite good with a real part of 49.31 ohms

and an imaginary part of -2.84.

Fig. 56 Return Loss of Thick GPS Antenna Design

Fig. 57 Impedance Measurement of Thick GPS Antenna

81

In order to understand how this antenna radiated, we looked into the electric field

on the patch itself. Here we can see very clearly how we are achieving linear polarization.

Both edges, pulse in phase one in the positive direction and one in the negative. As the

signal pulses so do the edges of the antenna. This exactly mirrors the behavior that we

would expect in an antenna that is radiating at the TM010 mode which was outlined in the

first chapter of this thesis, section 1.3.

Fig. 58 Electric Field on the Patch for the Thick GPS Case

We also, examined in detail the radiation parameters of this antenna, as well as

observed its gain and directivity. The directivity of this antenna was simulated to be

82

3.0897, and its gain was simulated to be 3.134. These numbers make perfect sense based

off its radiation patterns shown below.

Fig. 59 Radiation Pattern of Thick GPS Antenna

It is quite apparent from the results provided above that we were able to generate

a workable antenna in the GPS frequency on our thick high permittivity substrate.

However, we still wanted to add the ferrite films to see if we could further enhance the

size, and performance of this antenna. Our first study consisted of placing ferrite films

underneath the patch and seeing the resultant shift of the return loss. Remember, as

shown previously a shift of the return loss downward results in a miniaturization effect.

This is due to our ability to make the entire antenna smaller, while at the same time

maintaining its original frequency.

83

Fig. 60 Miniaturization effect of Ferrite Placement on Thick GPS Antenna

Fig. 61 Return Loss of Antenna vs. Ferrite Thickness

CF of Antenna with Ferrite Film Under the PatchCF of Antenna with Ferrite Film Under the PatchCF of Antenna with Ferrite Film Under the PatchCF of Antenna with Ferrite Film Under the Patch

1.52

1.53

1.54

1.55

1.56

1.57

1.58

1.59

0 1 2 3 4 5 6 7 8 9 10

Thickness of Ferrite Under Patch (um)Thickness of Ferrite Under Patch (um)Thickness of Ferrite Under Patch (um)Thickness of Ferrite Under Patch (um)

Frequency (GHz)

Frequency (GHz)

Frequency (GHz)

Frequency (GHz)

Return Loss of Antenna in dB vs. Thickness of Ferrite FilmReturn Loss of Antenna in dB vs. Thickness of Ferrite FilmReturn Loss of Antenna in dB vs. Thickness of Ferrite FilmReturn Loss of Antenna in dB vs. Thickness of Ferrite Film

-35

-30

-25

-20

-15

-10

-5

0

0 1 2 3 4 5 6 7 8 9 10

um of ferrite filmum of ferrite filmum of ferrite filmum of ferrite film

Return Loss

Return Loss

Return Loss

Return Loss

84

From the above figures we can clearly see the miniaturization effect of the ferrite

film underneath the patch. However, since the original antenna is so well matched we see

a negative shift of the return loss. This is to be expected. In our previous study, the

antenna was not well matched, and its physical dimensions did not promote good

radiation qualities. With our original antenna being so well matched, we would expect

introducing impedance mismatch would affect the return loss.

The figure below shows how the gain and directivity change as a result of the

change in ferrite thickness under the patch. You can clearly see that the shift is minor

from a directivity of 3.1 in the case of the antenna without ferrite films, to a directivity of

2.95 for the 8um of (Ni1-xCox)Fe2O4 Ferrite Case. The results for the gain are quite

consistent as well. Even though the changes are small there is a definite downward trend

to these parameters, as shown with the best fit lines.

Again, this is due to the case that our antenna was so well matched in the

beginning. For our previous study adding films to a not well matched antenna, matched

the antenna. This increased the return loss and therefore increased the gain as these

parameters are linked. For this study we see a lessening of the return loss, therefore we

would expect that the gain and directivities would decrease.

85

Fig. 62 Gain and Directivity of Thick Antenna with Ferrite Films

As previously stated this behavior is to be expected as adding ferrite films will

affect the matching between the antenna and substrate. This can be fixed however, by

changing the location of the feeding pin. For example, if we take the case of 6 um of

ferrite film underneath the patch, we can move the pin location to achieve over -35dB

return loss as shown in the following figures, which results not only in a better matching

than the non ferrite case, but also a equivalent bandwidth. This behavior is currently

being studied as we hope to increase the bandwidth of these antennas even more. As it

stands we can achieve approximately 3 MHz of bandwidth with these thick antenna,

while a significant enhancement over the thin case, we still need to increase this

bandwidth to make these antennas practical in industry.

Gain and Directivity vs Ferrite ThicknessGain and Directivity vs Ferrite ThicknessGain and Directivity vs Ferrite ThicknessGain and Directivity vs Ferrite Thickness

2.85

2.9

2.95

3

3.05

3.1

3.15

3.2

0 1 2 3 4 5 6 7 8 9 10

Ferrite Film Thickness (um)Ferrite Film Thickness (um)Ferrite Film Thickness (um)Ferrite Film Thickness (um)

Gain

Directivity

Linear (Directivity)

Linear (Gain)

86

Fig. 63 Return Loss of Thick GPS Antenna Design,

Adjusted Pin Location for Ferrite Film of 6um

Fig. 64 Impedance Matching of Thick GPS Antenna Design,

Adjusted Pin Location for Ferrite Film of 6um

87

2.3 Conclusion

We can conclude from this experiment many things. First, increasing the

thickness of the substrate dramatically improved the impedance matching. It also made

the antenna much more efficient. This was due to the ability to get a return loss much

lower than that of the original thin antenna case. Secondly the shape of the antenna,

which closely follows that of the Balanis design [19] outlined in chapter 1 of this thesis,

has dramatically improved the radiation performance. This can be attributed to the

antenna achieving radiation along the correct edges for TM010 mode.

For the ferrite film, we can make conclusions as well. First, the downward shift of

the center frequency as has been shown in Fig. 60 shows us how adding a ferrite film to

our antenna results in a lower center frequency. That lower central frequency allows us to

miniaturize our already small GPS antenna. This theory has been outlined in Chapter 1.

However, we can also see that adding the ferrite film to our antenna decreases its

return loss. This; again, is due to the fact that the original antenna was matched so well.

This is not a problem however, as we can easily determine a better pin location for each

case as shown for the 6um case. Therefore, the ferrite film allows us to get the

miniaturization that we are looking for in our antenna design.

88

Chapter 6: Thesis Conclusion

We have shown throughout this paper a valid process to making miniaturized

GPS antenna. We have done this by introducing how increasing the permittivity of a

substrate will decrease the dimensions of a patch antenna needed to radiate at a specific

frequency. We have also outlined how ultra high permittivity substrates (K~100) can be

fabricated. We have also shown how these substrates can achieve the desired magnitude

of other desirable antenna characteristics such as thermal stability, and low loss by

utilizing creative doping techniques. In doing this we have also shown the reader how

increasing the permittivity of the antenna while resulting in miniaturization will also

cause undesirable outcomes. These outcomes include the loss of antenna bandwidth, and

the inability of matching the antennas. These are two problems we then addressed in the

following chapters.

In order to prevent these things from happening we have introduced two distinct

ideas on dealing with them. The first was to use magnetic metallic films to improve the

radiation and help with the miniaturization of the antenna. We have shown that with a

2.1GHz patch antenna that we can achieve significant enhancement of these parameters.

The metal magnetic films helped with both the bandwidth and the impedance matching

between the copper patches and the alumina substrates that had been used. We were then

able to come to the conclusion that increasing the permeability of the layer between the

patch and the substrate would further help with this process.

89

To do this we would need to use some new type of material, high in both

permittivity and permeability, the material also had to function at frequencies at and

above GPS range. To this end we utilized a new type of ferrite material processed and

investigated at Northeastern. This material was a (Ni1-xCox)Fe2O4 Ferrite, and had a

resonant frequency above that of the GPS antenna, and many other self biased ferrite

films. We then incorporated this film into our antenna designs and again saw

enhancement of bandwidth, gain, and radiation performance of our antennas.

Next we examined different antenna designs, in particular how the size of the

antenna, the shape of the patch and the thickness of the substrate play in the radiation of

the patch. We showed how the thickness of the patch can increase the bandwidth of the

antenna, as well as that the shape of the patch effects the radiation.

Finally, we were able to simulate a new series of antennas which utilized all of

these concepts. A rectangular design, with a thick substrate that by utilizing ferrite films

we were able to make were under our design goal of .5inches by .5inches. This antenna

operated at GPS frequencies with a bandwidth of 3MHz; thus realizing our design goal of

miniaturization of antennas utilizing ultra high permittivity materials and other novel

magneto(di)electric materials.

90

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