Overcoming Mutual Blockage Between Neighboring Dipole Antennas ...

8 downloads 0 Views 627KB Size Report
Dec 14, 2012 - Dipole Antennas Using a Low-Profile. Patterned Metasurface. Alessio Monti, Student Member, IEEE, Jason Soric, Student Member, IEEE, ...
1414

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

Overcoming Mutual Blockage Between Neighboring Dipole Antennas Using a Low-Profile Patterned Metasurface Alessio Monti, Student Member, IEEE, Jason Soric, Student Member, IEEE, Andrea Alù, Member, IEEE, Filiberto Bilotti, Senior Member, IEEE, Alessandro Toscano, Senior Member, IEEE, and Lucio Vegni, Life Member, IEEE

Abstract—In this letter, we investigate the possibility of using the mantle cloaking approach to reduce mutual blockage effects between two electrically close antennas. In particular, we consider the case of two dipoles resonating at different, close frequencies and at 3 GHz). We separated by an electrically short distance ( show that by covering the two antennas with properly patterned metasurfaces printed on realistic substrates, it is possible to make each antenna invisible to the other and preserve their individual operation as if they were isolated. This new cloaking application is confirmed by realistic full-wave numerical simulations. Index Terms—Cloaking, dipole antennas, metasurfaces.

I. INTRODUCTION

I

N THE last decade, there has been a worldwide effort in the design of electromagnetic covers that may strongly reduce the visibility, scattering signature, and electromagnetic interference of objects of various material compositions and geometries over a desired range of frequencies. Several cloaking approaches have been proposed for acoustic [1], [2] and electromagnetic waves [3]–[9]. Recent progress has been also made in their realization at microwave frequencies [10]–[14]. Arguably, the best-known cloaking technique is based on conformal mapping [4]–[6], which uses an inhomogeneous cover to guide an impinging wave around an object. In this way, any interaction between the field and the object is avoided and, consequently, objects with arbitrary geometry and size can be made, in principle, invisible. The associated design complexity, narrow bandwidth, and susceptibility to losses and manufacturing tolerances have limited their applicability in realistic scenarios [13]. Another technique, in some sense related to conformal mapping, requires transmission lines or radial waveguides surrounding an array of objects or a single bulk object [6]–[8]. This technique appears more feasible than conformal mapping, but, also in this Manuscript received October 04, 2012; accepted November 02, 2012. Date of publication November 20, 2012; date of current version December 14, 2012. This work was supported in part by the NSF under CAREER Award No. ECCS0953311 and the AFOSR under YIP Grant No. FA9550-11-1-0009. A. Monti, F. Bilotti, A. Toscano, and L. Vegni are with the Department of Applied Electronics, “Roma Tre” University, 00146 Rome, Italy (e-mail: [email protected]). J. Soric and A. Alù are with the Department of Electrical and Computer Engineering, University of Texas at Austin, Austin, TX 78712 USA. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2012.2229102

case, the cloak isolates its internal region from its surroundings. A different approach, known as plasmonic cloaking, was proposed in [3] based on the scattering cancellation method. In this construction, a cover surrounding the object does not inhibit the interaction between the object and the external field, but produces an “anti-phase” scattering contribution able to compensate the scattering due to the object alone. This approach requires that the cover is conformal to the object and that the cover material is characterized, in the desired range of frequencies, by a negative (ENG), low-positive, or near-zero (ENZ) relative permittivity [3], [15]–[18]. Recently, it has also been shown that scattering cancellation can be achieved by using thin realistic metasurfaces, rather than volumetric bulk metamaterials [9], [19]. In this case, the metasurface is designed so that the current distribution induced by the external electromagnetic field scatters an out-of-phase field compared to the one due to the object alone. This approach is commonly referred to as “mantle cloaking,” and realistic implementations at microwave [19]–[22] and terahertz (THz) frequencies [23] have been recently proposed. We put forward here an interesting antenna application of cloaking, consisting in the reduction of the blockage effect between closely spaced antennas. It is well known that the presence of an obstacle in the near field of an antenna appreciably modifies its radiating (e.g., the radiation pattern shape) and electrical (e.g., the impedance matching) features. Several papers concerning the reduction of the blockage effect caused by passive objects have been proposed. For example, in [24] and [25], soft and hard surfaces have been shown to reduce the scattering of a cylindrical object (e.g., a strut or the feed) part of a parabolic antenna. More recently, in [26] and [27], metal-plate and transmission-line cloaks were shown to reduce the blockage effect of a passive object placed directly in front of a horn antenna. However, there are very few studies about the reduction of the mutual blockage effect between two antennas that are designed in free-space but are forced, for space constraints, to stay in close electrical proximity. In this situation, which is ever more common in modern crowded electromagnetic environments, the obstacle placed in the proximity of the antenna is another antenna, usually operating at a different frequency. Here, the performance degradation of both antennas can become dramatic, affecting antenna coverage as well as their resonant properties. The reduction of mutual blockage is not a straightforward task due to the fact that the object to hide is not passive and

1536-1225/$31.00 © 2012 IEEE

MONTI et al.: OVERCOMING MUTUAL BLOCKAGE BETWEEN NEIGHBORING DIPOLE ANTENNAS

owns an electromagnetic functionality. The cloak, thus, must not affect excessively the antenna performance at its operating frequency. In order to overcome this issue, in [28] the authors proposed the employment of an eccentric elliptical transformation-based cloak to restore the parameters of a two-dimensional (2-D), infinitely long (i.e., nonresonant) electrical source, whose radiation is blocked by an identical scatterer placed in its neighborhood. This solution, though fascinating, is difficult to realize experimentally, and it is based on a variety of ideal assumptions hardly met in realistic antenna configurations, which are commonly formed by three-dimensional (3-D) resonating elements. In this letter, we show that the mantle cloaking technique may be very suitable for the same purpose in realistic configurations. We propose to cover two electrically close 3-D half-wavelength resonating dipoles with realistic metasurfaces designed according to the approach presented in [9]. This way, it is possible to effectively reduce the blockage between the two radiators and recover their original radiation and matching properties as if they were isolated. The covers consist of properly patterned metallic surfaces printed on realistic dielectric substrates. II. MANTLE CLOAKS DESIGN We consider as our reference scenario the situation in which two transmitting dipole antennas, resonating at slightly different frequencies, are placed in very close electrical proximity of each other. The two dipoles are designed in free space to operate for different services (e.g., two dipoles of different mobile network operators hosted by the same telecommunication strut). The shorter dipole is designed to resonate in free-space at GHz, while the longer one resonates at GHz. Both antennas were optimized to be matched to a 75- feed. When placed in close proximity, in particular with a separation of just at 3 GHz, their radiation performance is largely compromised due to mutual blockage. This is shown in Fig. 1, where the antenna system has been characterized using the full-wave numerical simulator CST Studio Suite 2012. The radiation patterns of the two isolated dipoles at their respective resonance frequencies fed by 75- ports are shown in Fig. 1(a) and (b), respectively, while in Fig. 1(c) and (d) we show the same patterns when the dipoles are placed in close proximity to each other and separately driven. As expected, the radiation pattern of each antenna is dramatically affected by the presence of a closely spaced metallic scatterer (i.e., the other dipole) in its near-field region. In Fig. 2, we show the reflection coefficient at the input ports in the isolated and coupled scenarios. In this latter case, the two dipoles are no longer matched to the source, again due to mutual coupling. The presence of the other dipole, in fact, drastically affects the near-field distribution and, thus, the input reactance of both antennas. In order to reduce the coupling effects, we cover each dipole with a mantle cloak capable of preserving and restoring their isolated radiation characteristics and canceling their mutual coupling. The first antenna is covered with a mantle cloak working at , while the second dipole is cloaked at . The conformal mantle cloaks have been designed over a flexible dielectric substrate, whose external face is suitably patterned with a metallic surface, as shown in Fig. 3. The patterns of the

1415

Fig. 1. 3-D gain patterns of (a) the isolated first dipole at , (b) the isolated , (c) the first dipole at in presence of the second dipole, second dipole at in presence of the first dipole. and (d) the second dipole at

Fig. 2. Amplitude of the reflection coefficient at the input port of the first and second dipole in the isolated case (continuous and dashed line, respectively) and in the reference scenario (dotted line and dash-dotted line, respectively).

metasurfaces have been chosen after optimization to realize the required surface reactance at the design frequency [9]. Considering the specific field polarization, vertical metallic strips are the most suitable geometry to affect the local averaged reactance, and we have performed a proper optimization to minimize the total scattering cross section (SCS) of each 3-D dipole at the resonance frequency of the other dipole under transverse magnetic (TM) plane-wave excitation. The geometrical parameters of the two metasurfaces are reported in the caption of Fig. 3. We calculated an overall SCS reduction of nearly 15 dB for both cloaked antennas under ideal TM plane-wave excitation. In our reference scenario, however, each dipole is excited by the near field of the other antenna, which is markedly different from a plane wave. Still, the scattering cancellation effect is very robust to different excitations [29], and it works also in the

1416

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

Fig. 3. Two dipole antennas cloaked with two metasurfaces consisting of vertical metallic strips printed on thin, flexible, commercial substrates (DuPont , at 2 GHz and Roger TMM4 with 951 with , at 10 GHz, respectively). The geometrical parameters are: , mm, mm and (b) , mm (a) mm. The cover radii are and , respectively, and being the antenna radius.

Fig. 5. Amplitude of the reflection coefficient at the input port of the first and second dipole in the uncloaked case (continuous and dashed lines, respectively) and of the first and second dipole in the cloaked case (dotted and dash-dotted lines, respectively).

Fig. 6. (a) Gain pattern of the first antenna at 3.298 GHz on the H-plane and (b) gain pattern of the second antenna at 2.970 GHz in the isolated, uncloaked, . and cloaked scenario for a separation distance equal to

Fig. 4. Gain patterns of the first antenna at 3.298 GHz on the (a) H-plane and (b) E-plane in the case of isolated dipoles (continuous line), the uncloaked scenario (dashed line), and the cloaked one (dotted line). (c) and (d) show the same quantities for the second antenna at 2.970 GHz.

very near field of the object [30]. Therefore, we expect a large scattering reduction even in this extreme excitation scenario in which the two antennas are closely coupled and surrounded by a thin patterned conformal surface. III. NUMERICAL RESULTS The radiation and matching properties of the two cloaked antennas are shown in Fig. 4, where we show the gain patterns (dotted line) on the H- and E-plane compared to the uncloaked (dashed line) and isolated (continuous line) cases. It is evident

that by applying the patterned metasurface covers, we obtain a significant restoration of the original patterns, despite the very small distance between them. The two antennas are effectively invisible to each other at their respective operating frequencies. Furthermore, in Fig. 5, we show the amplitude of the reflection coefficient at the input ports of the two antennas in the cloaked case. As expected, around the resonant frequency of each isolated dipole, we obtain a significant improvement of the impedance matching of the two antennas compared to the uncloaked case. We have also considered the performance of the designed cloaks for different separation distances and particularly when the two dipoles further approach each other. In Fig. 6 we show the gain patterns on the H-plane for a separation distance between dipole centers equal to at 3 GHz (the gain patterns on the E-plane are omitted for brevity). Please note that the covers are unchanged and are the same as in the previous example. The cloak performance, in terms of restoration of original gain pattern and impedance matching, are essentially unchanged. These results are further evidence of the robustness of the mantle cloak against different excitations, including those in the very near field. We also analyze the operational bandwidth of the designed cloaks in terms of far-field radiation. For this purpose, in Fig. 7 we show the radiation patterns (on the H-plane) of the two

MONTI et al.: OVERCOMING MUTUAL BLOCKAGE BETWEEN NEIGHBORING DIPOLE ANTENNAS

Fig. 7. Gain patterns on the H-plane of the (a) first and (b) second dipole around their respective resonance frequencies.

dipoles in the neighborhood of the cloaking frequencies. It is clear that the obtained cloaking effect and the respective mutual blockage reduction is rather narrowband due to the different frequency dispersion of the cloak and the antenna itself. With proper optimization, it may be possible to obtain a more broadband effect, in particular if the operating frequency of the two antennas is not so close in frequency. IV. CONCLUSION We have proposed an innovative solution to sensibly reduce the mutual blockage effect between two transmitting 3-D dipole antennas placed in close electrical proximity. Our setup is based on the mantle cloaking approach, and it requires covering each dipole with properly patterned metallic metasurfaces printed on flexible dielectric substrates. The metasurface covering each dipole acts as an electromagnetic cloak at the resonant frequency of the other dipole. In this way, the performances of each transmitting antenna at their respective operating frequencies are not influenced by the presence of the other dipole, which is effectively invisible even in its very near field. A striking restoration of each antenna’s radiation features was presented, showing radiation patterns nearly unperturbed compared to the isolated antenna. Additionally, the matching performance of each antenna was shown to be substantially improved over a narrowband around each design frequency. The designed metasurfaces are robust to size and realistic losses; therefore, we envision practical implementations within current technology, which are currently being considered. Full-wave simulations confirming the desired behavior have been demonstrated, together with some considerations on the bandwidth of the proposed setup. We stress that the present design refers to the worst-case scenario in which the two neighboring antennas operate in a similar frequency spectrum. For larger differences between the two operating frequencies, broader bandwidths of operation and even better results are expected. REFERENCES [1] A. N. Norris, “Acoustic cloaking theory,” Proc. R. A., vol. 464, pp. 2411–2434, 2008. [2] M. D. Guild, M. R. Haberman, and A. Alù, “Plasmonic cloaking and scattering cancelation for electromagnetic and acoustic waves,” Wave Mot., vol. 48, pp. 468–482, 2011. [3] A. Alù and N. Engheta, “Achieving transparency with plasmonic and metamaterial coatings,” Phys. Rev. E, vol. 72, p. 016623, 2005. [4] J. B. Pendry, D. Schurig, and D. R. Smith, “Controlling electromagnetic fields,” Science, vol. 312, pp. 1780–1782, 2006.

1417

[5] U. Leonhardt, “Optical conformal mapping,” Science, vol. 312, pp. 1777–1780, 2006. [6] J. Li and J. B. Pendry, “Hiding under the carpet: A new strategy for cloaking,” Phys. Rev. Lett., vol. 101, p. 203901, 2008. [7] P. Alitalo, O. Luukkonen, L. Jylha, J. Venermo, and S. A. Tretyakov, “Transmission-line networks cloaking objects from electromagnetic fields,” IEEE Trans. Antennas Propag., vol. 56, no. 2, pp. 416–424, Feb. 2008. [8] S. A. Tretyakov, P. Alitalo, O. Luukkonen, and C. Simovski, “Broadband electromagnetic cloaking of long cylindrical objects,” Phys. Rev. Lett., vol. 103, p. 103905, 2009. [9] A. Alù, “Mantle cloak: Invisibility induced by a surface,” Phys. Rev. B, vol. 80, p. 245115, 2009. [10] D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, and D. R. Smith, “Demonstration of a metamaterial electromagnetic cloak at microwave frequencies,” Science, vol. 314, pp. 977–980, 2006. [11] B. Edwards, A. Alù, M. Silveirinha, and N. Engheta, “Experimental verification of plasmonic cloaking at microwave frequencies with metamaterials,” Phys. Rev. Lett., vol. 103, p. 153901, 2009. [12] D. Rainwater, A. Kerkhoff, K. Melin, J. C. Soric, G. Moreno, and A. Alù, “Experimental verification of three-dimensional plasmonic cloaking in free-space,” New J. Phys., vol. 143, p. 013054, 2012. [13] P. Y. Chen, J. C. Soric, and A. Alù, “Invisibility and cloaking based on scattering cancellation,” Adv. Mater., vol. 24, pp. 281–304, 2012. [14] P. Alitalo, A. E. Culhaoglu, A. V. Osipov, S. Thurner, E. Kemptner, and S. A. Tretyakov, “Bistatic scattering characterization of a threedimensional broadband cloaking structure,” J. Appl. Phys., vol. 111, p. 034901, 2012. [15] F. Bilotti, S. Tricarico, and L. Vegni, “Electromagnetic cloaking devices for TE and TM polarizations,” New J. Phys., vol. 10, p. 115035, 2008. [16] F. Bilotti, S. Tricarico, and L. Vegni, “Plasmonic metamaterial cloaking at optical frequencies,” IEEE Trans. Nanotechnol., vol. 9, no. 1, pp. 55–61, Jan. 2010. [17] A. Monti, F. Bilotti, and A. Toscano, “Optical cloaking of cylindrical objects by using covers made of core–shell nanoparticles,” Opt. Express, vol. 36, pp. 4479–4481, 2011. [18] A. Monti, F. Bilotti, A. Toscano, and L. Vegni, “Possible implementation of epsilon-near-zero metamaterials working at optical frequencies,” Opt. Commun., vol. 285, pp. 3412–3418, 2012. [19] P. Y. Chen and A. Alù, “Mantle-Cloaking using thin patterned metasurfaces,” Phys. Rev. B., vol. 84, p. 205110, 2011. [20] Y. R. Padooru, A. B. Yakovlev, P. Y. Chen, and A. Alù, “Analytical modeling of conformal mantle cloaks for cylindrical objects using sub-wavelength printed and slotted arrays,” J. Appl. Phys., vol. 112, p. 034907, 2012. [21] P. Y. Chen, F. Monticone, and A. Alù, “Suppressing the electromagnetic scattering with an helical mantle cloak,” IEEE Antennas Wireless Propag. Lett., vol. 10, pp. 1598–1601, 2011. [22] J. C. Soric, A. Alù, A. Kerkhoff, and D. Rainwater, “Experimental demonstration of a conformal mantle cloak for radio-waves,” in Proc. IEEE APS Int. Sym., 2012, pp. 1–2. [23] P. Y. Chen and A. Alù, “Atomically-thin surface cloak using graphene monolayers,” ACS Nano, vol. 5, pp. 5855–5563, 2011. [24] P. S. Kildal, A. A. Kishk, and A. Tengs, “Reduction of forward scattering from cylindrical objects using hard surfaces,” IEEE Trans. Antennas Propag., vol. 44, no. 11, pp. 1509–1520, Nov. 1996. [25] M. Riel, Y. Brand, Y. Demers, and P. D. Maagt, “Performance improvements of center-fed reflector antennas using low scattering struts,” IEEE Trans. Antennas Propag., vol. 60, no. 3, pp. 1269–1280, Mar. 2012. [26] P. Alitalo, C. Valagiannopoulos, and S. A. Tretyakov, “Simple cloak for antenna blockage reduction,” in Proc. IEEE APS Int. Symp., 2011, pp. 669–672. [27] P. Alitalo, J. Vehmas, and S. A. Tretyakov, “Reduction of antenna blockage with a transmission-line cloak,” in Proc. 5th EuCAP, 2011, pp. 2399–2402. [28] D. H. Know and D. H. Werner, “Restoration of antenna parameters in scattering environments using electromagnetic cloaking,” Appl. Phys. Lett., vol. 92, p. 113507, 2008. [29] A. Alù and N. Engheta, “Plasmonic materials in transparency and cloaking problems: Mechanism, robustness, and physical insights,” Opt. Express, vol. 15, p. 3318, 2007. [30] A. Alù and N. Engheta, “Cloaking and transparency for collections of particles with metamaterial and plasmonic covers,” Opt. Express, vol. 15, p. 7578, 2007.