7. Liquid Crystal and Liquid Crystal Polymer based Antennasshodhganga.inflibnet.ac.in/bitstream/10603/48026/13/13-chapter 7.pdf · Liquid Crystal and Liquid Crystal Polymer based

Liquid Crystal and Liquid Crystal Polymer based Antenna Chapter 7

152

Chapter 7

7. Liquid Crystal and Liquid Crystal Polymer based Antennas

7.1 Introduction: Depending on the temperature, liquid crystal (LC) phase exists in

between crystalline solid and an isotropic liquid. In this state the material can flow

like a liquid but at the same time molecules have orientation order. A typical LC

molecule has a rod like shape as shown in the Fig 7.1. The size of the molecule is

typically is few nanometres. This shape anisotropy causes anisotropy in terms of

dielectric constant.

Fig 7.1 Typical Liquid Crystal Molecule and its temperature dependency

Cross section of Liquid Crystal based molecule orientation with bias voltage is

shown in Fig 7.2. Depending on the RF field distribution and

n , LCs feature

anisotropic electrical properties. Thin polyimide film is coated on the inner surface of

the substrates to orient the LC molecules parallel to the surface initially.

Fig 7.2 LC Molecules orientation with applied bias voltage


153

At this level the RF field distribution is perpendicular to the director

n as shown in

the Fig 7.2 and the relative permittivity and loss tangent along the short axes are

effective. The orientation of the molecules will be same if the applied bias voltage is

less than the threshold voltage. If applied voltage exceeds certain voltage Vmax then

all molecules will be aligned parallel to the bias voltage. When voltage is released,

then molecules return to the initial state due to the polimide film. Required time for

this process is defined as switching time which depends on LC material, its

thickness, temperature and its oriention mechanism. All other states between ε|| and ε

┴ equivalently continuous tunability, can be achieved by varying the applied voltage

between the threshold voltage and Vmax. A general characteristic of LC that is

dielectric constant and loss tangent versus bias voltage is plotted in Fig 7.3.

Fig 7.3 εr and tan δ characteristics of LC material Vs bias voltage

Nematic phase state liquid crystal is a non linear dielectric material in which the

dielectric constant can be changed between two extreme states that are described by

the orientation of the liquid crystal molecules, either parallel or perpendicular to the

exited RF field. The effective dielectric anisotropy can be defined as

∆ε = ε|| - ε ┴

Where ∆ε = Dielectric anisotropy

ε|| = LC Permittivity with DC voltage

ε ┴ = LC Permittivity without DC voltage

Instead of placing a microstrip line on the top substrate, microstrip patches can be

realized as antenna elements. Reconfigurable reflect arrays also can be designed

based on the principle of variable patch dimensions. Instead of changing the

dimensions of the metalized patch, dielectric properties of LC under the patches are

tuned with a bias voltage. Although all the patches have identical physical

dimensions, they have different electrical properties. This results in different


154

backscattered phases. Beam forming is possible as different patch lengths from a

feed to the patches are compensated by the preadjusted phases of the patches.

Our intension is to use the liquid crystal and liquid crystal polymer based materials

as substrate materials in the design of microstrip patch antennas. By looking at the

basic operation mechanism in terms of dielectric anisotropy with change in bias

voltage between two electrodes, we can tune the antenna to our desired frequency

and digital controllability will be in our hand. Liquid crystal substrate material can be

attained with LC cavity by placing spherical spacers with small diameter between

two substrates. Although antenna consists of three dielectric layers which are top

substrate, LC layer and bottom substrate the thickness should be maintained low to

attain compactness in size.

7.2 Liquid Crystal Polymer Material

To reduce the overall size, structural complexity and the cost of RF systems,

multilayered substrate materials in which passive elements are Integrated as

distributed elements inside the substrate/packing layers is required. A solution for the

compact integration of these passive components, potentially with embedded active

chips as well is the system-on-package (SOP) approach. In SOP package, connecting

passive and active components on the board and encapsulating the assembly inside of

a robust package are two steps critical to reliable operation of the RF systems. The

package should create an acceptable environmental seal without significantly

affecting the electrical characteristics of the entire circuit. Specifically, the electrical

discontinuities created from the package should create minimal reflections and

minimal inductive or capacitive parasites to avoid throwing off the sensitive

matching circuit of active devices. The most important material characteristics for

use in SOP systems are low electrical loss and excellent seal integrity, generally

specified by permeability to moisture and gases.

So many engineered materials are excellent in performance, but expensive in the

fabrication. For example alumina has dielectric constant 10, which makes it

appropriate for so many antenna applications. In addition, alumina has a high thermal

coefficient of dielectric constant, which means its dielectric constant changes

significantly with temperature. This is why network analyzers, which frequently used


155

alumina substrates, are often left turned on continuously to ensure the dielectric.

Properties of the RF substrates are stabilized. A lamination of these microwave

composites and alumina materials is that none is capable of creating homogeneously

laminated compact 30 integrated RF modules. In other words, these microwave

boards must use other adhesive materials, which have significantly worse water and

gas permeability characteristics to achieve a compact stacked configuration. Fully

hermetic packaging with microwave boards is often required for military

specifications or in satellites, but at much greater financial expense. The term

hermetic means that zero water or gases will permeate the package.

Low temperature co-fired ceramic (LTCC) is the most commonly used ceramic

substrate material for compact RF-design systems. LTCC has very low electrical loss

and can be used in multilayer laminated modules that have densely integrated passive

and active devices stacked and connected vertically to save space and cost. The

major drawback with LTCC is with its high dielectric constant which decreases the

antenna radiation efficiency.

7.3 Liquid Crystal Polymer Substrate

LCP has drawn much attention for its outstanding packaging characteristics.

LCP is low-cost material with the best packing characteristics of any polymer has

generated great interest in using it as a substrate material for mm-wave applications.

A comparison of these packing characteristics Vs all other polymers are shown in Fig

7.4. Low water absorption should be there for microwave substrate materials for

better stability and reliability. Generally for organic materials the range of water

absorption characteristics is from 0.02% to 0.25% or more.

Fig 7.4 Water and oxygen permeability of different polymers


156

Fig 7.4 shows the water and oxygen permeability of different polymers, in which

liquid crystal polymer is having low water vapour transition. Not only for less water

absorption characteristic, but also for several reasons we prefer LCP material in the

design of RF and microwave modules. Some of the key features are

LCP is having excellent high-frequency electrical properties, stable ɛr and

low loss tangent 0.002-0.004 for frequency < 35GHz.

Quasi-hermetic

Low coefficient of thermal expansion(CTE)

Recyclable

Cost is less

Naturally Non-flammable(Environmentally Friendly)

Flexible

Multilayer all LCP laminations capabilities to create multilayer LCP RF

modules

Relatively low lamination processing temperature(=285 00 c)

Low dielectric constant for use as an efficient antenna substrate.

Fig 7.5 (a) Inset fed Microstrip antenna on LCP Substrate (b) Twisted LCP antenna (c) Bended

LCP antenna (d) Rolled LCP Antenna (Courtesy by Google Images)

Fig 7.5 shows Inset fed flexible liquid crystal polymer based microstrip antenna,

which can be twisted, bended and rolled without affecting its performance

characteristics.


157

7.3.1 FELIOS LCP

FELIOS liquid crystal polymer is a flexible circuit board material, which has high

frequency characteristics and low loss tangent after moisture absorption. Now a days

in so many applications like notebook PC’s and smart phones these materials are

been placed instead of traditional materials. The features of this material includes

a) High dimensional stability

b) Good peel strength of copper foil

c) Excellent high frequency properties

7.3.2 Ultralam ® 3850 (Liquid Crystalline Polymer Substrate Material)

Ultralam ® 3850 is the emerging liquid crystalline polymer circuit material from

Rogers Corporation. The features like excellent high frequency properties, good

dimensional stability, external low moisture absorption and flame resistant makes

this as one of the potential substrate material for RF circuit design. So many benefits

are associated with this material, which are listed below

Excellent and stable electrical properties for impedance matching

Uniformity in the thickness for maximum signal integrity

Flexible material for conformal applications i.e. bends easily

In humid environment it maintains the stable mechanical, electrical and

dimensional properties.

The applications includes high speed switches and routers, chip packing, MEM’s,

military satellites, radar sensors, hybrid substrates, handheld RF devices and in the

design of antennas.

0 2 4 6 8 10 122.5

3

3.5

4

4.5Frequency Vs Dielectric constant

Frequency

Die

lect

ric C

onst

ant

LCP at 50 CLCP at 23 CFR4 at 50 CFR4 at 23 C

Fig 7.6 Frequency Vs Dielectric constant of LCP and FR4 at 230 C and 500 C


158

The electrical and environmental properties of Ultralam ® 3850 at 10 GHz, 230 c are

tabulated as shown in Table 7.1.

Table 7.1 Electrical and Environmental Properties of Ultralam ® 3000 LCP material

S. No Parameter Value 1 Dielectric Constant 2.9 2 Dissipation Factor 0.0025 3 Surface Resistivity 1x1010 Mohm 4 Volume Resistivity 1x1012 Mohm cm 5 Dielectric Breakdown Strength 1378(3500) KV/cm(v/mil) 6 Water Absorption (230 c, 24 hrs) 0.04%

7.4 Package and Interconnecting

Traditionally with high dielectric constant materials, introducing air cavities inside of

multilayer RF modules for embedding chips or other elements creates impedance

discontinuities that cause reflections and can destroy RF performance. In addition

many package cavities rely on metal bonding rings around the cavity interface, which

necessitates more difficult feed through solutions such as re-routing and tapering the

transmission line underneath the seal. A unique possibility with LCP, because of its

low dielectric constant and multilayer lamination capabilities, is to form cavities in

the substrate before lamination to provide sandwiched all LCP constructions that can

pass transmission lines directly through the package interface with negligible effects

on the RF performance.

The LCP low dielectric constant would enable the superstrate packing to

accommodate chips, MEMS and other devices without any concern for the parasitic

packing effects. Because of the flexibility and low cost, the LCP can be used in

conformal antenna designs.

7.5 Flexibility Consideration

Flexibility is one of the key factors for LCP materials usage in the design of

conformal antennas. Tests were performed on the mechanical rollability and the

effects of rolling on antenna performance. The procedure for performing this antenna

testing included


159

1. Ensuring measurement repeatability when connecting/disconnecting antenna

in the default flat state

2. Performing flexure testing on the antenna, rolling it on to tubes with various

diameters

3. Re-measuring and observing once again the potential differences in

measurement or visual structural changes.

In this procedure we observed that there are very minute changes in the reflection

coefficient of antenna at resonating frequency and in all the cases, there is no

frequency shift is observed.

7.6 Liquid Crystal Patch Antenna

People are attracting towards the development of microwave tuneable devices for

various applications. To develop these devices, liquid crystals are gaining much

attention because of their anisotropic behaviour, which permits to change in the

resonant frequency and reflection phase. By using ferroelectric phase shifters and

varactor diodes, we can implement electronic beam scanning but they will increase

the system complexity and cost.

(a)

(b)

Fig 7.7 Rectangular patch antenna on LC Material, (a) Antenna Model, (b) Side View

A rectangular patch antenna is designed to operate at X band with N15 liquid crystal

material as substrate with thickness of 650 µm on ground plane of dimension 42X27

mm as shown in Fig 7.7. Electrodes are connected between patch element and

ground plane to apply biasing voltage. Voltage is applied through electrodes to

antenna from 1V to 20 V and it is observed that dielectric constant is varied between


160

2.17 to 2.27 in this range. Fig 7.8 shows the change in resonant frequency with the

change in bias voltage and Fig 7.9 shows change in reflection phase with change in

bias voltage and dielectric constant.

9.6 9.8 10 10.2 10.4 10.6 10.8-25

-20

-15

-10

-5

0

Frequency in GHz

Ref

lect

ion

Coe

ffici

ent i

n dB

18 V4 V

Fig 7.8 Return loss Vs Frequency of LC antenna at two stages of voltage

The simulated tuning range was 6% while the measurement is showing around 4%.

Losses in the liquid crystal material are giving antenna efficiencies ranging from 30-

35% at these frequencies when the liquid crystal was in unbiased state. If we

consider at millimetre wave frequencies, the LC exhibits lower loss tangents with

highly efficient in performance.

9.6 9.7 9.8 9.9 10 10.1 10.2 10.3 10.4 10.5 10.60

50

100

150

200

250

300

350

Frequency in GHz

Ref

lect

ion

Phas

e (d

eg)

4 VLC 2.10, tanD 0.065LC 2.27, tanD 0.06518 V

Fig 7.9 Frequency Vs Reflection phase of LC antenna

Fig 7.10 Radiation pattern for single element LC patch antenna


161

Fig 7.11 Radiation pattern for 2X2 array LC patch elements

Fig 7.10 shows the simulation radiation pattern of the single element liquid crystal

patch antenna and 2x2 array liquid crystal patch antenna. E-plane radiation pattern

seems to be directive and H-plane pattern of butterfly like in single element and

nulling at 2700 for 2x2 array antenna. Table 7.2 shows the antenna parameters for

single element and 2x2 array case. Gain is considerably increased with array

implementation but efficiency is constant.

Table 7.2 Liquid crystal antenna parameters for single element and 2x2 array

S No Parameter Single Element Patch 2x2 array patch 1 Peak Directivity 6.37 dB 19.5 dB 2 Peak Gain 4.85 dB 14.86 dB 3 Peak Realized Gain 4.05 dB 12.42 dB 4 Radiated Power 0.00346 w 0.0138 w 5 Accepted Power 0.00454 w 0.0181 w 6 Incident Power 0.00543 w 0.0217 w 7 Radiation efficiency 76% 76%

The results are giving strong evidence to apply liquid crystal materials in the design

of antennas for tuneable applications. The existing phase shifters that are been used

in millimetre wave applications, which have performance limitations can be replaced

with liquid crystals material based devices.

7.7 Balanced Antipodal Vivaldi antenna (BAVA) on LCP Substrate

Balance antipodal antenna is a modified version of antipodal antenna. The structure

seems to be like antipodal with two chords, in which it consists of three conducting

arms and the remaining part is ground plane. This balanced antipodal antenna

normally contains two chords in which each chord contains two planes. Top substrate

contains conducting arm on the plane and copper etching on the other plane. Bottom


162

substrate contains slot line on one plane and conducting arm on the other plane. The

conducting path is increased in this balanced type reducing the ground plane, surface

waves have been decreased and so cross polarization has been decreased.

The improvement in the cross polarization performance was brought about by

converting the usual antipodal Vivaldi into triple structure, by inserting additional

substrate and the metallization layer, which balance the electric field distribution in

flared slot.

Fig 7.12 Balanced Antipodal Vivaldi Antenna (a) HFSS Simulated Model, (b) Fabricated

Prototype

7.7.1 Design Steps: By taking operating frequency (fr), height of the substrate (h)

and dielectric constant εr, the antenna length and width can be calculated using the

formula

12

rrf

cLW

------ (1)

The radiating structure can be formed from intersection of quarters of two ellipses.

The primary radii r1 and r2, secondary radii rs1 and rs2 can be taken as

22

22

2

1

m

m

wwr

wwr

------- (2)

rs1=l, rs2=0.5r2

The transmission feeder width Wm with impedance Zo equal to 50 ohms can be

determined by

0

120Zhw

rm

------ (3)


163

The antenna dimensions are listed in table 7.3 Table 7.3 Balanced Antipodal Vivaldi Antenna Dimensions

S. No Parameter Value 1 Antenna Length 52 mm 2 Antenna Width 28.5 mm 3 Slot Width 8 mm 4 Slot Length 36 mm 5 Transmission Line Width, Wm 2.5 mm 6 Transmission Line Length, Lm 16 mm 7 Operating Frequency Range 6-18 GHz 8 Dielectric Material Used LCP with r =2.9 9 Feed Element Length 8 mm

10 Feed Element Width 6 mm

7.7.2 BAVA Parameters

Fig 7.13 Return Loss Vs Frequency of Balanced Antipodal Vivaldi Antenna

Fig 7.14 VSWR Vs Frequency of Balanced Antipodal Vivaldi Antenna

Fig 7.15 Measured Return loss Vs Frequency of Balanced Antipodal Vivaldi Antenna


164

Fig 7.13 shows the frequency Vs return loss of the antenna and Fig 7.14 shows the

VSWR Vs frequency curve. The measured results are taken on Agilent Vector

Network analyzer. Calibration is done using the standard loads supplied by the

manufacturer. Fig 7.15 shows the measured return loss Vs frequency curve on

network analyzer. All the measurements are carried out carefully by not disturbing

the cable setup, which is necessary for accurate measurement.

Fig 7.16 Simulated Radiation Pattern Plots of Balanced Antipodal Vivaldi Antenna at 6, 9, 12,

15, 16 and 18 GHz

Fig 7.17 Measured radiation Pattern Plots of Balanced Antipodal Vivaldi Antenna at 6, 9, 12,

15, 16 and 18 GHz


165

The proposed model has reflection coefficient below -10 dB through the entire

frequency band i.e. from 6-18 GHz and good radiation pattern with minimised back

and side lobes. Radiation patterns of the proposed model in simulation and

measurement is shown in Fig 7.16 and Fig 7.17 respectively.

4 6 8 10 12 14 16 18 20

2

4

6

8

10

12

14

Frequency Vs Gain

Frequency

Gai

n in

dB

SimulationMeasured

Fig 7.18 Frequency Vs Gain of Balanced Antipodal Vivaldi Antenna

Maximum gain obtained through simulation is 12 dB and practically it is 8.5 dB. By

considering all the factors, the designed balanced antipodal antenna performance is

excellent in wideband applications.

7.8 Wideband tapered step antenna on LCP Substrate

Performance study of wideband tapered step antenna on liquid crystal polymer

substrate material is presented. Bandwidth enhancement is achieved by adding step

serrated ground on the front side of the model along with the radiating patch. The

radiating patch seems to be the intersection of two half circles connected back to

back. The lower half circle radius is more than upper half circle radius.

Fig 7.19 Wideband tapered step antenna on LCP Substrate

Wideband tapered step antenna is designed on the liquid crystal polymer substrate

(Ultralam 3850, εr = 2.9) with dimensions of 20X20X0.5 mm. Coplanar waveguide


166

feeding is used in this model with feed line width of 2.6 mm and gap between feed

line to ground plane of 0.5 mm.

7.8.1 Antenna Parameters

Fig 7.20 shows the return loss curve for the LCP based wideband tapered step

antenna. Antenna is resonating between 5.2 to 16.6 GHz with bandwidth of 11.4

GHz in the simulation and Fig 7.21 shows the measured result from network

analyzer from 4.8-15.5 GHz with bandwidth of 10.7 GHz. There is a small difference

of 0.7 GHz in bandwidth is observed from simulate result due to poor quality in the

SMA connector with feed line and ground plane.

Fig 7.20 Simulated Return loss Vs Frequency of wideband tapered step antenna on LCP

substrate

Fig 7.21 Measured Return loss Vs Frequency of wideband step serrated antenna on LCP

substrate

Fig 7.22 shows the antenna three dimensional radiation view at 13.6 GHz. Radiation

pattern of omni directional in E-plane and quasi omni directional in H-plane with low

cross polarization levels can be observed from Fig 7.23.


167

Fig 7.22 Three dimensional view of radiation for wideband tapered step antenna at 13.6 GHz

Fig 7.23 Radiation pattern in E and H-plane of wideband tapered step antenna at 13.6 GHz

Fig 7.24 Current distribution of wideband tapered step antenna at 13.6 GHz

Fig 7.24 shows the simulated current distribution of the antenna at 13.6 GHz. The

current intensity is maximum at radiating element and feed line towards x-direction

with equal magnitude but opposite in polarity. Fig 7.25 shows the frequency Vs gain

plot for the liquid crystal polymer antenna. From this result we can observe that gain

is increasing up to 14 GHz and after reaching to the peak gain of 4 dB, the gain is

decreased at higher frequency.


168

Fig 7.25 Gain Vs Frequency of wideband tapered step antenna

7.8.2 Flexibility Testing

Actual reason for choosing LCP material in the design is to have a flexible model,

which should not produce odd results when it is placed on different surfaces. The

ability of this model is also tested by bending the antenna in different angles and in

each case the reflection coefficient result is noted. The current model is placed in

different tubes with various diameters for this testing.

4 6 8 10 12 14 16 18-40

-35

-30

-25

-20

-15

-10

-5

Frequency in GHz

Ret

urn

loss

in d

B

Measurement 1Measurement 2Measurement 3Measurement 4

Fig 7.26 Flexibility testing of LCP antenna with different angles

By consolidating all these cases of testing we observed not much variation in the

frequency of operation rather than a small shift in the frequency, which gives the

potential of the LCP material in the conformal applications. Fig 7.26 shows the

testing result of the antenna with different bending angles.

7.9 Comparison of different liquid crystal antennas

Table 7.4 shows the comparison of the liquid crystal based antennas performance

characteristics. Liquid crystal antenna can be used for tuneable applications and

liquid crystal polymer antenna can be used for conformal applications. Balanced


169

antipodal is covering large bandwidth but thickness of the substrate is more so

wideband tapered step antenna with 0.5 mm substrate thickness can be used for

conformal applications.

Table 7.4 Comparison of Liquid Crystal Antennas

S. No

Antenna Model

Dimensions in mm

Resonant Frequency in GHz

Bandwidth in MHz

Impedance Bandwidth %

Gain in dB

Efficiency %

Applications

1 Liquid Crystal Antenna

42x27x0.6 10 400 40% 4.5 76% Tunable Applications

2 Balanced Antipodal Vivaldi Antenna on LCP

52x28.5x0.8 6-18 GHz 12000 100% 8.5 94% Wideband applications

3 Wideband Tapered Step Antenna on LCP

20x20x0.5 4.8-15.5 10700 101% 4 95% Conformal applications

7.10 Chapter Summary:

Three models are studied namely liquid crystal antenna, liquid crystal polymer based

balanced antipodal Vivaldi antenna and wideband tapered step LCP antenna. The

dielectric anisotropy of liquid crystal material with change in temperature due to

applied bias voltage is the base for the first model liquid crystal antenna. By applying

DC voltage to electrodes connected between patch and ground plane, tunability in the

resonant frequency is obtained. Peak gain of 4.85 dB and efficiency of 76% is

achieved from this model. A balanced antipodal Vivaldi antenna is constructed on

liquid crystal polymer substrate material and it is resonating between 6 to 18 GHz

with 12 GHz bandwidth.

Another model of wideband tapered step liquid crystal polymer material based

antenna is designed to operate between 4.8 to 15.5 GHz with bandwidth of more than

10.7 GHz. Omni directional radiation with peak realized gain of 4 dB is attained

from this model. Flexibility of the antenna is tested by placing the model in different

tubes with different diameters and observed the constant reflection coefficient results

over the frequency range. So this model on liquid crystal polymer substrate can be

used in the conformal wideband applications.

Documents

7. Liquid Crystal and Liquid Crystal Polymer based Antennasshodhganga.inflibnet.ac.in/bitstream/10603/48026/13/13-chapter 7.pdf · Liquid Crystal and Liquid Crystal Polymer based