Hybrid Fractal Shape Planar Monopole Antenna ... - IEEE Xplore

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 3, MARCH 2014

IV. CONCLUSION By incorporating a varactor diode into its CLL-based NFRP element, a frequency-agile version of a passive fixed-value capacitor, efficient monopole antenna was obtained. This frequency-agile version increased the effective fractional impedance bandwidth by more than a factor of four from its original value. The final configuration emphasized the simplicity of its design; it was fabricated and tested. The measured results demonstrated that the frequency-agile NFRP monopole antenna prototype has good impedance matching, relatively high radiation efficiency, and stable and uniform radiation patterns over its entire frequency-agile range, in good agreement with the predicted values. Because of the favorable simulation results presented for the larger tunable capacitance range, alternate varactor diodes are being considered to achieve a prototype with these design parameters and confirm the predicted substantially larger frequency-agile range.

REFERENCES [1] C. A. Balanis, Antenna Theory: Analysis and Design, 3rd ed. Hoboken, NJ, USA: Wiley Interscience, 2005. [2] Y. Dong and T. Itoh, “Metamaterial-based antennas,” Proc. IEEE, vol. 100, no. 7, pp. 2271–2285, July 2012. [3] J. Oh and K. Sarabandi, “Low profile, miniaturized, inductively coupled capacitively loaded monopole antenna,” IEEE Trans. Antennas Propag., vol. 60, no. 3, pp. 1206–1213, Mar. 2012. [4] R. W. Ziolkowski, P. Jin, and C.-C. Lin, “Metamaterial-inspired engineering of antennas,” Proc. IEEE, vol. 99, no. 10, pp. 1720–1731, Oct. 2011. [5] L. J. Chu, “Physical limitations of omni-directional antennas,” J. Appl. Phys., vol. 19, pp. 1163–1175, Dec. 1948. [6] H. L. Thal, “New radiation limits for spherical wire antennas,” IEEE Trans. Antennas Propag., vol. 54, no. 10, pp. 2757–2763, Oct. 2006. [7] A. Petosa, “An overview of tuning techniques for frequency-agile antennas,” IEEE Antennas Propag. Mag., vol. 54, pp. 271–296, 2012. [8] M. Hassan and G. V. Eleftheriades, “A compact frequency-reconfigurable metamaterial-inspired antenna,” IEEE Antennas Wireless Propag. Lett., vol. 10, pp. 1154–1157, 2011. [9] S. Zhu, D. G. Holtby, K. L. Ford, A. Tennant, and R. J. Langley, “Compact low frequency varactor loaded tunable SRR antenna,” IEEE Trans. Antennas Propag., vol. 61, no. 4, pp. 2757–2763, Apr. 2013. [10] Y. Yu, J. Xiong, H. Li, and S. He, “An electrically small frequency reconfigurable antenna with a wide tuning range,” IEEE Antennas Wireless Propag. Lett., vol. 10, pp. 103–106, 2011. [11] A. Erentok and R. W. Ziolkowski, “Metamaterial-inspired efficient electrically small antennas,” IEEE Trans. Antennas Propag., vol. 56, no. 3, pp. 691–707, Mar. 2008. [12] P. Jin and R. W. Ziolkowski, “Multi-frequency, linear and circular polarized, metamaterial-inspired, near-field resonant parasitic antennas,” IEEE Trans. Antennas Propag., vol. 59, no. 5, pp. 1446–1459, May 2011. [13] N. Zhu, R. W. Ziolkowski, and H. Xin, “Electrically small GPS L1 rectennas,” IEEE Antennas Wireless Propag. Lett., vol. 10, pp. 935–938, 2011. [14] N. Zhu and R. W. Ziolkowski, “Active metamaterial-inspired broad-bandwidth, efficient, electrically small antennas,” IEEE Antennas Wireless Propag. Lett., vol. 10, pp. 1582–1585, 2011. [15] N. Zhu and R. W. Ziolkowski, “Design and measurements of an electrically small, broad bandwidth, non-Foster circuit-augmented protractor antenna,” Appl. Phys. Lett., vol. 101, p. 024107, Jul. 2012. [16] K. B. Alici and E. Ozbay, “Radiation properties of a split ring resonator and monopole composite,” Phys. Stat. Sol. (B), vol. 244, no. 4, pp. 1192–1197, Mar. 2007. [17] K. B. Alici and E. Ozbay, “Electrically small split ring resonator antennas,” J. Appl. Phys., vol. 101, p. 083104, Apr. 2007. [18] O. S. Kim and O. Breinbjerg, “Miniaturised self-resonant split-ring resonator antenna,” Electron. Lett., vol. 45, no. 4, pp. 196–197, Feb. 2009. [19] [Online]. Available: http://www.skyworksinc.com/uploads/documents/SMV1770_Series_200095I.pdf [20] E. Antonino-Daviu, M. Cabedo-Fabrés, M. Ferrando-Bataller, and V. M. R. Peñarrocha, “Modal analysis and design of band-notched UWB planar monopole antennas,” IEEE Trans. Antennas Propag., vol. 58, no. 5, pp. 1457–1467, May 2010.

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[21] Y.-Y. Bai, S. Xiao, M.-C. Tang, Z.-F. Ding, and B. Wang, “Wide-angle scanning phased array with pattern reconfigurable elements,” IEEE Trans. Antennas Propag., vol. 59, no. 11, pp. 4071–4076, Nov. 2011. [22] R. Ludwig and P. Bretchko, RF Circuit Design, Theory and Application, 1st ed. Englewood Cliffs, NJ, USA: Prentice-Hall, 2000. [23] M.-C. Tang and R. W. Ziolkowski, “A frequency agile, ultralow-profile, complementary split ring resonator-based electrically small antenna,” Microw. Opt. Technol. Lett., vol. 55, no. 10, pp. 2425–2428, Oct. 2013. [24] R. Cutshall and R. W. Ziolkowski, “Performance characteristics of planar and three-dimensional versions of a frequency agile electrically small antenna,” IEEE Antennas Propag. Mag., submitted for publication.

Hybrid Fractal Shape Planar Monopole Antenna Covering Multiband Wireless Communications With MIMO Implementation for Handheld Mobile Devices Yogesh Kumar Choukiker, Satish K. Sharma, and Santanu K. Behera

Abstract—A hybrid fractal shape planar monopole antenna covering multiple wireless communication bands is presented for multiple-input-multiple-output (MIMO) implementation for handheld mobile devices. The proposed structure is the combination of Minkowski island curve and Koch curve fractals. It is placed with edge to edge separation at 1.75 GHz. The T-shape strip is inserted and rectangular of slot is etched at top side of ground plane, respectively to improve the impedance matching and isolation between the antennas. A measured dB) are 14% impedance matching fractional bandwidths ( from 1.65 GHz to 1.9 GHz for the band 1 and 80% from 2.68 GHz to 6.25 GHz for the band 2. Acceptable agreement is obtained between the simulated and measured antenna performance parameters. These characteristics demonstrate that the proposed antenna is an attractive candidate for handheld mobile devices. Index Terms—Hybrid fractal antenna, multiple-input-multiple-output (MIMO), multiband wireless communications, planar monopole.

I. INTRODUCTION There is a great demand to enhance data throughput in handheld/portable devices for wireless communications such as live high definition television (HDTV) broadcast, online game, real-time video streaming, and mobile electronic devices [1]. Fractal antennas allow compact, multiband and broadband antenna designs [2], [3]. Most Manuscript received February 11, 2013; revised November 11, 2013; accepted December 08, 2013. Date of publication December 17, 2013; date of current version February 27, 2014. The work of Y. K. Choukiker was supported by the TEQIP-II, National Institute of Technology, Rourkela, Govt. of India. Y. K. Choukiker was with the Antenna and Microwave Laboratory (AML), Department of Electrical and Computer Engineering, San Diego State University, San Diego, CA 92182 USA. He is now with the Microwave and Antenna Design Laboratory, Department of Electronics and Communication Engineering, National Institute of Technology, Rourkela, India (e-mail: [email protected]) S. K. Sharma is with the Antenna and Microwave Laboratory (AML), Department of Electrical and Computer Engineering, San Diego State University, San Diego, CA 92182 USA (e-mail: [email protected]). S. K. Behera is with the Microwave and Antenna Design Laboratory, Department of Electronics and Communication Engineering, National Institute of Technology, Rourkela, India (e-mail: [email protected]). Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2013.2295213

0018-926X © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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Fig. 1. Three initial stages of generation (a) Koch curve fractal and (b) Minkowski curve fractal geometries. Fig. 3. Simulated scattering parameters and isolation of the MIMO antennas with different iterations.

Fig. 4. Simulated scattering parameters and isolation of the MIMO antennas with different configuration of the grooved area and T-shape strip.

Fig. 2. Geometry of the proposed hybrid fractal MIMO antenna: (a) front view, (b) top view, and (c) hybrid fractal design dimensions.

fractal objects have self-similar shape, with different scales [4]. The fractal shape can be carried out by applying the infinite number of iterations using multiple reduction copy machine (MRCM) algorithm [5]. The space filling property, when applied to an antenna element, leads to an increase of the electrical length. The more convoluted and longer surface currents result in lowering the antenna resonant frequency for a given overall extension of the resonator. The fractal miniaturization technique has already been applied to Koch wire monopole [6], combination of fractal geometries [7]–[11], and the Sierpinski fractal-shape antennas [12]–[14]. Although the essence of this technique falls into the inductive loading, the radiation patterns of the antennas derived from this technique are maintained because of the self-similarities of the fractals. Recently, theoretical and experimental published articles [15]–[23] confirm the superior data rate, multipath fading reduction and co-channel interference suppression capability when the antennas are implemented in MIMO arrangements. Moreover, the trend for mobile terminals nowadays is to accommodate the increased number of wireless communication applications. In [15], [16], MIMO implementation of the microstrip antennas is applied for the wireless digital television (DTV) media players covering long term evaluation (LTE) bands and for USB dongle for wireless LTE/WLAN bands, respectively. Various techniques have been reported to enhance the isolation between the MIMO antenna

Fig. 5. Simulated surface current distributions of the MIMO antennas: (a) Antenna 1 at 1.75 GHz and 4.5 GHz and (b) Antenna 2 at 1.75 GHz and 4.5 GHz.

elements. Further, there is demand for diversity antennas in the LTE/WiFi/Wimax/WLAN bands [17]. In [18], the authors reduced the isolation between the antennas using bent slit and a metal strip between the two antennas. In [19], the authors used neutralization lines for achieving the isolation. The isolation/mutual-coupling between two pots are good across the bandwidth. However, it cannot cover the most desired bands, such as LTE and UMTS in limited antenna volume. Another paper [20] reports a quad band MIMO antenna for wireless communication terminal, where the radiating element is combination of the C-shaped slot and T-shaped slit. In [21], the authors show the closely spaced element with large bandwidth. However, the relatively large antenna volume and strong coupling between its two elements restrict its applications. In [22], a dual band MIMO antenna with two back-to-back monopoles in symmetric configuration is presented. Here, radiating elements are placed on a large volume of metal case which makes it difficult to be applied in a hand held/portables mobile devices. For fractal MIMO implementation in [23], the authors used a Koch curve edge in microstrip patch. Another fractal MIMO antenna in [17]


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Fig. 6. Simulated 3D gain radiation patterns for the antenna 1 (a), (c), (e), and (g) and the antenna 2 (b), (d), (f), and (h). (a), (b) are at 1.75 GHz; (c), (d) are at 3 GHz; (e), (f) are at 4.5 GHz; and (g), (h) are at 6 GHz.

shows Koch curve and Minkowski curve based fractal MIMO implementation for multiband applications. To enhance data throughput for wireless communications, multiple-input-multiple-output (MIMO) antennas should be implemented in a handheld/portable device. Also, to support several wireless communication applications on a device, multiband antennas are desired. Such a task becomes even more challenging when, besides multiband operations, a high degree of miniaturization is required. Fractal structure, having self-similarity and space filling properties, can produce a very long length or a wide surface area in limited space. Therefore, in this communication, we propose a hybrid fractal shaped planar monopole antenna as a radiating element for the MIMO implementation. It is a combination of Koch curve and Minkowski island fractals with compact size of 10 10 mm . The proposed antenna covers LTE/WiFi/WiMAX/WLAN wireless communication bands with near omni-directional radiation patterns. Here, a defected ground plane structure is used (a combination of rectangular grooved area and T-shaped strip) to obtained high isolation between the two MIMO antenna elements. Parametric study is performed to analyze the effect of the T-shaped strip and grooved ground plane parameters on the operating frequency as well as the isolation between the two antennas. The Ansys high frequency structure simulator (HFSS) version 14 was used for modeling and analysis of the proposed antenna. In Section II, geometry of the proposed antenna and simulation results are presented. Experimental verification of proposed antenna is discussed in Section III. Finally, Section IV concludes major findings. II. ANTENNA GEOMETRY AND SIMULATIONS RESULTS A. Antenna Geometry The recursive procedure of the Koch and Minkowski island curve fractals are shown in Fig. 1. To obtain the self-similarity dimensions, the geometry is scaled down, but with identical copies of itself. If there are such copies of original geometry scaled down by a function , the similarity dimension is defined in the following: (1) For example, a square can be divided into four copies of 1/2 scale, copies of 1/n nine copies of 1/3 scale, 16 copies of 1/4 scale, or scale. Substituting in the above formula, the dimension of the geometry is ascertained to be 2. This approach can be followed in determining the dimension of fractal geometries. For construction of Koch and Minkowski curve fractals, one can start with a straight-line called initiator and it is divided into three equal parts . In case of

TABLE I DIMENSIONS OF PROPOSED HYBRID FRACTAL MIMO ANTENNA

the Koch curve, the middle segment is divided and replaced with two other segment of the same length. Besides, in case of the Minkowski island curve, the middle segment is replaced by two horizontal and a vertical segment of equal lengths. This is the first iterated version of geometry and is called “generator” for higher iterations, as shown in Fig. 1(a) and (b). This procedure is iterated recursively to result in self-similar fractal geometry by taking the order of iteration and the (1) as the input parameters. Basically, individual iteradimension tions are applied to both the Koch and Minkowski curves which are then combined to get hybrid fractal geometry. For further optimizations, dimension “ ” for both fractals are varied simultaneously. Geometry of the proposed hybrid fractal MIMO antenna is shown in Fig. 2(a) along with its final dimensions as listed in Table I. Koch curve fractal and Minkowski island curve fractal are applied to the edges of the square patch up to the second iteration. Its dimensions are also indicated in Fig. 2(c). The motivation behind using such geometry is to improve the space filling, a feature that translates into reduced antenna physical size and for the increased number of resonant frequency bands. The antenna is fed through the 50 SMA coaxial probe connected to the microstrip line with matching section over the grooved ground plane. As shown in Fig. 2, MIMO antenna consists at 1.75 GHz) beof the edge-to-edge separation of 28.02 mm ( tween the two symmetrical hybrid fractal radiating elements. The FR-4 substrate of size 100 50 mm with height is used. The are placed on one side of the substrate radiating elements and grooved ground plane of size 86 50 mm is located on the other side. It should be noted that the grooved area ( and is located and with fixed on top of the ground plane with T-shape strip ( width of 2 mm. It is introduced to improve the impedance matching and isolation between the antennas. B. Simulation Results As the number of iterations increase, the average electrical length of the monopole also increases, just like the inductive loading and slot loading techniques reported in [24], [25], which consequently, lowers the operating frequency of the proposed antenna and leads to an effective antenna miniaturization. However, for the iterations higher than the second, the reduction of operating frequency is not achievable since

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Fig. 9. Simulated and measured envelope correlation coefficients (ECC) of the proposed MIMO antenna.

Fig. 7. Photographs of the fabricated hybrid fractal MIMO antennas: (a) top view and (b) bottom view.

Fig. 10. Simulated and measured capacity loss of the proposed MIMO antenna.

Fig. 8. Simulated and measured scattering parameters of the proposed antenna. and ) and (b) isolation ( and (a) Reflection coefficient magnitude ( ).

the antenna design becomes quite complicated and its fabrication is difficult. of the antennas and The reflection coefficient magnitudes between the antennas are shown in Fig. 3. It is isolation observed that, when the number of iterations increase, the fractional dB definition) of the antenna matching bandwidths ( also increases. There are several visible multiple resonances within the bandwidth. For the iteration 2, impedance bandwidths are 14% from 1.65 GHz to 1.9 GHz for the band 1 and 80% from 2.68 GHz to 6.45 GHz for the band 2, which covers LTE band (1.7–1.9 GHz) and several wireless communication bands like WiFi/WiMAX/WLAN bands is below dB (2.68–6.45 GHz). In addition, the isolation dB for the band 1 and band 2, respectively. and

Fig. 4 shows the simulated and of the proposed monopole antenna for the grooved ground plane and T-shape strip for mm and the two cases. In case I, the grooved rectangular slot ( mm) is present without T-shape strip. It is observed that, the reflection coefficient magnitude and isolation are not good enough for the desired frequency bands (LTE/WiFi/WiMAX/WLAN). In case II mm, mm, mm and mm), it is ( observed that the reflection coefficient magnitude and isolation are well below the acceptable criteria for our desired operating bands. Thus, the proposed structure is providing very satisfactory performance for the both bands 1 and 2 with the case II design parameters. Fig. 5(a) and (b) shows the surface current distributions at 1.75 GHz and 4.5 GHz for the proposed MIMO antenna with grooved ground and T-shape strip. As can be seen, when antenna 1 is excited and antenna 2 is terminated in 50 load, and vice versa, the surface current flows in the feed line as well as in T-shape strip at both the frequencies. It can be noticed that there is negligible current on the second radiator due to the presence of T-shape strip, thereby improving isolation between the antennas. This tends to decouple current on antenna 2 and hence it enhances the isolation between the two radiators efficiently at 1.75 GHz and 4.5 GHz. Fig. 6(a)–(h) shows the gain 3D radiation patterns for the antenna 1 and antenna 2 at 1.75 GHz, 3 GHz, 4.5 GHz and 6 GHz. It can be seen that, the gain of the proposed monopole antenna at all the frequencies within the bands are more than 2 dBi. The 3D patterns are omni-directional towards the lower frequency band and becomes nearly omni-directional with multiple lobes towards the higher frequency end. Table II provides the measured gain, simulated gain and simulated total antenna efficiency at the band 1 and band 2. From the table, we can observe that, some variation in measured and simulated gain occurs but the difference is not consistent in nature. This may be attributed to fabrication tolerances associated with fractal shape, when simulation expected sharp edges, but fabrication provided rounded corners. This may result in a different surface current on the antenna structure than the simulated ones, hence variation in gain. Also, scattering effects due to the


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Fig. 11. Measured radiation pattern for the antenna 1 (a), (c), (e), and (g) and the antenna 2 (b), (d), (f) and (h). (a), (b) are at 1.75 GHz (XZ and YZ Co- and Cross-pols.); (c), (d) are at 3 GHz (XZ and YZ Co- and Cross-pols.); (e), (f) are at 4.5 GHz (XZ and YZ Co- and Cross-pols.); and (g), (h) are at 6 GHz (XZ and YZ Co- and Cross-pols.).

TABLE II GAIN AND TOTAL ANTENNA EFFICIENCY

field radiation pattern measurements and consequently, ECC calculation, the S-parameters based ECC computation is preferred. According of the two antenna systems can be computed to [15], the ECC using the following (2): (2)

feed cable and antenna under test (AUT) mount attributes to this variation. However, total antenna efficiency, which includes all the losses, are more than 80% throughout the band 1 and band 2. III. EXPERIMENTAL VERIFICATION Proposed hybrid fractal MIMO antenna is milled on the copper side of FR-4 substrate using LPKF-42 Protomat milling machine. Photographs of fabricated antenna are shown in Fig. 7. The antenna is measured using Anritsu (model #37486D) vector network analyzer (VNA) for impedance matching. The radiation patterns were measured in an anechoic chamber, both available at the antenna and microwave laboratory (AML) at SDSU. Measured and simulated values of the and isolation reflection coefficient magnitude are plotted in Fig. 8(a) and (b), respectively. These results exhibit reasonable agreement although there is a frequency shift that can be attributed to reflection from SMA connector and some uncertainty in the electrical properties of the substrate. As can be seen, measured impedance bandwidth is 14% from 1.65 GHz to 1.9 GHz for the band 1 and 80% from 2.68 GHz to 6.25 GHz for the band 2. The bandwidths achieved meet the requirements of LTE/WiFi/WiMAX/WLAN communication applications. The isolation between the two antennas dB and dB for the lower and higher frequency is below bands, respectively. For MIMO application, the envelope correlation coefficient (ECC) can be computed using either the far field radiation patterns [16] or scattering parameters method [15]. Due to complication of the 3D far

Fig. 9 shows the simulated and measured ECC across the desired frequency bands. As can be seen, the value of ECC is well below the practical threshold value of 0.5 for both the band 1 and band 2. Similarly, capacity loss (b/s/Hz) is another performance parameter which characterizes quality of a MIMO antenna system. Channel capacity is the tightest upper bound on the rate of information that can be reliably transmitted over a communications channel. This can be defined using the correlation matrix given in [27], and is calculated by using the following (3) [18], [26] which is found to be below 0.4 b/s/Hz (3) where

is the receiving antenna correlation matrix that is given by:

and

Fig. 10 shows the comparison of measured and simulated capacity loss values of the proposed MIMO antenna. It can be observed that, the capacity loss does not exceed 0.3 b/s/Hz and it is well below the threshold value 0.4 b/s/Hz for both the band 1 and band 2. Fig. 11 shows the measured radiation patterns at the band 1 and band 2 for XZ (Co-pol. and Cross-pol.) and YZ (Co-pol. and Cross-pol.) cut planes. During the measurement, only antenna 1 is excited while antenna 2 is terminated with a 50 broadband load. It can be seen from Fig. 11(a)–(h) that the patterns are omni-directional but towards the

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Fig. 12. Measured peak realized gain (dBi) of the antenna 1 for the band 1 and band 2 while antenna 2 is matched terminated.

higher frequency band, it is nearly omni-directional with some multiple lobes. In the proposed MIMO antenna, the radiators are placed symmetrically, therefore, it will provide complementary or diversity patterns when one antenna is excited at a time and the other one is matched terminated. The measured gain is plotted in Fig. 12. Due to symmetry of the antenna structure and their common fabrication tolerances; the gain values of the two radiators are almost the same. Therefore, only the gain of the antenna 1 is presented, while the antenna 2 is terminated in a 50 matched load. The measured peak gain is varying dBi with a maximum gain of around 2 dBi for the between 0 and band 1. Similarly, for the band 2, it is varying between 2.5 dBi and 4.85 dBi with a maximum gain of 7 dBi. IV. CONCLUSION A hybrid fractal planar monopole MIMO antenna has been investigated. By incorporating the Minkowski island curve and Koch curve fractals it is found that the operating frequency of the fractal antenna is lowered greatly which provides measured bandwidths of 14% for the band 1 (1.65–1.9 GHz) and 80% for the band 2 (2.68–6.25 GHz). The simulated and measured ECC computed using the scattering parameters is very low for the proposed antenna. Similarly, the capacity loss remains below 0.3 b/s/Hz throughout the communication bands. The simulated 3D radiation patterns and measured 2 D cut patterns show acceptable omni-directional patterns. Overall, the antenna performance is suitable for handheld devices covering several wireless communication bands (i.e., LTE/WiFi/WiMAX/WLAN). ACKNOWLEDGMENT Y. K. Choukiker thanks the Antenna and Microwave Lab (AML) at San Diego State University for offering him resources to perform the work. He is also thankful to B. Babakhani, and S. Fernandez for their support during the simulation and measurements.

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