1522 PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012

Performance Evaluation of Three Rectangular Patch Element ArrayAntenna Conformed on Small Radius Cylindrical Surface

Emad S. Ahmed and Jawad K. AliDepartment of Electrical and Electronic Engineering, University of Technology, Baghdad, Iraq

Abstract— The cylindrical geometry can offer certain desirable antenna characteristics thatare not provided by planar elements. In this paper, a three-element cylindrical conformal arrayantenna has been presented as a candidate for use in wireless communications and Radio Fre-quency Identification (RFID). Each element in the array is a microstrip fed rectangular patchantenna designed to resonate at 2.4 GHz. Once the desired results were obtained for a singleelement, each element in the conformal cylindrical array has been then designed using the samedimensions and parameters. Modeling and performance evaluation of the array has been carriedout using the commercially available electromagnetic software CST Studio SuiteTM 2009. Sim-ulations have been conducted to study the performance of the proposed conformal array as wellas the effects of small radius cylinder on mutual coupling and the radiation pattern of the array.The cylindrical radii in consideration are of about one quarter wavelengths or slightly more. Theradius of cylinder used in simulation is taken to be 0.24λ and 0.32λ. Compared with the existingcylindrical conformal antenna, the proposed array antenna possesses a reduction in cylindricalstructure radius with acceptable ominidirectionality and gain needed for wireless communicationsand RFID applications.

1. INTRODUCTION

Miniaturization in the integrated circuit technology and advancement in signal and data processinghave opened prospects for wide spectrum of applications which uses densely packed terminalsplaced in little volume. That is why such applications depend on availability of conformal shapedantennas, ensuring required directions of radio wave propagation and enabling hidden terminalmounting. The range of applications spans from sensors goes through wireless access modes andthen up to modern miniaturized spacecraft [1].

One of the most important innovations in modern antenna technology is the conformal antennaarray. Conformal arrays have good potential for application in aerospace vehicles with excel-lent aerodynamic characteristics. Cylindrical antenna arrays have attracted the greatest attentionamongst conformal antennas and their applications include mobile cellular base stations, airborneradar and mobile satellite communication terminals [2, 3].

Microstrip antennas are often used because of their thin profile, light weight and low cost.Furthermore, they can be made conformal to the structure. When the radius of the curved structureis large, the antenna can be analyzed as the planar one. However, for structure with smaller radii,more rigorous analysis methods should be used. If the antenna has a cylindrical shape, i.e., if oneprincipal curvature is zero, the antenna can be analyzed as a circular-cylindrical one. In the casewere both principal curvatures are different from zero, the antenna can be analyzed as a sphericalone [4].

The use of cylindrical substrate for microwave design is generally driven by the physical at-tributes of the system rather than by choice, since the analysis and fabrication are more compli-cated than for a comparable planar implementation. However, the cylindrical geometry can offercertain desirable antenna characteristics that are not provided by planar elements. There are alsovariety of configurations that can be realized, for example cylindrical conformal patch and slot an-tennas [5–7], microstrip [8], and coplanar transmission lines [9]. Cylindrical conformal structures,with radii greater than one half wavelengths, have been proposed for use as prospective candi-dates for mobile communications systems, cellular base stations, and Telemetry, Teleranging andTelecommand (TTC) communication that is essential to maintain space missions due to their fullfield of view advantage [1, 10, 11].

In this paper, a microstrip fed rectangular patch antenna resonates at 2.4 GHz is considered.The proposed planar patch antenna is used in array consisting of three equally spaced elements.The proposed antenna array is conformed on a finite cylindrical substrate of 1.57mm thickness andrelative permittivity of 2.2. Two different radii for the cylindrical structure are simulated usingCST Microwave Studio simulator. Results obtained on return loss, coupling between elements andradiation pattern are presented and discussed.

Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 1523

2. ANTENNA DESIGN AND CONFIGURAIONS

2.1. Single Element Antenna StructureThe configuration of the rectangular patch antenna is shown in Figure 1(a). The patch has beenmodeled in CST Studio and its dimensions have been adjusted to resonance at 2.4 GHz. A quarter-wave transformer was used to match 343 Ω input impedance to a 50Ω system. The final dimensionsof the entire microstrip patch are given in Table 1. Figure 1(b) shows the return loss response ofthe patch element antenna. It can be clearly indicated that the antenna was resonates at 2.4 GHzwith return loss of less than −10 dB within 40 MHz bandwidth.

2.2. The Proposed Conformal Antenna Array StructureThree patch elements were equally spaced on cylindrical substrate. The substrate material usedfor modeling has a thickness of 1.57mm. The dielectric constant of the substrate is εr = 2.2.The conductive material in the model is of 70.0µm thick copper. The radius of the cylinder iscomparable to one quarter wavelength and the height is H = 90mm. Inside the cylinder there is a

2 2.2 2.4 2.6 2.8 3-18

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Figure 1: (a) The layout of single element patch antenna structure, and (b) is its return loss S11 (dB)response for single element patch antenna.

Figure 2: A 3-D view of the modeled 3-element array.

Table 1: Antenna dimensions in mm.

W L W1 L1

60 88 41.08 39.03W2 L2 W3 L3

0.72 24.05 4.84 15

1524 PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012

continuous ground plane of 70.0µm thick copper. The model of the antenna array taken from thesimulation software is shown in Figure 2.

3. PERFORMANCE EVALUATION

The cylindrical structure of Figure 2 has been modeled through a commercially available finiteelement package CST Studio Suite. Cylinders with radii 30 and 40mm (0.24 and 0.32λ) have beenanalyzed while keeping the rest of the antenna parameters fixed. The simulation results of returnloss of all of the ports of the array and the coupling among the antenna elements are shown inFigure 3.

From Figure 3, it’s clearly observed that the coupling between elements for 30mm (0.24λ)radius cylinder is about −1 dB, while for cylinder of 40 mm (0.32λ) radius, the coupling is less than−18 dB. The small radius of the cylinder results in decreasing the spacing between the elements sothe mutual coupling between elements is increased.

Simulated radiation patterns at 2.4GHz for single element and 3-element array are illustratedin Figure 4.

The radiation patterns are significantly affected. In the elevation direction, the radiation pattern

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Figure 3: Simulated coupling of the 3-element array conformed on cylinders with radii of 30 mm (0.24λ) and40mm (0.32λ).

Eθ plane Hφ plane

(b)

Eθ plane Hφ plane

(a)

Figure 4: Radiation patterns: (a) for element in cylindrical array, the radius of cylinder is R = 30mm(0.24λ) and (b) radius of cylinder is R = 40 mm (0.32λ).

Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 1525

is strongly dependant on the cylinder radius but much less so in the azimuth direction. The Eplane (Eθ) and H plane (Hφ) fields, depicted in the figure, reveal that they still have an acceptablequasi ominidirectional radiation pattern.

4. CONCLUSION

This paper presents detailed performance evaluation concepts of a three rectangular patch elementconformal antenna arrays. There are few issues that should be taken into consideration whendesigning such antennas. Firstly the curvature of the cylindrical array affects the radiation patternof the antenna and the optimal radius should be found depending on the application on hand.Secondly the spacing between elements is very important to consider as it affects the level of mutualcoupling in the array. An acceptable mutual coupling was obtained for cylinder radius greater thanone quarter wavelength. The result shows that the resonant frequency is not affected by curvaturehowever the radiation patterns are significantly affected. The radiation pattern in the elevationdirection is strongly dependant on the cylinder radius but much less so in the azimuth direction.Simulation results shows that the proposed array antenna possesses an acceptable ominidirectionalradiation pattern needed for most wireless communications and RFID applications.

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