1522 PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012Performance Evaluation of Three Rectangular Patch Element Array Antenna Conformed on Small Radius Cylindrical Surface Emad S. Ahmed and Jawad K. Ali Department of Electrical and Electronic Engineering, University of Technology, Baghdad, Iraq Abstract— The cylindrical geometry can oﬀer certain desirable antenna characteristics that are not provided by planar elements. In this paper, a three-element cylindrical conformal array antenna 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 patch antenna designed to resonate at 2.4 GHz. Once the desired results were obtained for a single element, each element in the conformal cylindrical array has been then designed using the same dimensions and parameters. Modeling and performance evaluation of the array has been carried out using the commercially available electromagnetic software CST Studio SuiteT M 2009. Sim- ulations have been conducted to study the performance of the proposed conformal array as well as 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. The radius of cylinder used in simulation is taken to be 0.24λ and 0.32λ. Compared with the existing cylindrical conformal antenna, the proposed array antenna possesses a reduction in cylindrical structure radius with acceptable ominidirectionality and gain needed for wireless communications and RFID applications. 1. INTRODUCTION Miniaturization in the integrated circuit technology and advancement in signal and data processing have opened prospects for wide spectrum of applications which uses densely packed terminals placed in little volume. That is why such applications depend on availability of conformal shaped antennas, ensuring required directions of radio wave propagation and enabling hidden terminal mounting. The range of applications spans from sensors goes through wireless access modes and then up to modern miniaturized spacecraft . One of the most important innovations in modern antenna technology is the conformal antenna array. Conformal arrays have good potential for application in aerospace vehicles with excel- lent aerodynamic characteristics. Cylindrical antenna arrays have attracted the greatest attention amongst conformal antennas and their applications include mobile cellular base stations, airborne radar 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 structure is 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 one principal curvature is zero, the antenna can be analyzed as a circular-cylindrical one. In the case were both principal curvatures are diﬀerent from zero, the antenna can be analyzed as a spherical one . 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 oﬀer certain desirable antenna characteristics that are not provided by planar elements. There are also variety of conﬁgurations that can be realized, for example cylindrical conformal patch and slot an- tennas [5–7], microstrip , and coplanar transmission lines . 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 and Telecommand (TTC) communication that is essential to maintain space missions due to their full ﬁeld 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 1.57 mm thickness and relative permittivity of 2.2. Two diﬀerent radii for the cylindrical structure are simulated using CST Microwave Studio simulator. Results obtained on return loss, coupling between elements and radiation pattern are presented and discussed.
Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 15232. ANTENNA DESIGN AND CONFIGURAIONS2.1. 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 50 Ω 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 1.57 mm. The dielectric constant of the substrate is εr = 2.2.The conductive material in the model is of 70.0 µm thick copper. The radius of the cylinder iscomparable to one quarter wavelength and the height is H = 90 mm. Inside the cylinder there is a 2 0 -2 -4 Return Loss, S11(dB) -6 -8 -10 -12 -14 -16 -18 2 2.2 2.4 2.6 2.8 3 Frequency, GHz (a) (b)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 39.03 W2 L2 W3 L3 0.72 24.05 4.84 15
1524 PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012continuous ground plane of 70.0 µm thick copper. The model of the antenna array taken from thesimulation software is shown in Figure 2.3. PERFORMANCE EVALUATIONThe cylindrical structure of Figure 2 has been modeled through a commercially available ﬁniteelement package CST Studio Suite. Cylinders with radii 30 and 40 mm (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 30 mm (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 2.4 GHz for single element and 3-element array are illustratedin Figure 4. The radiation patterns are signiﬁcantly aﬀected. In the elevation direction, the radiation pattern 10 10 0 0 S11 -10 S11 -10 S22 S22 S33 -20 S33 -20 S21 Return Loss, dBReturn Loss, dB S21 S31 -30 S31 -30 S32 S32 -40 -40 -50 -50 -60 -60 R=40 mm -70 R=30 mm -70 -80 -80 -90 2 2.2 2.4 2.6 2.8 3 2 2.2 2.4 2.6 2.8 3 frequency, GHz Frequency, GHzFigure 3: Simulated coupling of the 3-element array conformed on cylinders with radii of 30 mm (0.24λ) and40 mm (0.32λ). E θ plane H φ plane E θ plane H φ plane (a) (b)Figure 4: Radiation patterns: (a) for element in cylindrical array, the radius of cylinder is R = 30 mm(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 1525is 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. CONCLUSIONThis 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.REFERENCES 1. Pawel, K., O. Przemyslaw, and H. Pawel, “TTC patch antennas made in a conformal form with small radius,” Proceeding of ‘Eu CAP 2006’, Nice, France, Nov. 6–10, 2006. 2. Wang, Q. and Q.-Q. He, “An arbitrary conformal array pattern synthesis method that includes mutual coupling and platform eﬀects,” Progress In Electromagnetics Research, Vol. 110, 297– 311, 2010. 3. Josefsson, L. and P. Persson, Conformal Array Antenna Theory and Design, Wiley-Inter Sci- ence, 2006. 4. Niksa, B. and S. Zvonimir, “Radiation properties of spherical and cylindrical rectangular mi- crostrip patch antennas,” Automatika Journal, Vol. 43, No. 1–2, 69–74, 2002. 5. Jain-Ming, J., J. A. Berrie, R. Kipp, and S. Lee, “Calculation of radiation patterns of microstrip antennas on cylindrical bodies of arbitrary cross section,” IEEE Transaction, Antennas and Wave Propagation, Vol. 45, No. 1, 126–132, 6. Ho, C. H., P. K. Shmaker, K. Smith, and J. W. Liao, “Printed cylindrical slot antenna for commercial applications,” Electronic Letters, Vol. 32, No. 3, 151–153, 1996. 7. Pirai, M. and H. R. Hassani, “L-probe fed circular polarized wideband planar patch antenna on cylindrical structure,” Progress In Electromagnetics Research C, Vol. 3, 161–167, 2008. 8. Huang, J., R. Vahldieck, and H. Jin, “Microstrip discontinuities on cylindrical surfaces,” IEEE MTT-S Symposium, Vol. 3, 1299–1302, Jun. 1993. 9. Su, H. and K. Wong, “Dispersion characteristics of cylindrical coplanar waveguides,” IEEE Trans. Microwave Theory and Techniques, Vol. 44, No. 11, 2120–2122, 1996.10. Ning, Y., S. Y. Tat, N. Xiao-Chun, and L. Le-Wei, “Analysis of probe-fed conformal microstrip antennas on ﬁnite grounded substrate,” IEE Trans. Ant. and Propag., Vol. 54, No. 2, 554–562, 2006.11. Geng, J. P., J. J. Li, R. H. Jin, S. Ye, X. L. Liang, and M. Z. Li, “The development of curved microstrip antenna with defected ground structure,” Progress In Electromagnetics Research, Vol. 98, 53–73, 2009.