Tunable Patch Antenna Using a Liquid Crystal Substrate

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TUNABLE PATCH ANTENNA USING A LIQUID CRYSTAL SUBSTRATE

Ranjan Bose] andA/oka Sinha

Department of Electrical Engineeringland Department of Physics

2,

lIT Delhi, Hauz Khas, New Delhi, INDIA -110016

Phone: +91-11-26591048, Fax: +91-11-26581606, Email: [email protected]

Keywords: Tunable Patch Antenna, Liquid Crystal

ABSTRACT

Patch antennas are attractive in many mobile

communication applications since they are light-weight

and simple to manufacture. However, the traditional patch

antenna suffers from narrowfrequency bandwidth. In this

paper, wepropose the use ofa liquid crystal substratefor a

patch antenna whose frequency can be tuned by changing

the biasing voltage across the substrate. It has beenfound

by simulations that by varying the biasing voltagefrom 0 Vthrough 11 the operatingfrequency o fthe circularpatch

antenna can be variedfrom 1.08 GHz through 2.35 GHz.

We have also proposed two novel applications for such

tunable antennas: (i) antenna basedmodulation (ARM) and

(ii) antenna basedfrequency diversity (ABFD).

1. INTRODUCTION

Modem printed circuit fabrication techniques have made it

possible to bui ld low profile antennas that are extremely

useful. Such antennas are referred to as microstrip or

printed circuit antennas [1]. A microstrip antenna is made

up of two parallel conduc tors that are separated by a

dielectric substrate, as shown in Figure 1. The lowerconductor usually acts as a ground plane and the upper

conductor is a pa tch, which is why such antennas are also

called patch antennas. The patch can be of various shapes

such as rectangular, circular, square, elliptical, dipole and

tr iangula r among others. Patch antennas are inherently

resonant antennas characte rized by extremely low

bandwidths. In addition, they are usual ly light in weight

and easy to install because of which they are frequently

used in mobile-phones, aircraft, satellite and missile

applicat ions. Simplicity, low manufacturing cost and the

flexibility to configure to specialized geometries are some

of the other advantages of patch antennas [2].

The tradi tional patch antenna suffers from low efficiencyand narrow frequency bandwidth. Thus, improved patch

antenna designs have been pursued for many years. One

part of the patch antenna that can be optimized is the

substrate [3]. This can be done by designing different

regions with constant permitt ivity [4]. Patch antennas on

inhomogeneous substrate with the permittivity

continuously varying with the height coordinate have been

1-4244-1539-Xl08/$25.00 ©2008 IEEE

proposed in [5, 6]. These antennas show improved

bandwidth and directivity. Patch antenna with

inhomogeneous dielectric substrate has also been

investigated in [7] under the assumption of a fixed current

distribution on the patch. In all these approaches the

permittivity of the substrate varies along the thickness of

the substrate. In effect, the behavior of the patch antenna

with inhomogeneous substrate can be approximated by a

homogeneous substrate with the effec tive value of the

permittivity, &eff. The reported increase in the bandwidth of

such patch antennas is marginal. Recently, tunableferroelectric antennas for fixed frequency scanning

applications have also been proposed where the scan angle

is varied using a bias voltage [8].

In this paper we propose a technique that allows us to

design and construct a tunable patch an tenna whose

resonance frequency can be varied using a bias voltage. By

tunabil ity we imply that the same antenna can be tuned to

resonate (hence radiate) over a range of frequencies. Such

tunable antenna technology will be an enabler for software

defined radios (SDR). The paper is organized as follows.

In Section 2 we describe the patch antenna, with the liquidcrystal substrate, used to demonstrate the concept. In

Section 3 we explain the modeling of the l iquid crystalsubstrate. The experimentally measured capacitance values

of an actual Ferroelectric Liquid Crystal cell is used in this

paper. Section gives the simulation results for a circular

patch antenna on a Liqu id Crystal substrate. Beside the

obvious application in SDRs, we propose two novel

applications of the tunable patch antennas in Section 5.

The paper concludes in Section 6.

2. ANTENNA DESCRIPTION

For the sake of illustration we have considered a circular

patch antenna. The resonance frequency, j,., of a circular

patch antenna is given by [1]

1.J (l )

where c 3 xl 08 mis, r is the radius of the patch and is

the permittivity. In order to achieve tunability of the

resonance frequency, we use a sui table l iquid crystal (LC)

as the substrate material. By varying the dc bias voltage

across the LC substrate , the of the LC changes [9]. The



change in the value of £ leads to the change in the

resonance frequency of the patch antenna. It should be

noted that upon applying the bias voltage, the permittivity

of the cylinder underneath the circular patch differs from

the permittivity of the remainder of the substrate. Figure 2

shows a typical profile of the relat ive permitt ivity for a

patch antenna with a LC substrate with a certain bias

voltage. Our proposed LC based patch antenna differsfrom the earlier patch antennas on inhomogeneous

substrates in the following two key aspects:

(i) For our case £ £ while for the other cases £ £

where is the height coordinate along the thickness

of the substrate.

(ii) For our case the resonance frequency can be tuned

using a bias voltage while for the other cases the

resonance frequency of the patch antenna is fixed.

For our tunable antenna, the total voltage (VIOl) on the

patch comprises of the dc biasing voltage bias) and the

information bearing small-signal voltage (Vsig) that actually

excites the antenna. Thus,

where, Cdc is the permittivity at zero bias voltage and is

the variance. The Gaussian fit with Cdc = 11.859 and =25 is also plotted in Figure 4 using the dotted line. From (1)

and (4) we obtain

)J::a;) (5)

2nrF:From (5) we note that the resonance frequency of the

antenna varies as e V b ~ a s with the bias voltage. Using (5)and the calculated permittivity values (Figure 4), we obtain

the range of resonance frequencies over which the LC

based circular patch antenna can be tuned. This is shown

in Figure 5. Thus, it is possible to vary the operating

frequency of the same circular patch antenna from 1.1098

GHz through 2.7184 GHz simply by varying the biasing

voltage from 0 V through 11 V across its LC substrate. We

have also plotted (using dotted line) the theoretical f,.

obtained using the Gaussian fit.

3. MODELING THE LIQUID CRYSTAL

SUBSTRATE

As a specific example, we analyze a circular patch antenna

on a ferroelectric liquid crystal (FLC) substrate. The

characteristics of this FLC are listed in Table I. A patch

antenna on a liquid crystal substrate is easy to construct.

Effectively, it is a liquid crystal cell filled with an

appropriate liquid crystal mixture and a metal contact of a

desired shape, serving as the patch.

The experimentally measured values of the capacitance (in

nF) of the FLC cell as a function of the applied voltage

across the cell is plotted in Figure 3. The cell thickness

was 1.4 Jlm and the ITO electrode area was 4 cm2•

Treating the LC cell as a parallel plate capacitor, we have

C (3)

4. SIMULATION RESULTS

Simulations were carried out on actual circular patch

antenna geometries to verify our claim. We have modeled

the circular patch antenna using the software CST

Microwave Studio®, which is a 3D electromagnetic field

simulation software. The patch antenna geometry is shown

in Figure 6. The scattering parameter SII for bias voltages

OV, 3V, 6V and 10V are plotted in Figure 7. SII represents

the return loss of the antenna, and the trough in the S11 plot

is indicative of the resonance frequency of the antenna [1].

The results from actual antenna simulations along with the

theoretically predic ted values are listed in Table II. We

observe from the table that the resonance frequency values

obtained from actual antenna simulations are lower than thetheoretical ly predicted values, though the values match

much better at low Vbias' This is because the f,. calculated

using (1) assumes a constant value of the permittivity of the

substrate while for actual antenna simulations, the

dielectric cylinder underneath the circular patch is

surrounded by a material of higher permittivity. We have

also lis ted the maximum radiated power (in dBi), where

dBi is with reference to an isotropic antenna.

Let us define the fractional bandwidth of the tunable

antenna as

B fmax - fmin Af (6)

where, fmax is the maximum resonant frequency, fmin is the

minimum resonant frequency andIe is the center frequencyof operation. For the tunable patch antenna discussed in

Table II, B = (2.35 -1.08)/1.71 0.74 or 74%.

(2)

(4)

VIOl bias sig

where C is the capacitance of the LC cell, is the

permittivity of the free space, £ is the permittivity of the

dielectric, A is the area of the plate (in our case the ITO

electrode area) and d is the thickness of the cell. Using (3),

the permittivity of the LC cell is ca lculated for different

Vbias and plotted in Figure 4. From the figure we observe

that the values of the permittivity of the LC filled parallelplate capacitor, as a function of the bias voltage bias) is

approximately Gaussian. Thus, we can write

_(V7s )

( )

20-2~ i a s Gdce



5. TWO NOVELAPPLICATIONS

In this section we describe two novel applications of the

tunable patch antenna. The first application is an antenna

based modulator (ABM) which can carry out Frequency

Shift Keying (FSK). As opposed to the traditional

communication systems where the modulator block is

separate from the antenna, here we propose to use theantenna as a modulator as well. Using the tunable patch

antenna, the bias voltage can be used to select the

frequency that is transmitted in accordance with a given

data stream. Let the input waveform to the antenna be

vet) A i cos(27r/;t) bias (t)i=O

where Ai is a constant scale factor and is lh frequency of

the M-ary FSK. The bias voltage, Vbiait) , takes discrete

values and is changed according to the input bit-stream in

order to tune the antenna at the desired Thus, for a

particular value of Vbiai t), only one particular frequency is

transmitted thereby carrying out the MFSKmodulation.

The second application of tunable patch antenna is to

mitigate fading by using frequency diversity. Let two input

waveforms to the antenna, at different instances in time, be

VI (t) met) c o s ( 2 ~ t ) bias

and

2 (t) met)COS(2Jif2 t ) bias

where m(t) is the message signal, Vbias corresponds to the

bias voltage resulting in the resonance frequency bias

corresponds to the bias voltage resulting in the resonance

frequency ii, and the frequencies and ii are chosen suchthat ~ = ii ~ f c o h , the coherence bandwidth of the

channel. Since the frequencies are separated by a band

greater than the coherence bandwidth of the channel, they

will fade independently [10]. This example can be

extended to independently fading carriers to provide

diversity of order Thus, the tunable patch antenna can

be used for antenna based frequency diversity (ABFD).

6. CONCLUSION

We have proposed, for the first time, a tunable patch

antenna with a liquid crystal substrate. The operating

frequency of this patch antenna can be tuned simply by

changing the biasing voltage across the substrate. It has

been shown that the resonance frequency of the tunable

antenna varies as -.; e V h ~ a s with the bias voltage. We have

carried out simulations using actual patch antenna

geometries and experimentally measured permittivity

values of a ferroelectric liquid crystal substrate. The

simulation results agree well with the theoretically

predicted values. The proposed technique is simple,

practical and has far-reaching consequence in the area of

wireless communications. We have also described two

novel applications of such tunable antennas: (i) antenna

based modulation (ABM) and (ii) antenna based frequency

diversity (ABFD).

REFERENCES

[1] C.A. Balanis, Antenna Theory: Analysis and Design,

John Wiley Sons, 1997.

[2] W.L. Stutzman and G.A.Thiele, Antenna Theory and

Design, John Wiley Sons, 1998.

[3] N. G. Alexopoulos, P. B. Katehi, and D. Rutledge,

"Substrate Optimization for Integrated Circuit Antennas,"

IEEE Trans. Microwave Theory Tech., Volume 83, Issue 7,

Jul. 1983. pp. 550 - 557.

[4] C. S. Lee, V. Nalbandian and F. Schwering, "Dual

frequency microstrip antenna with inhomogeneously filled

dielectric substrate," Microwave Opt. Techn. Lett., vol. 6,

No. 11, 1993,pp.629-632.

[5] L. Vegni, F. Bilotti and A. Toscano, "Microstrip disk

antennas with inhomogeneous artificial dielectrics," J.

Electro. Waves Applic., vol. 14, No.9 , 2000. pp. 12031227.

[6] A . Toscano and L. Vegni, "Full-wave analysis of planar

stratified media with inhomogeneous layers," IEEE Trans.

Antennas Propagat., Volume 48, Issue 4, April 2000. pp.

631-633.

[7] G. Kristensson, P. Waller and A. Derneryd, Technical

Report LUTEDX/(TEAT-7100)/1--48/(2001), Lund

Institute of Technology, Department of Electroscience, P.O.

Box 118, S-221 00 Lund, Sweden, 2001.

http://www.es.1th.se.

[8] G. Lovat, P. Burghignoli and S. Celozzi, "A Tunable

Ferroelectric Antenna for Fixed-Frequency Scanning

Applications," IEEE Ant. and Wireless Prop. Let., vol. 5,

2006, pp. 353-356.

[9] L.M. Blinov, E.P. Pozhidaev, F.V. Podgornov, S.A.

Pikin, S.P. Palto, A. Sinha, A. Yasuda, S. Hashimoto and

W. Haase, " 'Thresholdless' hysteresis-free switching as an

apparent phenomenon of surface stabilized ferroelectric

liquid crystal cell," Physical Review E, vol. 66, 21701,

2002.

[10] T. S. Rappaport, Wireless Communications: Principles

and Practice, 2ndEd., Prentice-Hall, New York, 2001.



Table II. Resonance frequency values for the tunable. I h d'ffj b'lrcu arpatc antenna or erent las vo ages.

Theoretical Actual antenna

Bias Values simulationsVoltage using Max.

(V) from the (GHz) Radiated

Eq. Gaussian Power

fit (dBi)(GHz) (GHz)

0 1.1098 1.1098 1.08 4.84

3 1.2157 1.2143 1.19 5.18

6 1.8328 1.5907 1.72 5.96

10 2.4816 3.0167 2.22 6.61

11 2.7184 3.7217 2.35 6.73

Table I. Characteristics of the FLC.

Attribute Value

Transition Cr-l0°C-SmC-58°C-

Temperatures SmA-80°C-Iso

Polarization 100 nC/cm2

Tilt angle 23.5 deg

Viscosity 0.7 Poise

z

Patch

(metal) Substrate

(dielectric)

z

Ground

-------LbJ-----Figure I - Cross section of a patch antenna Figure 2 - A typical profile of the permittivity for a

patch antenna with an LC substrate and a bias voltage.



1010

4

oo.....- ..a.- -'--

2

8

10 ': :- .

1010

1. J. .

25 _L_ -1

..

.......l.. _ .l.. .

.1' ••••••••••••••••••••••••• •••••••••••••••••••••••• •••••••••••••••••••••••• •••••••••••••••••••••••••t"••

O ' - - - - - - -- - - - . . a . - - -.........- - - - - - - - '

~ 2 OJ(J

c

$ 150

o 10

Figure 3 - Measured values of the capacitance (in nF) of the FLC

cell as a function of the appl ied vo ltage across the cell wi th

thickness d 1.4Ilm.

Figure 4 - Calculated values of the permittivity of the FLC cell as

a function of the applied voltage across the cell and the Gaussian

fit with (j 5 (dotted curve).

dim.... in mrn

1010

1L....-_...I--_--..L-__L....-_-'--_.......L.._-...J

3 . 5 ~ o, , , , .. " ..

0 ,

¥3 · · · : , ,, ,, . .· , · ·, , · , .

c

2.5 , 11'- , : ( : ,,; tI\ ...

u:

2 ..· · ,; .. ·"" ",,,· .. ·;· · ·; · . ·; · . ; , .o

Figure 5 - Resonance frequencies as a function of the bias voltage.

The dotted line represents the predicted values from the Gaussian

fit.

Figure 6 - The geometry of the circular patch antenna modeled

using CST Microwave Studio®.



Vbias = 0 V

J,. 1.08 GHz

Vbias 3 V

J,. = 1.19 GHz

Vbias 6 V

J,. = 1.72 GHz

Vbias = 10 V

J,. = 2.22 GHz

Vbias 11 V

J,. 2.35 GHz

Figure 7 - The Sll (return loss) plots for bias = OV, 3V, 6V, IOV and IIV obtained from actual antenna simulations

of the circular patch antenna with FLC substrate.

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Tunable Patch Antenna Using a Liquid Crystal Substrate