Dual-Polarized Reduction of Dipole Antenna Blockage ... - IEEE Xplore

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 11, NOVEMBER 2015

4827

Dual-Polarized Reduction of Dipole Antenna Blockage Using Mantle Cloaks Jason C. Soric, Member, IEEE, Alessio Monti, Member, IEEE, Alessandro Toscano, Senior Member, IEEE, Filiberto Bilotti, Senior Member, IEEE, and Andrea Alù, Fellow, IEEE

Abstract—We theoretically and experimentally explore the use of mantle covers to cloak dipole antennas and reduce their blockage on nearby antennas. The proposed structures allow reducing the observability in a given frequency range and overall scattering of conventional dipole antennas operating in a different frequency band, ideally suited to reduce the mutual influence among antennas placed in close proximity to each other. We prove this concept in the case of a low-band (LB) dipole antenna placed in close proximity to a high-band dipole antenna, and study several thin cover designs that are shown to be effective in reducing the LB blockage, without disrupting the performance of both antennas in terms of radiation pattern and impedance matching. An optimized cover is proposed to strongly reduce the interference and shadowing over a large bandwidth targeted for one polarization, and we also experimentally demonstrate the operation of dual-polarized covers for near-field horn and log-periodic antenna excitation. Index Terms—Antenna blockage, cloaking, metasurface, miniaturized element frequency selective surface (MEFSS), RCS reduction.

I. I NTRODUCTION

I

N MANY applications, there is significant need to make antennas low-observable or radio-transparent. For instance, in tomography, imaging, and energy harvesting, low-visibility receiving antennas may significantly enhance the resolution and the global performance of sensing and monitoring systems. Another relevant example is represented by high-complexity or crowded communication systems, in which interference or blockage effects caused by closely located antennas may significantly affect the performance of a given radiator [1]–[3]. In these contexts, reducing the electromagnetic presence of an antenna or a sensor by tailoring its geometry, loading, or using a cover has been explored and demonstrated in several examples [4]–[10] (and references therein). In recent years, we have shown that ultrathin impedance surfaces may be applied to cover dielectric and conductive passive objects (not owing any electromagnetic functionality) in order to suppress their

Manuscript received December 31, 2014; revised May 10, 2015; accepted August 29, 2015. Date of publication September 03, 2015; date of current version October 28, 2015. This work was supported in part by the NSF CAREER Award no. ECCS-0953311, in part by the AFOSR YIP Award no. FA955011-1-0009, and in part by the DTRA YIP Award no. HDTRA1-12-1-0022. (Corresponding author: Alessio Monti.) J. C. Soric and A. Alù are with the Department of Electrical and Computer Engineering, University of Texas at Austin, Austin, TX 78712 USA (e-mail: [email protected]). A. Monti is with the Niccolo Cuscano University, Rome 00166, Italy. A. Toscano, and F. Bilotti are with the Department of Engineering, University of Roma Tre, Rome, Italy. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2015.2476468

overall scattering signature at the frequency of interest [11]– [18]. These ultrathin surfaces may significantly reduce the total integrated scattering cross-section (SCS) of targets of moderate size (2a ≤ λ), where a is the cross-sectional radius and λ is the free-space wavelength. For conducting objects, the bandwidth and suppression level of such mantle cloaks are mainly dependent on the conformability of the cover to its target, where more conformal designs lead to a stronger scattering suppression (>15 dB) over a narrow bandwidth (∼3%) [17]. Conversely, covers with a larger separation from the target, may achieve a more shallow suppression (∼5 dB) up to 30% fractional bandwidths [18]. A key feature of the scattering cancellation technique applied to these examples [11], [19] is the ability of the cloaked object to interact with the background region [7]–[10], rather than being isolated as in other approaches to cloaking. These features are ideally suited for antenna applications, including blockage reduction from passive obstacles, elimination of the mutual coupling between closely spaced antennas [8], and the realization of low-visibility receiving antennas for sensing and monitoring applications [7], [9]. In this work, we expand on this topic and discuss the potential that these mantle cloaks may specifically have in radiofrequency communication systems and dense antenna arrays, providing optimized designs and experimental evidence for mantle cloaks optimally tailored for antenna systems. We envision practical dipole antennas operating over a wide-frequency band and covered by suitable and easy-to-manufacture ultrathin covers that may strongly reduce their visibility to and blockage effects toward nearby antennas operating in different frequency bands and on both polarizations. These concepts, originally inspired by the ones of conventional (FSS) and miniaturized element (MEFSS) frequency-selective surfaces [20]–[22], apply the inherently nonresonant scattering cancellation technique [11] to practically realizable antenna systems. In such applications, tradeoffs are generally necessary in terms of bandwidth, efficiency, overall scattering suppression, and other specific requirements for the application of interest. Yet, we demonstrate here that the mantle cloaking technique offers unique features for the purposes at hand, and large flexibility to antenna designers. We introduce designs that can overcome the bandwidth limitations generally associated with metamaterials and metasurfaces and address the limitations arising when we consider antennas placed in close proximity, including polarization coupling and bandwidth limitations. Finally, we show near- and far-field experiments validating our proposed cloaking technique for realistic antenna configurations.

0018-926X © 2015 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.

4828


II. M ANTLE C OVERS FOR A NTENNA A PPLICATIONS Let us consider the case of an antenna placed in close proximity to another antenna or scatterer. In this configuration, the antenna suffers from unwanted interference, impedance mismatch, perturbations on its radiation pattern, and detrimental mutual coupling. In this section, we show that suitably designed cloaks may be used to minimize these effects. We focus on the problem in which a high-band (HB) antenna is placed very close to a larger low-band (LB) antenna. This is a situation of common interest in multiband antenna arrays, e.g., for base stations of cellular communication systems. The need for compact antenna systems typically requires that the different radiating elements are placed in close proximity to each other, significantly affecting their radiation properties. In the scenario of interest, an HB antenna is placed very close to an LB antenna, and they both independently radiate in two different frequency bands. We expect that either antenna may act as a partial reflector in the other antenna band, thereby redirecting the radiation pattern of the antenna network. Here, we show that, when the blocking LB antenna is covered by a conformal mantle cover ZS , tailored to cancel the dominant scattering in the HB, the HB antenna does not feel the presence of the neighboring element, and it radiates as if isolated. On the contrary, typically the LB antenna is weakly affected by the HB antenna presence, due to its smaller electrical size, being able to radiate well in the LB. In particular, the larger is the separation between the two frequency bands of operation, the weaker is the blockage effect naturally caused by the smaller antenna on the larger one. A. Vertically Polarized Mantle Covers Under Plane-Wave Excitation To be sufficiently close to a real scenario, we consider here a typical case for cellular base stations, where the HB dipole antenna works, for instance, in the range 1.7−2.7 GHz and is placed very close to an LB dipole antenna working, for instance, at 0.698−0.968 GHz. This scenario is particularly interesting because the LB dipole is paired to a rather broadband HB antenna and, thus, should be radio-transparent to the latter over the same, quite large, frequency range. This is a nontrivial task, and it involves more complications compared to the idealized case of two narrowband dipoles numerically studied in [8]. In order to match the LB bandwidth and beamwidth requirements, the longer dipole has length 2l = 0.72λl , diameter 2a = 0.04λl , with a feed gap g = 0.08λl , where λl is the central LB wavelength at 833 MHz, as in the inset of Fig. 1. This figure shows the total SCS for a 50-Ω-loaded and an open-circuit LB dipole under plane-wave excitation, which is defined as the total integrated bistatic scattered power at all angles for a particular excitation [23], [24]. First, we compare the SCS between loaded and unloaded LB antennas across a broad bandwidth. A clear resonance is seen in Fig. 1, near 970 MHz, with no significant dependence on the loading condition [25]. Therefore, only the structural scattering of the LB element needs to be considered for the cover design, and rigorous Mie theory may be applied to the obstructing open circuit LB dipole. This is an

Fig. 1. Total scattering for the open-circuit and loaded LB dipole across the frequency range of interest. Both bare and covered dipoles are shown for a vertically polarized plane-wave excitation (Einc = ˆ zE0 exp[j(ωt − k0 y)]). (Inset) Geometry of the dual-polarized patch surface.

important preliminary step that allows us to dramatically reduce the design efforts. A horizontal-strip capacitive surface is first considered in order to reduce the scattering of the antenna, with dimensions D = 11.6 mm, wT M = 0.3 mm, wT E = 0 mm, and ac = 18.0 mm, which can provide an average surface impedance ZS = −j247 Ω, able to suppress the scattering at the target frequency 2.7 GHz (as compared to ZS = −j249 Ω calculated from Mie theory using the dominant omnidirectional electric scattering mode, as discussed in [11] and [12]). We extract the effective surface impedance at normal incidence for both polarizations numerically [23] using the simple extraction formula η 0 . (1) ZS = − s11 −1 l) − j cot (k 0 s11 +1 In Fig. 2, we show the surface impedance extracted for each polarization to compare the different covers. The surface impedance extraction method is simply based on an infinite planar sheet model, using the finite element method (FEM) with high-density adaptive meshing [23], [26], where each of the surfaces is backed by a groundplane (electrical short circuit s.c.) separated by an air substrate of l = ac − a = 9.6 mm. Obviously, this extraction neglects the effects of curvature, but we show in the following that it can serve as a good starting point for the design optimization, and shows excellent agreement (2-Ω difference) with our 2-D analytical model, as we only consider normal incidence here, k0 and η0 are the wavenumber and wave impedance in free space. In (1), we may also safely assume that the cover is lossless, due to the high conductivity of metals in the radio-frequency range considered here, which has been numerically verified. In Fig. 1, we only consider excitation with vertical polarization (V-pol), which corresponds to an electric field polarzE0 exp[j(ωt − ized along the cylindrical dipole axis (Einc = ˆ k0 y)]), while the horizontal polarization (H-pol) is orthogonal to it at normal incidence, and it has a smaller interaction

SORIC et al.: DUAL-POLARIZED REDUCTION OF DIPOLE ANTENNA BLOCKAGE

Fig. 2. Comparison of the extracted surface impedance for patch and strip infinite surfaces at normal incidence. (Inset) Transmission-line equivalent used for the dual-polarized extraction at normal incidence.

with a thin vertical dipole. The horizontal strip cover considered in this first geometry is formed by opening thin air gaps ˆ in a uniform copper shell. In along the azimuthal direction (φ) this design, no gaps are present in the longitudinal direction, as opposed to the patch array schematically shown in the inset of Fig. 1, which will be useful for dual-polarization designs, as it will be clear later. These thin slits cause an electric field discontinuity at the surface providing an effective capacitive response, following (1) [13], [26]. As discussed in [18] for the case of cylindrical metallic rods without antenna functionality, the proposed radius of the cover, which is significantly larger than the rod, is ideal to increase the bandwidth of operation using a single-layer cover and is, thus, also useful in the case of antenna cloaking considered here. As seen in the figure, the scattering is indeed largely reduced in the HB window, without changing the scattering response of the LB antenna at its own operating frequency. This is a clear indication that, as it will be shown later, the LB antenna operation is not affected while its scattering is negligible, remaining radio-transparent over a large band at the HB frequencies. The fractional bandwidth for 10-dB scattering suppression is 14% in the HB band, with a maximum suppression of 13.5 dB at 2.56 GHz. In Fig. 3, we also consider the SCS variation with frequency, for excitation with both polarizations. As expected, in the bare case, we see the dominance of V-pol scattering, while the H-pol is lower at any frequency and, in particular, 20 dB (1.7 GHz) to 10 dB (2.7 GHz) lower across the HB band. Therefore, the low H-pol scattering of the bare LB dipole is in the order of the cloaked dipole residual scattering for V-pol excitation. However, when we cover the LB antenna with a horizontal strip capacitive surface, whose radius is quite larger than the one of the dipole, in order to achieve broadband invisibility within the HB window, the H-pol scattering significantly increases, since the whole object (antenna + cover) is now electrically larger, bringing the SCS up to the level of the bare LB dipole in the upper HB (black dots in Fig. 3). In the next section, we consider introducing vertical gaps in the cloak to add capacitive response to the surface also for the horizontal polarization, in order to mitigate this effect.

4829

Fig. 3. Total scattering for the open-circuit LB dipole. We compare the effects of the vertical-polarized (strips) and the dual-polarized (patches) cloaks on each polarization excitation.

B. Dual-Polarized Excitation

Mantle

Covers

Under

Plane-Wave

Using mantle covers consisting of rectangular patches, with slits both in the azimuthal and vertical directions, we can drastically improve the polarization performance of the considered broadband cloaks [27]. We note here that more exotic patterns exist; however, the simple patterns considered here give better bandwidth and angular response, which are both critical in practical deployments [13]. Considering the schematic geometry in Fig. 1, 12 vertical cuts of wT E = 1 mm were introduced every 30◦ in the original horizontal strip cover design, in order to reduce the H-pol scattering highlighted in Fig. 3. This figure shows the comparison between strip and patch covers, both for V- and H-pol scattering. The additional vertical slits provide a large reactance ZS = −j565 Ω at 2.7 GHz for H-polarized waves, which substantially reduces the H-pol scattering, but increases the scattering for V-pol excitation. The effect of the vertical slits is clear in Fig. 2: the surface impedance presents a high reactance across the entire band, which almost completely suppresses the cover presence to H-polarized wavefronts, leaving only the minimal residual scattering from the dipole itself. However, the patch array increases the polarization coupling and slightly decreases the bandwidth and more so the suppression level. The suppression level for the dual-polarized cloak is now 8 dB at 2.52 GHz with a 5-dB fractional bandwidth of around 18%. In order to demonstrate the dual-polarized cloaking effect in free space, Fig. 4 shows different scattering profiles for a cross-dipole, obtained by combining two orthogonal dipoles as in Fig. 1, under plane-wave illumination. Due to the aspect ratio of the cloak, one row of patches at the antenna feed needs to be removed in this combined design. The effect of this removal slightly increases the SCS by 0.6 dB (not shown here for brevity). The top row in Fig. 4 shows the SCS patterns of the bare cross dipole at different target frequencies in the HB, and the bottom row shows the effect of the cloaking cover. Here, a vertically polarized wavefront illuminates the structure from the x ˆ direction (φ = 0◦ , θ = 90◦ ) with normalized electric field

4830


Fig. 4. SCS patterns for crossed LB dipoles under plane wave excitation.

polarized along ˆ z. Due to symmetry considerations, the crossed dipoles have a quasi-isotropic response. The patterns highlight the significant scattering reduction at all angles and over a broad range of frequencies. We have chosen the upper HB frequencies, since they contribute the most interference between antennas, and are the focus of this work. In each case, it is clear that the omnidirectional scattering mode of the dipole is almost completely suppressed, leaving only higher order scattering contributions. Our single-layer cover designs are indeed tailored to cancel the dominant scattering order over a broad bandwidth, achieving a significant performance without complex design schemes. Finally, we show in Figs. 5 and 6, the far-field gain patterns of the LB antenna with and without cloaks, highlighting how the patterns are nearly identical. Here, we have used conventional lightweight/cost aluminum in the LB dipole and for the cover. Due to the nonresonant nature of the mantle cover, moderate conductive and dielectric losses have little effect on the performance of the cloaks across a broad bandwidth; highlighting a significant advantage of our methodology. Moreover, using a subwavelength pattern of rectangular patches, we are assured to be far from the self-resonance of the cloaking cover. Figs. 5 and 6 confirm that, due to the high reactance exhibited by the cover at the dipole resonance frequency for both polarizations (Fig. 2), the cloak has little effect on the radiation features of the LB antenna elements. The LB transparency of this cover is a key feature of our design, as there is no need to redesign the LB element to be covered for matching or beamwidth specifications. We note that such a cover essentially decouples the optimization of the cloak and of the antenna, significantly reducing the complexity in the overall design process of a complex communication system.

III. E XPERIMENTAL V ERIFICATION We have realized and tested the performance of the cloaked antenna system based on the previous theory, as described in the following sections. We have analyzed both near-field distributions and the radiation properties of different antennas placed

Fig. 5. Gain of covered and bare LB dipoles at 698 MHz.

Fig. 6. Same as Fig. 5 at 968 MHz.

very close to the cloaked dipole to verify the impact of the designed cloak in a basic communication link. A. Near-Field Patterns The optimized dual-polarized patch cloak described in the previous section, and tailored for an LB dipole antenna, was fabricated using 1 oz copper foil cut using a Roland GX-24 vinyl cutter. Delrin spacers were placed on both ends of each dipole arm to provide the required air gap between the mantle cloak and the LB dipole arms. The illuminating microwave source is a Pasternack 10-dBi standard gain horn placed in close proximity to each testing scenario. In Fig. 7, we show the experimental setup, characterized by a distance from the center of each antenna to the horn aperture of only 0.17λ, where λ is the free-space wavelength at 2.7 GHz. The antennas were essentially placed in such a way that their cover is nearly touching the horn (c.f., Fig. 7), to demonstrate that the scattering suppression works independent of the excitation, even in the very near-field of the source. In each experiment, we programmed a Fanuc robotic arm ending with an E-field probe to perform an accurate raster scan


Fig. 7. Bare and covered LB dipoles placed directly in front of a standard gain horn. The E-field probe is shown directly above each testing setup, illustrating the region where the probe skips to a different plane in each test.

4831

Fig. 9. Snapshot in time of the real part of Re{ˆ zE exp[j(ωt − k0 y)]} at 2.3 GHz (bare, covered, free space). The same scale is used throughout.

Fig. 10. Snapshot in time at 2.4 GHz (bare, covered, free space).

Fig. 8. Measured near-field scattering suppression achieved with the patch array cover for both polarizations.

in the plane crossing the center of the LB dipole arm (for details of our near-field scanning system, refer to [14]). The raster scans are taken over approximately a 3λ × 3λ scan area with sampling distances Δx = Δy = 0.05λ. The LB dipole is loaded with standard 50-Ω terminations in each testing scenario. Fig. 8 shows the level of scattering suppression integrated throughout the raster scan. This figure of merit (FOM) used to quantify the agreement between the cloaked antenna case to the background measurement, without any device in front of the horn, is |Ecov − E0 |2 dS. (2) σ= |Ebare − E0 |2



scan

where Ecov , Ebare , and E0 are the time-harmonic fields measured pixel-by-pixel in the raster scan around the cover, bare, and free-space fields, respectively. This quantity provides a raw descriptive metric of how well the cover can reduce the overall near-field scattering, reflections, and field distortion, compared to the bare antenna. We emphasize here that this FOM is not equivalent to the total scattering width of the object, but it is directly related to it in the sense that a small far-field scattering necessarily corresponds to small-field perturbations around the object under test. In Fig. 8, we see a strong reduction in near-field scattering obtained by the patch array, with a 10-dB fractional bandwidth of 12%, and a maximum suppression of nearly 18 dB at 2.69 GHz. Even though direct comparison to

the SCS of the simulated patch array could be misleading for the reasons outlined above, the suppression lineshape and level show excellent agreement. As expected, the H-pol scattering suppression is more limited (red dots in Fig. 8), due to the fact that the original scattering from the bare dipole is very small for this polarization. The fact that σ stays around 0 dB over the frequency range of interest ensures that the cloak does not add significant scattering in the H-pol excitation. Note that the use of the horizontal strip cloak described in Section II-A would introduce significant additional H-pol scattering compared to the bare case. Figs. 9–12 show near-field scanning images for different frequencies of the extracted electric field, providing more insights

4832


Fig. 14. Input matching comparison between the covered and bare LB antenna. Fig. 13. Far-field gain measurement.

into the performance of the patch array cloak in the presence of very near-field and nonuniform excitations. In each figure, the white box refers to the region that the scanner avoided, since it corresponds to the location of the antenna. In each case, the bare dipole strongly distorts the total electric field radiated by the horn throughout the raster scan area. We can see how this disturbance allows some radiation from the microwave source to propagate, but it is far less than the one observed in free space, and introduces significant beam squint from the microwave source. On the other hand, the patch array cloak cancels a significant portion of the scattering due to the LB antenna, and allows the horn to radiate as in free space in all considered frequencies. The frequency band between 2.5 and 2.8 GHz has a suppression level better than 10 dB (Fig. 8), and this is consistent with the near-field restoration in Figs. 9–12. B. Broadband Gain Restoration of a Log-Periodic Antenna Link As a second experimental verification, we have compared the gain between two PCB log-periodic antennas in the presence of the bare and covered LB antennas placed directly within their line-of-sight path. This experiment demonstrates the gain restoration in a link connection by removing the shadow and scattering created by the bare LB antenna. First, we consider two log-periodic antennas separated by R = 1 m (∼9λ). Assuming the two antennas to be identical, the gain was calculated using the usual Friis transmission equation (Fig. 13, black dotted line) [28] 2

|s21 | =

λ0 4πR

2 GT x GRx .

(3)

Here in (3), s21 is the measured transmission between the two antennas, λ0 is the wavelength, and GT X = GRX are the gain of the transmitting and receiving antennas, respectively. Next, the gain was measured using (3) when a bare LB antenna was placed at 0.3λ from the transmitting log-periodic antenna, as shown in the inset of the figure, using the previously measured receiver gain GRx . The cloak was then placed over the blocking LB antenna, and the transmitting gain was again calculated

using (3) with the same receiver gain GRx . Across a broad frequency range 2−4 GHz, we see strong improvement when the cloak is applied compared to the blocking LB antenna. Specifically, at 2.69 GHz, an improvement of 3.65 dB was measured, which matches that of the log–log measurement of 4.94 dBi. This result shows that the cloak is effective to different forms of excitation, and allows realizing antennas capable of radiating in a low-frequency band, yet remaining essentially radio-transparent in the desired high-frequency band. Finally, to further emphasize the radio-transparency of the cloak at the resonance frequency of the dipole on which it is applied, in Fig. 14, we show the comparison between the measured reflection coefficient with and without cloak across the LB frequency range. It is evident that the matching properties are not affected by the presence of the cloak, while, given the cloak isotropy, also the radiation patterns are not influenced, as shown above (c.f., Figs. 5 and 6). Essentially, the cloak does not influence the radiation properties in the LB, due to the high surface reactance values exhibited by the cover at the resonance frequency of the dipole.

IV. C ONCLUSION In this paper, we have shown a simple, inexpensive, and lightweight cover applicable on a conventional cylindrical dipole antenna to strongly reduce the scattering of dual-polarized sources over a wide bandwidth, while not affecting its radiation performance in the band of interest. The proposed cover, formed by a dense array of metallic patches, may be used with dual-polarized sources in very close proximity. While the cover thickness allows broadening the bandwidth of scattering suppression, it also affects the performance for cross-polarized fields, requiring special attention to both incident polarizations. By applying the patch array to an LB dipole antenna, we have shown that the cloaked antenna radiation performance is almost unaffected compared to the bare case. We have validated our results with two different illuminations, a microwave horn in the very near field and a log-periodic antenna in a radio-link scenario. More broadly, these concepts open a new venue to design compact antenna and sensor systems, where interelement antenna interference may be strongly reduced without sacrificing performance in bandwidth, beamwidth, or matching.


R EFERENCES [1] 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. [2] J. Vehmas, P. Alitalo, and S. A. Tretyakov, “Transmission-line cloak as an antenna,” IEEE Trans. Antenna Wireless Propag. Lett., vol. 10, pp. 1594– 1597, Dec. 2011. [3] M. Riel, Y. Bramd, Y. Demers, and P. de 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. [4] A. O. Karilainen and S. A. Tretyakov, “Circularly polarized receiving antenna incorporating two helices to achieve low backscattering,” IEEE Trans. Antenna Propag., vol. 60, no. 7, pp. 3471–3475, Jul. 2012. [5] C. A. Valagiannopoulos and N. L. Tsitsas, “Integral equation analysis of a low-profile receiving planar microstrip antenna with a cloaking substrate,” Radio Sci., vol. 47, p. RS004878, Apr. 2012. [6] N. M. Estakhri and A. Alù, “Minimum-scattering superabsorbers,” Phys. Rev. B, vol. 89, p. 121416(R), Mar. 2014. [7] A. Alù and N. Engheta, “Cloaking a sensor,” Phys. Rev. Lett., vol. 102, p. 233901, Jun. 2009. [8] A. Monti, J. Soric, A. Alù, F. Bilotti, A. Toscano, and L. Vegni, “Overcoming mutual blockage between neighboring dipole antennas using a low-profile patterned metasurface,” Antennas Propag. Lett., vol. 11, pp. 1414–1417, Dec. 2012. [9] R. Fleury, J. C. Soric, and A. Alù, “Physical bounds on absorption and scattering for cloaked sensors,” Phys. Rev. B, vol. 89, p. 045122, Jan. 2014. [10] J. C. Soric, R. Fleury, A. Monti, A. Toscano, F. Bilotti, and A. Alù, “Controlling scattering and absorption with metamaterial covers,” IEEE Trans. Antennas Propag., vol. 62, no. 8, pp. 4220–4229, Aug. 2014. [11] A. Alù, “Mantle cloak: Invisibility induced by a surface,” Phys. Rev. B, vol. 80, p. 24115, 2009. [12] P. Y. Chen and A. Alù, “Mantle cloaking using thin patterned metasurfaces,” Phys. Rev. B, vol. 84, p. 205110, 2011. [13] Y. R. Padooru, A. B. Yakovlev, P. Y. Chen, and A. Alù, “Analytical modeling of conformal mantle cloaks for cylindrical objects using subwavelength printed and slotted arrays,” J. Appl. Phys., vol. 11, p. 034907, 2012. [14] J. C. Soric, P. Y. Chen, A. Kerkhoff, D. Rainwater, K. Melin, and A. Alù, “Demonstration of an ultralow profile cloak for scattering suppression of a finite-length rod in free-space,” New J. Phys., vol. 15, p. 033037, Mar. 2013. [15] P. Y. Chen, C. Argyropoulos, and A. Alù, “Broadening the cloaking bandwidth with non-Foster metasurfaces,” Phys. Rev. Lett., vol. 111, p. 233001, Dec. 2013. [16] J. C. Soric and A. Alù, “Wideband tunable and non-Foster mantle cloaks,” presented at USNC-URSI Nat. Radio Sci. Meeting, Boulder, CO, USA, Jan. 8–12, 2014. [17] R. Schofield, J. C. Soric, D. Rainwater, A. Kerkhoff, and A. Alù, “Scattering suppression and wideband tunability of a flexible mantle cloak for finite-length conducting rods,” New. J. Phys., vol. 16, p. 063063, Jun. 2014. [18] J. C. Soric, A. Monti, A. Toscano, F. Bilotti, and A. Alù, “Multiband and wideband bilayer mantle cloaks,” IEEE Trans. Antennas Propag., vol. 63, no. 7, pp. 3235–3240, Jul. 2007. [19] A. Alù and N. Engheta, “Achieving transparency with plasmonic and metamaterial coatings,” Phys. Rev. E, vol. 72, no. 1, p. 016623, Jul. 2005. [20] B. A. Munk, Finite Antenna Arrays and FSS. Hoboken, NJ, USA: Wiley/IEEE Press, 2003. [21] K. Sarabandi and N. Behdad, “A frequency selective surface with miniaturized elements,” IEEE Trans. Antenna Propag., vol. 55, no. 5, pp. 1239–1245, May 2007. [22] M. A. Al-Joumayly and N. Behdad, “A generalized method for synthesizing low-profile, band-pass frequency selective surfaces with non-resonant constituting elements,” IEEE Trans. Antenna Propag., vol. 58, no. 12, pp. 4033–4041, Dec. 2010. [23] CST Microwave Studio. (2015) [Online]. Available: http://www.cst.com [24] C. A. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles. Hoboken, NJ, USA: Wiley, 1983. [25] M. Gusstafsson, J. B. Andersen, G. Kristensson, and G. F. Pedersen, “Forward scattering of loaded and unloaded antennas,” IEEE Trans. Antenna Propag., vol. 60, no. 12, pp. 5663–5668, Dec. 2012. [26] S. A. Tretyakov, Analytical Modeling in Applied Electromagnetics. Norwood, MA, USA: Artech House, 2003.

4833

[27] A. Monti, J. Soric, A. Alù, A. Toscano, and F. Bilotti, “Anisotropic mantle cloaks for TM and TE scattering reduction,” IEEE Trans. Antennas Propag., vol. 63, no. 4, pp. 1775–1788, Apr. 2015. [28] C. A. Balanis, Antenna Theory: Analysis and Design, 3rd ed. Hoboken, NJ, USA: Wiley, 2005. Jason C. Soric (M’10) received the B.S., M.S., and Ph.D. degrees in electrical engineering from the University of Texas at Austin, Austin, TX, USA, in 2006, 2010, and 2015, respectively. He was an Engineer with Harris Microwave Communications Division, San Antonio, TX, USA, where he was the Lead of Tx/Rx ATE bench testing. He has interned with Ball Aerospace and Technologies Corp. and Raytheon Space and Airborne Systems, where he focused on conformalphased arrays and the near-field testing of AESA radar systems, respectively. In 2013, he spent a summer at ETS-Lindgren, Cedar Park, TX, USA, where he focused on PA and AGC characterization and OTA LTE channel emulation testing for CTIA and 3GPP. His research interests include active/passive metamaterials and metasurfaces, minimum scattering antennas and sensors, exotic wave control, and material characterization. Dr. Soric received Third place in the Student Paper Competition at the 2012 IEEE International Symposium on Antennas and Propagation and USNC-URSI National Radio Science Meeting in Chicago, IL, USA, and was selected as a 2014 finalist for the Student Paper Competition of the same conference in Memphis, TN, USA. In 2012, he also received the First place at WMCS hosted at Baylor University, Waco, TX, USA. Alessio Monti (S’12–M’15) was born in Rome, Italy, on February 16, 1987. He received the B.S. degree (summa cum laude) and the M.S. degree (summa cum laude) in electronic and ICT engineering both from Roma Tre University, Rome, Italy, in 2008 and 2010, respectively. From 2011 to 2013, he attended the Doctoral School in Electronic Engineering, Roma Tre University. Currently, he is an Assistant Professor with Niccolò Cusano University, Rome, Italy, where he teaches courses on antenna theory and microwave engineering. His research interests include the design and the applications of microwave and optical artificially engineered materials and metasurfaces, the design of cloaking devices for scattering cancellation operating at microwave and optical frequencies, and the study of the electromagnetic properties of the plasmonic nanoparticles arrays. Dr. Monti is a Member of the Secretarial Office of the International Association “Metamorphose-VI” and of the Technical Program Committee (TPC) of the 8th and 9th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics. He has also been serving as a Technical Reviewer of the many high-level international journals related to electromagnetic field theory, metamaterials, and plasmonics. Alessandro Toscano (M’91–SM’11) was born in Capua, Italy, on June 26, 1964. He received the Laurea degree in electronic engineering from “La Sapienza” University, Rome, Italy, in December 1988 and the Ph.D. degree in electronic engineering from “La Sapienza” University, Rome, Italy, in September 1993. In December 2011, as the winner of a public contest, he became Full Professor of electromagnetic field theory with the Department of Engineering, “Roma Tre” University, Rome, Italy. His contributions include: 1) analysis and design of innovative antennas loaded with chiral and bianisotropic materials; 2) development of finite element—boundary integral methods to bear concepts in mathematical physics and applied electromagnetics to solve longstanding problems involving nonconventional materials; and 3) design of metamaterial inclusions and metamaterial-based components to solve practical problems in acoustics, electromagnetics, and optics. His work to date has resulted in more than 100 journal papers, and more than 200 conference papers. Of these, around 150 have appeared in the IEEE journals and conferences. His research interests include metamaterials and nonconventional media with the ultimate aim to respond to the need to develop new technologies making use of the electromagnetic fields to design new components and to protect the environment and the human health. Dr. Toscano is a Member of the Academic Senate.

4834


Filiberto Bilotti (M’98–SM’06) received the Laurea and Ph.D. degrees in electronic engineering from “Roma Tre” University, Rome, Italy, in 1998 and 2002, respectively. Since 2002, he has been a Full Professor of electromagnetic field theory with the Department of Engineering, “Roma Tre” University. He organized the First International Congress on Advanced Electromagnetic Materials and Metamaterials in Microwaves and Optics-Metamaterials 2007, Rome, Italy, October 2007, served as the Chairman of the Steering Committee and has been elected General Chair of the same conference for the period 2008–2014 and 2015–2018, respectively. He is the author of more than 370 papers in international journals, conference proceedings, and book chapters. His research interests include microwave and optical applications of artificial electromagnetic materials, metamaterials, and metasurfaces. Dr. Bilotti served as a Member of the Technical Program, Steering, and Organizing Committee of several national and international conferences, as organizer and chairman of special sessions focused on the applications of metamaterials at microwave and optical frequencies, as an Associate Editor of the IEEE T RANSACTIONS ON A NTENNAS AND P ROPAGATION (2013– present) and Metamaterials Journal (2007–2013), as a Member of the Editorial Board of the journals EPJ Applied Metamaterials (2013–present), International Journal on RF and Microwave Computer-Aided Engineering (2009–present), Scientific Reports—Nature (2013–present), and as a Technical Reviewer of the major international journals related to electromagnetic field theory and metamaterials. He was an Elected Member of the Board of Directors (2007– 2013) and currently is the President (2013–2016) of the Virtual Institute for Artificial Electromagnetic Materials and Metamaterials (METAMORPHOSE VI, the International Metamaterials Society). He is a Member of the Optical Society of America and has been the recipient of the Raj Mittra Travel Grant Senior Researcher Award in 2007 and of the Finmeccanica Group Innovation Award, in 2014.

Andrea Alù (S’03–M’07–SM’12–F’14) received the Laurea, M.S., and Ph.D. degrees from the University of Roma Tre, Rome, Italy, in 2001, 2003, and 2007, respectively. He is an Associate Professor and David & Doris Lybarger Endowed Faculty Fellow in Engineering with the University of Texas at Austin, Austin, TX, USA. He is currently a Visiting Professor at the FOM Institute (AMOLF), The Netherlands. From 2002 to 2008, he was periodically working with the University of Pennsylvania, Philadelphia, PA, USA, where he has also developed significant parts of his Ph.D. degree and postgraduate research. After spending 1 year as a Postdoctoral Research Fellow with UPenn, in 2009, he joined the Faculty of the University of Texas at Austin, Austin, TX, USA. He has organized and chaired various special sessions in international symposia and conferences. He has served as Technical Program Committee Chair of Metamaterials’2013 and Metamaterials’2014, and is the current Technical Program Committee Chair for the upcoming 2016 IEEE International Symposium on Antennas and Propagation. He is the coauthor of an edited book on optical antennas, over 300 journal papers, over 450 conference papers, and over 20 book chapters. His research interests include metamaterials and plasmonics, electromagnetics, optics and photonics, scattering, cloaking and transparency, nanocircuits and nanostructures modeling, miniaturized antennas and nanoantennas, RF antennas and circuits, and acoustics. Dr. Alù is a Member of the Applied Research Laboratories and of the Wireless Networking and Communications Group with UT Austin. He is a Full Member of URSI, a Fellow of OSA and APS, a Senior Member of SPIE, and a Member of MRS. He has been serving as OSA Traveling Lecturer since 2010, as IEEE AP-S Distinguished Lecturer since 2014, and as the IEEE joint AP-S and MTT-S chapter for Central Texas. He is currently on the Editorial Board of Physical Review B, Scientific Reports and Advanced Optical Materials, he serves as Associate Editor of four journals, including the IEEE A NTENNAS AND W IRELESS P ROPAGATION L ETTERS and the Optics Express. He has guest edited special issues for the IEEE J OURNAL OF S ELECTED T OPICS IN Q UANTUM E LECTRONICS, the Journal of the Optical Society of America B, Optics Communications, and Metamaterials and Sensors on a variety of topics involving metamaterials, plasmonics, optics, and electromagnetic theory. Over the last few years, he has received several research awards, including the NSF Alan T. Waterman Award (2015), KNAW Visiting Professorship from the Royal Netherlands Academy of Arts and Sciences (2015), the OSA Adolph Lomb Medal (2013), the IUPAP Young Scientist Prize in Optics (2013), the IEEE MTT Outstanding Young Engineer Award (2014), the Franco Strazzabosco Award for Young Engineers (2013), the URSI Issac Koga Gold Medal (2011), the SPIE Early Career Investigator Award (2012), an NSF CAREER Award (2010), the AFOSR and the DTRA Young Investigator Awards (2010, 2011), Young Scientist Awards from URSI General Assembly (2005), and URSI Commission B (2004, 2007, and 2010). His students have also received several awards, including two student paper awards at IEEE Antennas and Propagation Symposia (in 2011 to Y. Zhao, in 2012 to J. Soric).