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MINIATURIZATION METHODS FOR NARROWBAND AND ULTRAWIDEBAND ANTENNAS

John L. Volakisvolakis.1@osu.edu

University of MichiganAnn Arbor, MI 48109-2122

http://www-personal.engin.umich.edu/~volakis

The Ohio State University ElectroScience LabElectrical & Computer Engineering Dept.

Columbus, Ohio 43212-1191http://www.ece.osu.edu/~volakis

.

.

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ECE Dept at Ohio State

• 50 Faculty members (No. 14 in size)• 1000 Undergraduate students (OSU has 50,000 students)• 340 Graduate students (no. 15/16 in M.Sc./Ph.D graduation)

• $7.1 M instructional budget• $17M Research Expenditure in 2002-2003 (among top 4

public universities)

• Areas: RF and Electromagnetics, Solid State & Electronic Mat., Control, Intelligent Transportation, Mixed Signals, VLSI, Comm. Systems & Wireless, Computer Vision, Networking, Robotics, Multimedia, Signal Processing, Power Engineering, High Performance Computing

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The ElectroScience Laboratory is a major Center of Excellence in the College of Engineering.• Graduated over 311 Ph.Ds. and Over 557 M.Sc. Degrees• Graduate 17 Ph.Ds/M.Sc. degrees per year Major research areas:

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ProfileFaculty: 10Research Scientists: 16Active Emeriti 4Students: 67Undergraduates ~17Annual Research Expenditure: ~$5.8Million (FY04), $7.(FY05)EM Courses Taught: 21 grad. courses

RF technology Frequency Selective Surfaces and VolumesComputational electromagneticsBioelectromagneticsWireless Propagation and SystemsElectromagnetic compatibility New materials (metamaterials) Sensors (systems and components)Microwave/Millimeter Circuits and SystemsMulti-physics engineering for sensors, bio- and RF-MEMSGround penetrating radarsRadar scatteringRemote SensingOptics and Photonics Satellite and ultra-wideband (UWB) communicationsAntenna analysis and design:

antenna miniaturizationUWB antennasreflector and feed designlarge phased arraysreconfigurable, and adaptive antennasnavigation systems

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UG Students Building UWB Radars

New radar technology to expand operationalfrequency to 100GHzand improve accuracy of futurecompact ranges.

E4991A RF

Materials CharacterizationMeasurement Radar: 400MHz-100GHz

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RF and Large Scale Modeling and Simulations

6

Metamaterials for Microwave Devices

7

Why so much interest in antennas and RF technology?

Ubiquitous wireless comm.More than 50% of a system-on-a-chipconsists of passive RF devices

•Automotive sensors•Air quality monitoring•Ground water monitoring•Home applications•RFID

Sensors

Wireless System Receivers

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Presentation Focus• Miniaturization Issues and Techniques

– Material Loading and Design• Polymers• Low Loss Ceramics (LTCCs and HTCCs)• Artificial magnetics and low loss ferrites.• Mixtures of materials to yield new properties

– Impedance matching bandwidth• Termination techniques

– Bandwidth vs. Size• Challenges for sizes below 0.2λ

• Metamaterials– Combinations of materials is known to exhibit novel properties for RF

applications.– Magnetic photonic crystals (MPCs), textured dielectrics, left-handed

materials, all fitting within the class of meta-materials exhibit novel and useful properties yet to be harnessed.

– Propagation, k-ω diagrams, Q and BW, are all affected by the choice and combination of materials.

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• Materials is a largely unexplored direction for antenna and RF applications– Materials design can enable antenna miniaturization,

bandwidth increase and multi-functionality – Dielectric modifications, and magnetic materials can

introduce new RF properties

LTCC samplesBarium-tetra-titanate, εr = 37 Rare earth titanate , εr = 85Calcium/Barium titanate, εr = 90

High-Temperature CeramicsLow-Temperature Cold Fired Ceramics (LTCC)Epoxy Bonded Ceramics Ceramic components

2 mm

Materials for EM Applications

• Sensors Systems (System or Lab on a Chip)• Will require the integration of several materials.

• Radio Frequency (RF MEMS)

New materials can mitigate failure in MEMS for reliable long term operations

substrate

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• “pliable” substrates with εr of up to 20.• Mixing of ceramic powder (BBNT) with

Silicone (PDMS)

• Fabricated via Particle Dispersion

Polymer-Ceramic Composites for Conformality and Miniaturization

D

H

~21-22

~5.5

~3.5

Measured ε

8.36-9.5 6.73.39 55.45 Particle 2

21-22.4 204.56 85.59 Particle 3

4.85-5.91~32.71 55.32 Particle 1

Expected εBaTiO3 [% Vol]H [mm]D [mm]

Mixture of BBNT (ceramic) / PDMS (silicone)

Particle 1Particle 3

Particle 2

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Algorithmic and Hardware Speed Ups are Fueling Design into Mainstream

Concurrent Hardware and Algorithmic Speed-Ups are Fueling Computational Algorithms into Mainstream Design

5-fold CPU Speed-Up Over Last 5 years

Processor CPU Speed

Unprecedented Algorithmic Speed-ups (from cubic down to linear)

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Antenna Miniaturization Techniques

1. Slow down current flow by material loading• Homogeneous material

(broadband)• Inhomogeneous, anisotropic

material (narrowband)• Magnetic Material (narrowband,

bias required)2. Control current phase velocity by

controlling L and C of equivalent transmission line model

L

L

L

L

C CC

effeffp CL

v 1=

eff

effc C

LZ =

k

ω

β∂∂

=k

cvg

βk

cvp =

slow w

ave

fast w

ave

k

ω

Temex Microwave Ferrite Materials Catalog, http://www.temex.net/temex/catalog/TEM01.pdf

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The Payoff in Material Optimization (Dielectric and Metallic Optimization)

GAcase2.01a.s-band(Four patch array with changeable PEC surfaces)

• 6cm × 0.2cm

• 13 Dielectric and 16 PEC positions

• Dielectrics used for runsε = 1, 2.2, 6.2, 10.2, 20, 25, 30, 40

•Frequency range 1.5-3.5GHz(0.3λ0-0.7λ0)

-3-2

-10

12

3 -3-2

-10

12

3

00.10.2

Dielectric Map

Diel ID

100

102

104

106

108

110

112

-3-2

-10

12

3 -3-2

-10

12

3

00.10.2

PEC Map

PEC ID

Air

PEC

2

4

6

8

10

12

14

16

Feed Locations

Gai

n (d

Bi)

BW (%)

Size (λ)1

1

2,3

2, 3

4

0.3 0.4 0.5 0.6 0.7 0.80

5

10

15

All non-zero data pointsMax

Size (λ)

Gai

n (d

Bi)

Number of non-zero data points= 2428

5,6

Optimal Gain Points

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Antennas Design Challenges

• Impedance Matching and Control• Bandwidth and Multi-functionality• Much Smaller Size• Material Availability

Minimize Reflections

• Wave Slow Down for miniaturization• Material Design for impedance Matching

PARAMETERS:

HOW? 2-8 times Smaller

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Achieving Optimal BW/Q Performance with Material Design

),( με ),( με

),( με

Current along 2” slot spiral for various loadings---shrinking radiation circle

f where size is 0.09λ

Theoretical gain performance of 6”(perfect real impedance matching)

Mixing smoothes-out impedance

• Spiral antenna impedance exhibits erratic trends with miniaturization via loading. • However, combined (εr,μr) superstrates have significant affect on impedance, and so

do properly placed lumped loads

),( με

,r rε μLTCC Superstrate

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Q is a Larger Issue for Small Antennas

• Q is typically small for wavelength size apertures, implying mostly real impedances (ZA), which are readily matched.

• Broadband (wavelength size) apertures, like the spiral, are also associated with nearly constant Re(ZA)=RA

• Matching becomes more challenging for small antennas due to the necessarily larger values of Q.

• However , higher Q implies large reactive impedances, that also change with frequency, implying greater challenges for matching small antennas.

• Our most recent design is at the limit of miniaturization

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0

441Q

RZRQjRZ

A

AAA

+=Γ−⇒

≡±=

measured data

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Impedance Changes as Antenna Size is Reduced…

• Resonant Antenna Example

m=1 m=1.25

•Each dipole, carrying a normalized current• compressed λ/2 sinusoid

m=2m=miniaturization factor

Impedance is significantly reduced with size reduction, even if dipole remains resonant

Re(Z) scales as m (λair / λcurrent ), but amplitude is scaled by m2

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Miniaturization via Artificial Transmission LineIn-Line Capacitive and Inductive Loading

effeff

eff

eff

CLv

CL

GZ

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0

==

==

με

εμ

Unit Length ATL

Llump

:b

λ<<

Llump

Clump

baCbCa

C

baLbLa

L

lumplineTeff

lumplineTeff

+

×+×≈

+

×+×≈

−

− 2

LT-line, CT-line: inherent to structure

a

See paper: xxx

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Use inductor only

Use capacitor/dielectric only

Use L & C together to maintain impedance

Z: characteristic impedance of ATLZ0: characteristic impedance of untreated T-line

(miniaturization factor λ0/ λ)

eff

eff

CL

Z =

L=7.5 nH C=0.14 pF

L=15 nH C=0.28 pF

L=22.5 nH C=0.43 pF

L=37.5 nH C=0.72 pF

L=60 nH C=1.15 pF

L=75 nH C=1.44 pF

Maintaining Constant Impedance While Slowing-Down Wave Speed

Slower Speed

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Proposed Miniature Spiral Design

Prediction for Miniature Spiral

+5

0

-15

Frequency (MHz)Total Gain (dBi)

( ) ( ) )("2800

inchesWM ATLe

××

≈βεα

Wc

4≈

Wc

2>+5

0

-15

Frequency (MHz)Total Gain (dBi)

( ) ( ) )("2800

inchesWM ATLe

××

≈βεα

Wc

4≈

Wc

2>+5

0

-15

Frequency (MHz)Total Gain (dBi)

( ) ( ) )("2800

inchesWM ATLe

××

≈βεα

Wc

4≈

Wc

2>

• Square Spiral Dimensions– W = 6 inches– h = 0.9 inches– εr = 10– MATL = √10

3.0−≈ eεα ( ) 3.02 −≈ ATLMβ

Typical 6” Spiral with ground plane (1” separation)

> 1000+5≈ 5000≈ 96-15

Frequency (MHz)

Total Gain(dBi)

0

0ε

μ>>Z

1>>rε

ATL square spiral arms low-dielectric filler or air

40dBiW λ

≈

r

dBihε

λ20

15−≈

Variable Metallic and Magnetic Properties Backing

Key FeaturesVariable Material for MatchingEmbedded L/C to emulate ferritesVariable Ground PlaneVariable Spiral Growth

Original was 400-450MHz

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Embedded

ATL Implementation with Embedded Inductive Loading

• Embedded inductive loading is a lower loss alternative to lumpedelements; it can be printed using standard PCB fab processes

• Achieved so far a MATL factor of ~2; this needs to be increased to 3 to meet the goal of -15dBi gain at 100MHz

Shift corresponds to MATL = 1.8

Unloaded Spiral Section

Imbedded Inductive Loading Section(Solenoid)

via

Top Layer Trace

Bottom Layer Trace

Top View of PCB Top Layer

3D View

Unloaded Spiral Section

Imbedded Inductive Loading Section(Solenoid)

via

Top Layer Trace

Bottom Layer Trace

Top View of PCB Top Layer

3D View

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Significant 60% Reduction with ATL Alone

FREE STANDING

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Integration of Ferrite Backing, Artificial LC & Dielectric Loading

Multiple Growth Rate Spiral with Imbedded ATL loading

0.75”

6.5”Ferrite Backing

Cross Section:6”εr = 9 0.625”

> 1000+5≈ 5000≈ 96-15

Frequency (MHz)

Total Gain (dBi)

Original was 400-450MHz

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Optimization of size, loading and gain

10-2 10-1 100-2

-1

0

1

3

4

5

1.76

Circumference / λair

Dire

ctiv

ity (d

B)

Broadside Directivity of a Circular Loopm=1

m=2m=4m=10

Radiation profile impact

Pattern impact

Freq.

Gain

Diminishing return

dB3≈

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Design Optimization

0.2 0.3 0.44

2

0S11 (dB)

frequency (GHz)0.2 0.3 0.4

40

20

0

20Gain (dBi)

frequency (GHz)

Initial designvariables

FieldAnalysis

Optimization Solver (SLP)

Converge?

Variable change

Optimal topology

Sensitivity

YES

NO

via density function

•Within 240-320MHz, antenna size ranges from 0.12λ-0.16λ

•BW requirement is ~30%

•Gain requirement ~ 5dB

•EDO COTS is 0.29λ-0.39λ in size•BW is 20%•Gain max is 0-2.5dB

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Example Patch Design•Designed broadband miniaturized antenna via automated topology design•Achieved AQF= 5.6 via volumetric graded metamaterials;•Fabricated and validated designs

Initial standard material

1.3 1.4 1.5 1.6 1.7 1.8 1.9 230

25

20

15

10

5

0

frequency (GHz)

S11

(dB

) Measurements

Optimized Graded Design Fabricated Design

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Controlled Design Approach

Total Dimensions:15cm × 15cm × 6cm

Gain ~ 4dBiBW ~ 18% @ 500MHz

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Optimization Approach

Video Clip

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Fabricated Prototype

Top layer (textured

dielectrics)

Loaded cavity

Feed layer

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Final Design Gain Measurements

• Gain is consistent with return loss

• Maximum measured gain = 4.5dBi at 498MHz

• 23%BW at 375MHZ• This design is 2 times smaller

than 14.2” COTS• Has 2.5dB more Gain (still CP)• Only ∼λ/20 Thick

0.4 0.43 0.46 0.49 0.53 0.56 0.59 0.62 0.6515

10

5

0

5

SimulationMeasurement

frequency (GHz)

Gai

n (d

Bi)

Ohio State Univ. Compact Range

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.815

10

5

0

5

SimulationMeasurement

frequency (GHz)

Gai

n (d

Bi)

Design 1

Design 2

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Novel Crystals• Antennas can exploit unique crystal

properties constructed from available materials to increase their received power by as much as 15 dB.

• Key crystal properties– Incoming signal is nearly all

‘transmitted’

– Crystal causes wave speed slow down (500 to 100000 slower, depending on materials

– Crystal causes huge amplitude increase by a factor of 10 to 100), implying comparable increase in antenna sensitivity

– Large dielectric constant leads to miniaturization and allows for packing very large arrays for higher power reception.

• By comparison, ordinary dielectrics cause signal reflection at dielectric interface (mismatches), lowering sensitivity and antenna gain. Thus, small antennas in high contrast dielectrics have low sensitivity.

Pulse Compression &Huge Amplitude Increase

Incoming Signalis transmitted intothe crystal with little reflection

Ordinary Antenna

Antenna in Crystal

Video Clip

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Using Crystalsfor Miniature High Gain Antennas

Regular band edge:

ω − ω0 ~ (k − k0) 2

Degenerate band edge:

ω − ω0 ~ (k − k0) 4

Double band edge

ω

ωω

k kk k0 k1

ω(k)

ω0

kStationary Inflection Point

ω − ω0 ~ (k − k0) 3.

(ω − ω0)1/4(ω − ω0) −1/4(ω − ω0) 3/4(DBE)1(ω − ω0) −1/3(ω − ω0) 2/3(SIP)

(ω − ω0)1/21(ω − ω0) 1/2(RBE)

TransmittanceSaturation Amplitude

Group velocity

Degeneracy at ω = ω0

Degenerate Band EdgeRegular Band EdgeStationary Inflection Pointin Magnetic Photonic Crystals

.

.

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… the Bottom Line…A2F A1

φ1φ2

A2F A1

…...

(ε0,μ0)

⎥⎦

⎤⎢⎣

⎡

)(

)(

i

i

HE

(ε0,μ0)

⎥⎦

⎤⎢⎣

⎡

)(

)(

t

t

HE

N CELLS

1st Unit Cell

Nth Unit Cell

φ1φ2

⎥⎦

⎤⎢⎣

⎡

)(

)(

r

r

HEM0

• Photonic crystals (MPCs), textured dielectrics, left-handed materials, all fitting within the class of meta-materials, exhibit novel and useful properties yet to be harnessed.

• Propagation, k-ωdiagrams, Q and BW, are all affected by the choice and combination of materials.

Video Clip

Small Dipole

34

Fabry-Perot & Photonic Crystal Resonators

Free Space

Dielectric Slabs Free

Space

Basic Fabry-Perot Resonator:

Frequency

Tran

smitt

ance

Fabry-Perot Peaks

Magnetic Photonic Crystal:

K

Freq

uenc

y

K

Freq

uenc

y

Fabry-Perot Peaksdue to finite thickness Semi infinite

Tran

smitt

ance

Frequency

Tran

smitt

ance

Frequency

Photonic Crystal:Wave slow down

when trans. vanishes

Good Transmissionwith wave slow down

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Frequency

Tran

smitt

ance

Slow Propagation

Tran

smitt

ance

Frequency

Tran

smitt

ance

Frequency

Frequency

Tran

smitt

ance

Band Edge vs. Magnetic Photonic Crystalswith realistic loss tangents

410−=δtan

Photonic Crystal:50 unit cells

410−=δtan

Magnetic Photonic Crystal:40 unit cells

A A F

ϕ1 ϕ2

100=ε

1=ε

Greater BW as compared to the regular photonic crystals

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B. Temelkuran, M. Bayindir, E. Ozbay, R. Biswas, M. M. Sigalas, G. Tuttle, and K. M. Ho, “Photonic crystal based resonant antenna with a very high directivity,” Journal of Applied Physics, vol. 87, no. 1, pp.603–605, Jan. 2000.

Detected power inside the crystal cavity

Detected power without the crystal

-- Dipole voltage and current increase withhigher electric fields ....---Similar to a dipole in a resonating cavity.---frequency and BW is controlled by Fabry-Perot resonances---MPCs and DBEs allows for greater BW, and matching around the SIP

Simple example: PEC ground planesi iE ,H

/ 4λ

i2 E

i2 H

How is power increased?

It is well known that the dipole reception is bestwhen placed λ/4 away from GP

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Total Received Power within DBE

0 50 100 150

-60

-50

-40

-30

-20

-10

0

8.5dB17.4dB

dB

Ө (in degrees)

)R/V 10log( Normalized INOC 82

11 elements5 elementsSingle elementSingle element in ε = 100 d = 1λ

11th

unit cell

z

x

Eθ

0d 0.93= λ

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Experimental Verification

Several Issues– Availability of materials (particularly

anisotropic)– Low Loss– Manufacturing accuracy– Crystal size– Maintaining the slow-wave

phenomenon in 3D configurations– Experimental Set-up

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Repeated sequence of parallel plates:A1 Anisotropic dielectric A2 A1 rotatedF Faraday rotation plate

Available Crystals to Form Periodic Assembly

TiO2/Al2O3

CaV-doped YIG

2 in

10-40 units

A1 layerAl2O3|TiO2

F layerCVG garnet

0.02 in12.5º

A2 layerRotated Al2O3|TiO2

1 unit2 in

Laminate alternating Al2O3|TiO2substrates, followed by cutting sheets in the perpendicular direction.

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Cheaper Approach: DBE CrystalsUsing Misaligned Printed Circuit Structures

Metallic strips with an offset angle, φ

Regular Band Edge Degenerate Band Edge Double Band Edge

Isotropic hoste.g. free space

a2

a3=1Δa3=0.01

W=0.1

a1=1Δa1=0.01

k-ω diagram for dy2=1.5

0.40.60.4dy2

k-ω diagram for dy2=1.2 k-ω diagram for dy2=0.6

y

x z

0 pi/2 pi 3pi/2 2pi0

0.05

0.1

0.15

0.2

0.25

ky

wa/

c

0 pi/2 pi 3pi/2 2pi0

0.05

0.1

0.15

0.2

0.25

ky

wa/

c

0 pi/2 pi 3pi/2 2pi0

0.05

0.1

0.15

0.2

0.25

ky

wa/

c

Tuning free space thickness degenerate band edge behavior

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡=

1 0 0 0 02.1 0

0 0 .18

Aε

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AGILENT TechnologiesE8362B 10MHz-20GHz

PNA Series Network Analyzer

Receiving AntennaPyramidal Horn

2.9”x2.128”

Transmitting AntennaPyramidal Horn

1.85”x1.28”

Crystal Sample4.3”X4.3”x2.3”

Holder (Foam)

d1≈0.5” d2 ≈ 10” Φ°

Experimental Set-Up for Measuring DBE Assembly

•Channel calibrated without the crystal

•Optimum polarization found by rotating the crystal by an angle Φ°

•S21 measured

8 Layer Crystal

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Transmission Characteristics:Demonstration of kω Diagram

7 8 9 10 11 12 13 140

0.5

1

1.5

Frequency(GHz)

|S21

|

FSDA, Mesh3experiment, d1=0.5"

0 pi/2 pi 3pi/2 2pi0

5

10

15

20

ky

Freq

uenc

y(G

Hz)

RBE

DBE

Double BE

•Transmission is associated with Fabry-Perot peaksclose to band edges

•Simulations validated by the experiment

•Edge effects are minimized by placing the crystal closer to the transmitter

•Over transmission due to focusing effect

Simulations: FSDA∞

∞∞

∞

•Plane wave excitation on infinite extend crystal

Experiment:

•Finite extend crystal excited by a horn

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Sleeve Monopole

h<λ/5Optimum Orientation

y

zx

d1≈0.5”5.554.543.532.521.510.50

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

P os it ion, z (cm )

|Ey|

FSDAE xperim ent

7 8 9 10 11 12 13 14 15 16-30

-25

-20

-15

-10

-5

0

5

10

Frequency(GHz)

|S21

| dB

3rd layer4th layer5th layer

Field Amplitude Measurement vs. Simulations

•Transmitting antenna is rotated to receive optimum polarization

•Field is sampled at mid-positions of each free space layer

•Measured field amplitude at the DBE frequency follows simulations

DBE Frequency

y-pol

44

Received Power Gain greater than 7dB

Experiment 2Antenna Reception

•Monopole antenna reception increases by a remarkable 7.4226 dB when embedded in the middle of the 4-layer Crystal

•Rutile crystals will be employed for improved performance.

9 9.5 10 10.5-2

-1

0

1

2

3

4

5

6

7

Frequency(GHz)

|S21

| dB

Without the crystalWith the crystal

substantial bandwidth(consistent growth)observed up to 7.4dB)

Antenna probe

1.1” thick crystal

45

Closing Remarks• Material design holds a great promise in new devices,

and will be of interest for many years• Design loops are not limited by the EM formulation, but

require stable, very fast and convergent solvers—this is a new challenge for the CEM community

• Manufacturing of the engineered materials (at the micro or nano-scale level) present an added and concurrent challenge

• Materials design is a multidisciplinary field (Math, EM, mechanical, thermal, material science)