Integration of a Compact Circularly Polarized Antenna

European Journal of Scientific Research ISSN 1450-216X Vol.120 No.2 (2014), pp.153-161 © EuroJournals Publishing, Inc. 2014 http://www.europeanjournalofscientificresearch.com/

Integration of a Compact Circularly Polarized Antenna Array with a Reconfigurable Symmetric Stub Phase Shifter using Liquid Crystals Substrates

Sayed. Missaoui Corresponding Author, Department of Technology, Higher Institute of Applied Sciences and Technology of Kasrine, Kairouan University, Tunisia Department of Physics, Faculty of Science Tunis, Tunis El Manar University,Tunisia E-mail: [email protected]

Sihe m. Missaoui Department of Physics, Faculty of Science Tunis, Tunis El Manar University,Tunisia

Mohsen. Kaddour Department of Physics, Faculty of Science Sfax, Sfax University, Tunisia

Abstract In this article, the integration of a compact Circularly Polarized (CP) antenna with a reconfigurable symmetric stub phase shifter using Liquid Crystal (LC) substrates for microwave applications is presented. LC is used as the substrate, which enables the fabrication of a flexible, low cost technologies, and light weight antenna. With these LCs, a simulation slope average 3°/GHz/cm, and 80% of the agility has been achieved with comparatively low control volt-ages less than 6 V. The reflection return loss has been greatly improved by about 15 dB, along with the variation of the simulation resonance frequency of 310 MHz, both before and after applying a continuous voltage with a bandwidth of 373 MHz.

Keywords:

Liquid Crystals, Circularly Polarized Antenna, Symmetric Stub Phase Shifter, Agile Structure, Microwave Application

1. Introduction Liquid Crystal have attracted significant interest in the past decade d ue to their unique optical properties and high degree of design CP antennas [1][2]. Many different technological advances have been necessary to facilitate this increased appearance of personal communication devices: faster and more highly integrated circuits [3][4], compact high-resolution display screens and light weight powerful batteries. Tunable phase shifters are the key elements in radar applications and modern telecommunications, particularly for antenna arrays, where the phase of the different ante nna elements are adjusted [5][6]. Often mounted right in the tip of the chain just before the antenna, their losses have an important impact on the system performance. Up to now, several approaches have been proposed to study the electrically tunable phase shifters [7][8][9]. One of the sought properties for this device is the possibility of external control. The use of LC to control signals at microwave, mm wave and THz wavelengths has many practical advantages in terms of cost, integration technologies ar e witnessing higher performance demands and constraints. LC material is a key innovation for building low-loss phase shifters and other control circuits at millimeter-wave frequencies. The materials presently used as mm- wave substrates are either expensive or show unsatisfactory performance. Identified as advanced candidate materials for flexible mm- wave substrates purposes and due to a unique combination of superior features and performance, LCs have attracted considerable attention in commercial wireless applications. It’s have anisotropic and intriguing properties, such as dielectric

Integration of a Compact C P Antenna Array with a Reconfigurable Symmetric Stub Phase Shifter using LCs Substrates

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anisotropy as well as elastic constants and flexoelectric coefficients. Those properties are essentially due to the orientational order of the LC phase, and the knowledge of the orientational order is then important to get good agility [10][11]. This article proposes devices using planar technology based on LC materials. This hoice allows the LC anisotropy property to be controlled by an electric field. Numerical results for the CP antenna and symmetric stub phase shifter with an LCs cavity are compared with the existing data to confirm the accuracy of the proposed analysis.

2. Liquid Crystal as tunable material for microwave applications In This paper, LCs are used in the nematic anisotropic tunable. The orientation with electric field is schematically presented in Figure 1.

Figure 1: Configuration permittivities  r // and  r 

Anisotropy is then defined as the difference between parallel and perpendicular permittivities and ensues from the following relation:     r , //   r ,  (1)   //    0  0 

0

 0

0   0    

(2)

where  r , // and  r ,  are, respectively, the parallel and perpendicular relative dielectric permittivities. r

 r , //

tan  //

 r,

tan  

tan  //

Vth

Vma x

Vbais

Figure 2: characteristics of the relative permittivity and the loss tangent of LC materials with DC

Such a general feature of CL is the dielectric constant and loss tangent with an applied voltage is shown in Figure 2. The antennas array with a phase variation at each radiating element is shown in Figure 3. The phase variation between the elements is the cause of the misalignment. The most common solution is to control the direction of the beam by the phase of the signals feeding the elements of the antenna and tunable phase shifters are then required. The phase shifters control the phase of the excitation current to direct the antenna beam to the desired region in space.

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3. Design of the CP antenna The phase variation required to direct the beam in the plane H of the opening which is obtained by 4 tunable phase shifters. They have been integrated in a hybrid manner (Figure 4) between the bias circuit and the antenna. Direction of radiation Wave front

 



θ

l sin 

c0

l Phase variation

7ΔΦ

6ΔΦ

5ΔΦ

4ΔΦ

3ΔΦ

2ΔΦ

ΔΦ

0°

Power Divider/Combiner

Figure 3: antenna array phase control using agile phase shifter

Glass cover and RF phase shifter

DC-control connector and feed network

4×8 Patch antenna array

Figure 4: Photograph of the phase shifter - patch antenna array [12]

The layout of the selected circularly polarized antenna is shown in Figure 6. The patch consists of a square radiator of width L = 2.1 cm. The antenna cre ates CP by slightly truncating two diagonal corners of patch and placing the feed point along one of the primary axes, oblique to the line of the truncated corners. This geometrical perturbation creates the two degenerate modes needed for circular polarization. The truncated corners are reduced in size by a length D = 2.0 cm. The substrate is made of a LC material inserted by capillarity with a dielectric constant permittivity of  r  2 . 9 and a loss tangent of 0.002.The Ceramic material with a relatively high dielectric constant of  r  20 .7 . The loss


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tangent of the ceramic is   0 . 00093 . The high dielectric constant allows for a reduction in patch size. The substrate itself is also a square having dimensions slightly larger than that of the metal patch. The width of the substrate is W = 25 mm and its height is h = 4 mm. The antenna is fed by a coaxial probe from the underside of the antenna. The feed is located at a position (xf;yf)=( 12.5;15.5) mm. This offset feed position is critical for creating CP. The CP antenna is excited with a 50 Ω microstrip line. W=25 D=2 X=12.5 εr=20.7

L=21 H=4 Y=15.5 tanδ=0.00093 Trancuted corners

Ceramic substrate

CP Patch Antenna LC

Figure 6. Design of the CP antenna based on LCs

Figure 7 shows the results of simulated HFSS and calculated VSWR with and without applied DC voltage and the dielectric permittivity LC is 2.9. It can be seen that the simulated resonance frequency variation (ΔFr) between with and without applied DC voltage is 11.26 MHz correspond to à frequency agility of 0.7%. The simulated impedance bandwidths with DC voltage and calculated without LC for a 0:3 VSWR, are 66.66 MHz and 7 MHz, respectively. The frequency error between simulated and calculated results is essentially owed to outside radiation of the substrate and of which one doesn’t hold account in the simulation.

Calculated (FDT D) without LC [13] Simulated without DC (HFSS) Simulated with DC Voltage (HFSS)

Figure 7. Simulated and calculated VSWR with and without applied DC voltage

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Eθ(φ=0°) Calculated (FDTD) without LC [13] Eθ(φ=0°) Simulated without DC Voltage (HFSS) Eθ(φ=0°) Simulated with DC Voltage (HFSS)

Figures 8. Simulated and calculated radiation patterns of the CP antenna for the plan (φ=0°)

Eφ(φ=90°) Calculated (FDTD) without LC [13] Eφ(φ=90°) Simulated without DC Voltage (HFSS) Eφ(φ=90°) Simulated with DC Voltage (HFSS)

Figures 9. Simulated and calculated radiation patterns of the CP antenna for the plan (φ=90°)

Figures 8 and 9 depicts the simulated and calculated radiation patterns of the CP antenna based on LCs for the plan (φ=0°). The radiation patterns of simulated and calculated are nearly the same for the plane φ=0° and φ=90°, we noticed that the two electric fields Eφ and Eθ are equal nominally for an angle θ nearly hopeless that is to say for the highest elevation. It is clearly seen from the radiation pattern comparison that, the peak gain with and without applied DC Voltage is respectively 4dB and 5dB, therefore the found gain with LC is improved.


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4. Study and Design of a Symmetric Stub Phase Shifter with the Insertion of Liquid Crystal Various phase shifters have been made in view of reducing the length of the active part and enhancing the interaction between the wave and the matter. Figure 10 shows the design of the symmetric stub phase shifter based on an LC. The lower part results from the engraved lines on a PTFE substrate of a low relative permittivity. Due to the low relative permittivity of PTFE, the electric field lines are less concentrated than in an LC. The structure optimization allows one to take into account the changes in the LC behavior. The upper part is a brass- made micro- machined cavity used for confining the LC over a coplanar line made on the PTFE substrate. A suitable control voltage of about 10 V is applied in order to obtain the desired tilt of the nematic LC molecules. Thus, the LC relative permittivity variation entails a phase and bandwidth shift as well as the generation of agility. The phase shifter is needed as a key component in phased-array agile antennas. This component is preferable when the return loss is minimal. Figures 11 and 12 depict the results of simulated and measured return losses with and without applied DC voltage. It can be seen that the return loss achieved −25 dB from 20 to 22 GHz. The resonance frequency variation (ΔFr) between simulated and measured is 500 MHz. The bandwidths simulated and measured at −20 dB are 463 MHz and 330 MHz, respectively. It can be seen from Figure 12 that the return loss achieved −40 dB from 20 to 22 GHz. The resonance frequency variation (ΔFr) between the simulated and measured is 317 MHz. The bandwidths simulated and measured at −20 dB are, respectively, 836 MHz and 600 MHz. This little variation between measurement and simulation data may result from a gap in the precision of the values found for the LC dielectric permittivities. Figure 13 illustrates the phase shift variation versus the applied DC voltage at 20 GHz for the symmetric stub phase shifter with an LC cavity. The greatest phase shift is obtained for an applied 10 V DC voltage; then LC is supposed to be saturated. The simulated and measured slope average, respectively, are 3°/GHz/cm and 2.77°/GHz/cm, and 80% of the agility is obtained for a 6-V field amplitude. Good agreement is observed in the two plots. Symmetric stub ligne

Liquid Crystal

Ground plane

Substrate PTFE

Figure 10. Structure of a symmetric stub phase shifter with an LC cavity

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simulated (HFSS) measured (N. Martin., 2003) [14]

Figure 11. Simulated and measured return losses without applied DC voltage.

simulated (HFSS) measured (N. Martin., 2003) [14]

Figure 12. Simulated and measured return loss with applied DC voltage

simulated (HFSS) measured (N.Martin., 2005) [15]

Figure 13. Simulated and measured Phase-shift versus DC voltage at 20 GHz


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5. Conclusion This paper presents the fundamentals of LC material and its applications for reconfigurable devices. An extremely light- weight, low-cost, flexible, and deployable integration of a compact CP antenna array with a reconfigurable symmetric stub phase shifter using LCs substrates has been shown. Firstly, structure of the CP agility antenna based on the coaxial feed line configuration were designed and simulated. Secondly, structure of a symmetrical stub phase shifter was analyzed. The accuracy of simulation was verified by comparison with experimental data. The observation of the phase deviation confirms the potential frequency agility of the devices that use LCs.

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