Switched Beam Antenna Employing Metamaterial ... - IEEE Xplore

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 8, AUGUST 2012

3583

Switched Beam Antenna Employing Metamaterial-Inspired Radiators Giorgio Bertin, Member, IEEE, Filiberto Bilotti, Senior Member, IEEE, Bruno Piovano, Roberto Vallauri, Member, IEEE, and Lucio Vegni, Life Member, IEEE

Abstract—We present a novel switched beam antenna (SBA) consisting of four identical metamaterial-inspired electrically short printed monopoles, vertically placed at the corners of a grounded square board. The antenna is designed to operate in the frequency range 1600–2700 MHz, with global dimensions 120 mm 120 mm 30 mm. The SBA has been first numerically simulated and optimized and, then, fabricated and tested. Numerical and experimental results show a good agreement. The obtained switched beam capabilities, the achieved realized gain levels, and the synthesized radiation pattern shapes on the horizontal plane, make the proposed SBA a good candidate as a radiating element of a receiving module for wireless telecommunication systems (DCS, UMTS, Wi-Fi, LTE) in areas with reduced signal coverage and/or high interference levels. Index Terms—Electrically small antennas, metamaterials, mobile antennas, mobile communications, switched beam antennas.

Fig. 1. Geometric scheme of the proposed SBA (top view).

I. INTRODUCTION

I

N wireless communications, two major requirements are: a) ensuring coverage extension, and b) providing data throughput enhancement. However, there are some scenarios were both requirements are difficult to be fulfilled, such as at the cell edge, where useful signals are weak and interfering signals may significantly affect data communication, and inside buildings, where signals are attenuated by the walls. In these critical contexts, a significant improvement of the communication quality can be achieved by equipping terminals with directive antennas, in order to enhance the link budget for those signals coming from the right direction, and reduce interfering signals coming from other angular sectors. However, considering that a wireless apparatus should not have a preferred orientation, so that it can receive signals coming from any direction, a method for steering the radiating beam could be adopted. Usually, a beam steering system is realized by using a set of differently oriented directive antennas and a switching network connecting the selected antenna to the input/output port. Another method adopted for steering the beam consists in using a phased array of monopoles, fed by equi-amplitude and proper phase signals. Feeding network could be a cascade of Manuscript received September 29, 2011; revised February 03, 2012; accepted March 08, 2012. Date of publication May 23, 2012; date of current version July 31, 2012. G. Bertin, B. Piovano, and R. Vallauri are with Telecom Italia Lab., 10148 Turin, Italy (e-mail: [email protected]). F. Bilotti and L. Vegni are with the Department of Applied Electronics of “Roma Tre” University, 00146 Rome, Italy (e-mail: [email protected]) 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.2012.2201111

Fig. 2. Radiation patterns on the horizontal plane at 2000 MHz by varying the phase excitations of the four monopoles as reported in Table I.

power dividers and phase shifters and the orientation of the radiated beam depends on the phase shifter values. In this paper, we propose a new switched beam antenna which follows this latter method and makes use of four antenna monopoles, vertically placed over a grounded support and positioned at the corners of a squared board, whose side length is at the reference frequency. The proposed SBA is designed to fulfil three basic requirements: a) large operating bandwidth comprising DCS 1800 (1710–1880 MHz), UMTS/HSPA (1920–2170 MHz), and LTE (2500–2690 MHz), b) reduced size, and c) operation in vertical polarization. In order to cope with the first two requirements, the monopoles are designed following the recent metamaterial concepts, which have been interestingly applied to a variety of antennas and radiating systems [1]–[15]. In particular, several innovative layouts of

0018-926X/$31.00 © 2012 IEEE

3584


TABLE I ARRAY DIRECTIVITY AND 3-DB BEAMWIDTH

TABLE II MONOPOLE SIZE AND FREQUENCY BAND OF OPERATION

TABLE III PEAK GAIN OF THE PRINTED MONOPOLES Fig. 3. (a) Geometric sketch of the Type A printed monopole. (b) Magnitude of the reflection coefficient at the SMA port.

The paper, that presents design, fabrication, and testing of the proposed SBA, is organized as follows. In Section II, we briefly describe the basic operation of the SBA. In Section III, we report three different designs of metamaterial-inspired printed monopoles fulfilling the bandwidth and size requirements. In Section IV, we present the design of the SBA (array and feeding network) and the measured results of the fabricated SBA prototype.

printed monopoles based on transmission-line metamaterials have been recently reported [12]–[15]. Typically, such layouts are matched in a broad frequency range and characterized by an electrically small size at the lowest frequency of operation. However, polarization and radiation pattern shapes of the reported layouts are usually strongly dependent with frequency. Broadband performance, in fact, is typically achieved with the help of the ground plane (printed on the same face of the board hosting the monopole) and is strongly affected by its dimensions. Basically, at some frequencies, radiation occurs mainly due to the currents flowing in the monopole, while at other frequencies, the main contribution is given by the currents excited on the ground plane. In contrast, the new configurations proposed in this paper, while keeping the broadband operation and size requirements, present also radiation properties, which are rather independent on the frequency and on the dimensions of the ground plane.

II. BASIC OPERATION OF THE PROPOSED SBA The idea is to develop an antenna array composed by four electrically short monopoles according to the scheme shown in Fig. 1. The monopoles are placed at the four corners of a square of side (e.g., 75 mm at 2000 MHz) and are fed by signals with equal amplitudes and different phases. Depending on the number of available phase values, different patterns can be generated. In order to assess the concept, we consider at first ideal monopoles with directivity 1.8 dBi (at 2000 MHz) and a regular dipolar radiation pattern with omni-directional coverage on the horizontal plane and 90 of 3-dB beamwidth (BW) in the vertical plane. If we consider, as first attempt, only two phase values (0 and 180 ), the generated radiation patterns are the ones shown in Fig. 2. In Table I directivity and 3-dB beamwidth on the vertical and horizontal planes are reported, considering the frequency range 1800–2200 MHz.

BERTIN et al.: SWITCHED BEAM ANTENNA EMPLOYING METAMATERIAL-INSPIRED RADIATORS

3585

Fig. 5. (a) Geometric sketch of the Type B printed monopole. (b) Magnitude of the reflection coefficient at SMA port.

Fig. 4. Gain pattern of the printed monopole of Fig. 3 on the (a) horizontal and (b) vertical planes. Co-polar (co-) and cross-polar (X-) components are the vertical and the horizontal ones, respectively.

The main results of this preliminary exercise are: a) the antenna system exhibits the required improvement in terms of directivity with respect to the single monopole; b) only two phase values (0, 180 ) are needed to ensure a reasonable and complete coverage on the horizontal plane. In the next Section, we will present three possible implementations of electrically short monopoles to be used in the SBA configuration of Fig. 1. III. DESIGN OF THE PRINTED MONOPOLES The need for an electrically short and wideband monopole, led us to consider the application of metamaterial-inspired radiators in printed technology. Particularly, in several papers it has been shown that by applying the phase-compensation it is possible to reduce the size of a resonant component. The idea has been put forward by Engheta in [16] and can be intuitively summarized as follows. Since the phase of a wave propagating in a double-negative (DNG) metamaterial (i.e., a material exhibiting both negative permittivity and permeability at a given frequency) is reversed compared to the case of a wave propagating in a regular material, it is always possible to combine

a regular material with a given thickness and a DNG material with a proper thickness in such a way that the total phase difference exhibited by the wave is zero when passing through the combination of the two materials. This theoretical result opened the door to many innovative designs in antennas and microwave components [4]–[8], [17]–[19]. Interestingly, as far as planar metamaterials are concerned, it has been demonstrated that the role of the permittivity is played by a capacitance, while the role of the permeability by an inductance. Therefore, a planar metamaterial can be realized by using properly loaded transmission-lines in printed technology [3], [20]–[23]. In this regard, a DNG metamaterial is represented by a composite transmission line system, whose electromagnetic behaviour is dominated by a shunt inductance and a series capacitance [3]. Now, by pairing a regular transmission-line and a DNG-like transmission-line, it is possible to design the system in order to achieve the phase-compensation. In this way, a quarter-wave printed monopole can be made electrically short, simply by loading it with a proper transmission-line metamaterial [14]. Usually, a metamaterial is considered as an ordered repetition of unit-cells, but it has been demonstrated that a single unit-cell, consisting of a series capacitance and a shunt inductance with proper values, can be enough to attain the phase compensation in printed monopoles (see e.g., [14]). This relevant result has been exploited to design wideband electrically short monopoles for wireless communications. However, as previously anticipated, the wideband behaviour is usually obtained by playing with the ground plane dimensions. The result of this procedure is that the antenna is indeed

3586


Fig. 7. (a) Geometric sketch of the Type C printed monopole. (b) Magnitude of the reflection coefficient at the SMA port.

Fig. 6. Gain pattern of the printed monopole of Fig. 5 on the (a) horizontal and (b) vertical planes. Co-polar (Co-) and cross-polar (X-) components are the vertical and the horizontal ones, respectively.

matched within a broad frequency range, but its radiation properties (e.g., polarization and radiation pattern shape) depend on the frequency. In fact, at some frequencies the radiation is mainly due to the monopole, while at other frequencies to the ground plane, and at other further frequencies to both of them. Simple physics considerations reveal that the broadband impedance matching is obtained by combining two different mechanisms: a) phase compensation due to the loading series capacitance and shunt inductance (in this case, radiation is due to the monopole), and b) ground plane impedance tuning (in this case, radiation is due to the ground plane and the loaded monopole plays only the role of a tuning stub for the radiating ground plane). The final outcome is that antenna polarization is not pure and radiation pattern shape depends on the frequency. In addition, another important aspect is that the antenna behaviour (both impedance matching and radiation) strongly depends on the size of the ground plane. The direct implementation of the metamaterial-inspired printed monopoles presented in the literature to the case of the SBA reported in Fig. 1, thus, is not straightforward and requires, as an additional step, the design of wideband and

Fig. 8. (a) Geometric sketch of the modified Type B printed monopole (lengths mm). are expressed in mm). (b) Final SBA structure (

electrically short printed monopoles with robust performances in terms of radiation pattern shape and polarization. In this Section, we propose three different layouts satisfying this condition, which represent good candidates for the implementation of the SBA of Fig. 1. In the following sub-sections, the three configurations will be presented. The achieved results, in terms of monopole size, frequency bandwidth, and gain are summarized in Tables II and III. A. Metamaterial-Inspired Printed Monopole: Type A The geometrical sketch of the first metamaterial-inspired printed monopole is reported in Fig. 3(a). The series capacitance has been implemented through an interdigited capacitor placed between the coplanar waveguide and the monopole, while the shunt inductance has been obtained through a metallic strip connecting the monopole to the ground plane. The monopole


3587

Fig. 11. Feeding network layout showing RF lines in red and DC polarization lines in green.

TABLE IV PHASE SHIFT CONFIGURATION OF THE FEEDING NETWORK

Fig. 9. Computed and measured of a single monopole (a) and isolation with ) between pairs of monopoles (b). (

However, we found that a reduction of the ground plane size does not sensibly affect the antenna performance. B. Metamaterial-Inspired Printed Monopole: Type B

Fig. 10. (a) General scheme of the feeding network with variable phase shifters. (b) Implementation with switched lines.

is printed on FR-4 dielectric board and fed by a 50 coaxial cable through an SMA connector. This configuration has been numerically simulated and optimized by using two codes based on the finite element method and the finite integration technique, respectively. The optimization had the aim to match the monopole at the SMA port and minimize the radiation by the ground plane. The reflection coefficient magnitude at the input port, reported in Fig. 3(b), shows a frequency band of operation ( dB) from 1900 to 2340 MHz, which does not completely fulfil the requirement. However, in the radiation patterns reported in Fig. 4, it is possible to recognize the almost pure radiation typical of a monopole. Finally, we remark that at the lowest operation frequency, the monopole dimensions are and considering the ground plane.

In order to further increase the operation bandwidth, especially at the lower frequencies, we needed to slightly increase the dimensions of the monopole. However, due to the reduced influence of the ground plane on the design, we could also reduce the dimensions of the ground plane itself. The new proposed configuration is reported in Fig. 5(a). In this case the needed series capacitance and the shunt inductance values are larger. For this reason, the inductive strip now terminates at the end of the monopole and the interdigited capacitance has been moved to the centre of the monopole. The antenna is again assumed to be printed on FR-4 substrate and fed by a 50 coaxial cable. The calculated matching properties of the monopole are reported in Fig. 5(b) and show that the antenna is now able to cover a wider frequency band comprising DCS 1800, UMTS, Wi-Fi, and part of LTE. Interestingly, the radiation patterns reported in Fig. 6, show that, again, the radiation performance of the antenna in terms of polarization purity and pattern shape are rather stable with the frequency, confirming that the main radiation contribution is

3588


Fig. 14. Picture of the fabricated SBA prototype.

Fig. 15. Measured reflection coefficient for all configurations in Table IV. Fig. 12. Measured reflection coefficients at the common port and measured transmission coefficient between the common port and port 1 for all configurations.

Fig. 13. Measured differential phase between the common port and port 1. The value of the phase shifter PS1 is set such that the nominal differential phase is 180 at 2 GHz.

given by the monopole. Finally, the monopole size at the lowest frequency of operation is and, considering the ground plane, . C. Metamaterial-Inspired Printed Monopole: Type C The third configuration we propose here is similar in geometry, dimensions, and performance to the previous one, but characterized by the operation at an additional resonant frequency,

which is enabled by the presence of a pair of spiral slots on the monopole (Fig. 7(a)). At this resonant frequency, that can be tuned by playing with the spiral parameters, the antenna exhibits further sub-wavelength dimensions. The calculated matching properties of the monopole are reported in Fig. 7(b). The impedance bandwidth of operation is the same as the one of the previous configuration, while the additional operation frequency is at 1395 MHz (with 13 MHz of bandwidth). At this lowest frequency, thus, the dimensions of the monopole are and, considering the ground plane, . This additional mode of operation can be used to provide further services and can be tuned by properly changing the spiral parameters, as reported, for instance, in [24], [25]. The radiation properties of the antenna in the frequency range 1730–2585 MHz are very similar to the ones reported already for the Type B monopole in Fig. 6. Therefore, for the sake of brevity, they are not reported here. However, due to the presence of the spiral slots, we observe that the peak gain of the antenna is slightly reduced, compared to the antenna reported in the previous sub-section. A comprehensive comparison is given in Table III. IV. SBA IMPLEMENTATION According to the basic idea introduced in Section II, the SBA implementation requires the design of the array of monopoles and the feeding network. For electrical characterization purpose, three types of circuits have been manufactured: a) board with monopoles alone, b) feeding network alone, and c) SBA


3589

Fig. 16. Contour plots of the measured gain pattern at 2140 MHz for configuration 2: (a) co-polarization; (b) cross-polarization.

Fig. 17. Contour plots of the measured gain pattern at 2140 MHz for configuration 5: (a) co-polarization; (b) cross-polarization.

with monopoles and feeding network integrated in the same board.

Finally, Fig. 9(b) reports the measured and calculated isolation between couples of adjacent and opposite monopoles.

A. Four Monopole Array: Design and Test On the basis of the results reported in Section III, we have chosen the Type B printed monopole (Type A does not completely satisfy bandwidth requirements and Type C exhibits the same performances as Type B, but with a slightly reduced gain), which is a good compromise between performances and manufacturing complexity (e.g., required photo-etching accuracy). The monopole has been slightly modified and re-optimized, as reported in Fig. 8(a), in order to fit with the SBA support which acts as ground plane. Fig. 8(b) shows the test circuit with the monopoles connected to SMA connectors placed on the back side of the board. The measured reflection coefficient of the four monopoles are quite similar, hence in Fig. 9(a) only one curve is shown. The array is able to fulfil the frequency band specifications, by covering DCS 1800, UMTS, Wi-Fi, and LTE bands. At the lowest operation frequency (1600 MHz), the monopoles are character. ized by the following dimensions:

B. Feeding Network: Design and Test The feeding network purpose is to feed the monopoles with equal amplitudes and the phase values reported in Table I. The scheme shown in Fig. 10, makes use of 3 dB power dividers and 0 /180 switched line phase shifters. The differential electrical length corresponds to a phase shift of 180 at 2000 MHz. The network has been designed and manufactured on a three-layer FR-4 substrate, each 0.5 mm thick. The ground is internal and placed at the interface between layers 1 and 2, while the RF lines and the polarization lines are placed on the top of layer 1 and on the bottom of layer 3, respectively. The circuit layout manufactured to check the switching network is reported in Fig. 11. The RF signal microstrip lines are depicted in red, while the polarization lines in green. As anticipated in Section II, six different configurations of the switching network are considered, obtained by setting the values of the four phase shifters, according to Table IV. Fig. 12 reports, for all configurations, the reflection coefficient at the common port and the transmission coefficient

3590


Fig. 18. Realized gain measured patterns at 5 frequencies on the principal planes (vertical on the left and horizontal on the right) for configurations 1, 3, and 5.


TABLE V MEASURED REALIZED GAIN OF THE SBA WITH 120 MM 120 MM (BOLD), 100 MM 100 MM (ROUND PARENTHESES), 80 MM 80 MM (SQUARED PARENTHESES) SUPPORTING BOARD DIMENSIONS. SIMULATED DIRECTIVITY WITH A 120 MM 120 MM BOARD DIMENSIONS IS REPORTED FOR COMPARISON (SLASH PARENTHESES)

between the common port and port 1 only, being very similar the behaviour noticed by varying the antenna ports from 1 to 4. Transmission curves are grouped into two families, to short or long line of phase shifter PS1. In the band 1710–2690 MHz, insertion loss, mainly due to microstrip lines and switches, are in the range 1.4–3.3 dB. This quite high loss, which directly impacts on the antenna gain, could be significantly reduced by adopting a low-loss substrate instead of FR-4. For instance, the simulations we have conducted (not reported here for the sake of brevity) with a RO3035 dielectric slab lead to an improvement of the insertion loss of about 0.5–1.0 dB. Further improvements can be obtained from a better selection of the switches and a re-design of the network. However, this goes beyond the scope of the present paper, which is aimed to propose the concept of a reduced size SBA working in a broad frequency range with excellent polarization purity. Finally, in Fig. 13 we report the measured differential phase between the common port and the port 1, when the phase shifter PS1 is set to 180 . Similar results are obtained in the case of the other ports and are not reported here for sake of brevity. C. SBA Prototype Measurements This sub-section reports the measured results obtained from the fabricated SBA prototype shown in Fig. 14. For what concerns the impedance matching performances (Fig. 15), the SBA covers all the required services (DCS 1800, UMTS, Wi-Fi, and LTE at 2600 MHz) with a reflection coefficient magnitude lower dB. than Pattern and gain measurement have been carried out in an anechoic chamber by means of a spherical near-field test range. All beam configurations 1–6 reported in Table IV have been investigated at 1850, 1950, 2140, 2400, and 2650 MHz In order to give a better understanding of the typical SBA patterns, for configurations 2 and 5, which are representative of the whole configuration set, the contour plots of the realized gain pattern, mea-

3591

sured at the intermediate frequency of 2140 MHz, are reported in Figs. 16 and 17. The distribution of maxima and minima of the gain patterns are consistent with the expected ones discussed in Section II. The realized gain peak level at 2140 MHz is close to 3.5 dBi for configuration 2 and 2.0 dBi for configuration 5. This means that for configuration 2 the array arrangement leads to an increase of around 1.5 dBi with respect to the single Type B radiator (see Table III), while for configuration 5 we get a realized gain peak level comparable to the case of the single monopole radiator. It is worth noticing that the realized gain takes into account also the insertion loss due to the feeding network (2–2.5 dB at 2140 MHz) discussed in the previous sub-section. Finally, in Fig. 18, we report the vertical and horizontal realized gain patterns for configurations 1, 3, and 5 at the five measurement frequencies. The realized gain patterns for configurations 2, 4, and 6 are similar to the ones of configurations 1, 3, and 5, respectively, and are not reported here for sake of brevity. The measured peak values of the realized gain of the SBA at the different frequencies and for the different configurations 1–6, are summarized in Table V. Before concluding, we remark that, other SBA prototypes have been fabricated and tested with the aim to check the effect of reducing the supporting board dimensions. We found that, as expected, the realized gain is affected (see Table V), due to the smaller electrical separation between the monopoles. Finally, we notice that the experimental results here presented prove the effectiveness of the proposed SBA for application in 3G/LTE routers for wireless internet connection. Typical application is for indoor use in places (i.e., at cell edge), where throughput achievable with wireless dongles is too much low. A preliminary measurement campaign has been carried out to compare a commercial apparatus and the same apparatus connected to the proposed SBA. Inside the cell coverage, data throughput of the two solutions were comparable, while at the cell edge the device equipped with the proposed SBA performs significantly better (40–100% of throughput improvement has been achieved). This behaviour is explained by the step like curve of the receiver throughput versus signal to noise-interference ratio. This result, obtained by using a non-optimized and rather lossy switching network, further confirms the effectiveness of the proposed SBA and stimulates us to continue working on the design of a better switching network to get closer to the directivity values reported in Table V, in view of a possible commercial exploitation of the product. V. CONCLUSION In this paper, we have presented a novel SBA consisting of four identical metamaterial-inspired electrically short printed monopoles, vertically placed at the corners of a grounded square board. The antenna operates in the frequency range 1600–2700 MHz and is equipped with a switching network able to change the phase at the four monopole ports and, consequently, the radiation pattern shape and the pointing direction on the horizontal plane. The electrically short monopoles have been designed according to the metamaterial concept of phase compensation and implemented through series capacitive and shunt inductive loads. An SBA prototype, employing one of

3592


the proposed monopole layouts, has been fabricated using a board of 120 mm 120 mm. Both electrical and radiating properties have been measured, showing a good agreement with the preliminary numerical design. The obtained switched beam capabilities, realized gain levels, and radiation pattern shapes on the horizontal plane make the proposed SBA a good candidate for the radiating element of a receiving module of wireless telecommunication systems (DCS, UMTS, Wi-Fi, LTE) in areas with reduced signal coverage and/or high interference levels. REFERENCES [1] N. Engheta and R. W. Ziolkowski, Metamaterials: Physics and Engineering Explorations. Hoboken-Piscataway, NJ: Wiley-IEEE Press, 2006. [2] R. Marqués, F. Martín, and M. Sorolla, Metamaterials With Negative Parameters: Theory, Design and Microwave Applications. Hoboken, NJ: Wiley-Interscience, 2008. [3] C. Caloz and T. Itoh, Electromagnetic Metamaterials, Transmission Line Theory and Microwave Applications. Hoboken-Piscataway, NJ: Wiley-IEEE Press, 2005. [4] R. W. Ziolkowski and A. D. Kipple, “Application of double negative materials to increase the power radiated by electrically small antennas,” IEEE Trans. Antennas Propag., vol. 52, pp. 2626–2640, 2003. [5] R. W. Ziolkowski and A. Erentok, “Metamaterial-based efficient electrically small antennas,” IEEE Trans. Antennas Propag., vol. 54, pp. 2113–2130, 2006. [6] A. Alù, F. Bilotti, N. Engheta, and L. Vegni, “Sub-wavelength, compact, resonant patch antennas loaded with metamaterials,” IEEE Trans. Antennas Propag., vol. 55, no. 1, pp. 13–25, 2007. [7] F. Bilotti, A. Alù, and L. Vegni, “Design of miniaturized metamaterial patch antennas with -negative loading,” IEEE Trans. Antennas Propag., vol. AP-56, no. 6, pp. 1640–1647, 2008. [8] S. Tricarico, F. Bilotti, and L. Vegni, “Multi-functional dipole antennas based on artificial magnetic metamaterials,” IET Microw. Antennas Propag., vol. 4, pp. 1026–1038, 2010. [9] A. Erentok and R. W. Ziolkowski, “Metamaterial-inspired efficient electrically small antennas,” IEEE Trans. Antennas Propag., vol. 56, pp. 691–707, 2008. [10] F. Bilotti and C. Vegni, “Design of high-performing microstrip receiving GPS antennas with multiple feeds,” IEEE Antennas Wireless Propag. Lett., vol. 9, pp. 248–251, 2010. [11] K. B. Alici and E. Ozbay, “Electrically small split ring resonator antennas,” J. App. Phys., vol. 101, p. 083104, 2007. [12] F. Qureshi, M. A. Antoniades, and G. V. Eleftheriades, “A compact and low-profile metameterial ring antenna with vertical polarization,” IEEE Antennas Wireless Propag. Lett., vol. 4, pp. 333–336, 2005. [13] G. V. Eleftheriades, M. A. Antoniades, and F. Qureshi, “Antenna applications of negative-refractive-index transmission-line structures,” IET Microw. Antennas Propag., vol. 1, pp. 12–22, 2007. [14] M. A. Antoniades and G. V. Eleftheriades, “A folded-monopole model for electrically small NRI-TL metamaterial antennas,” IEEE Antennas Wireless Propag. Lett., vol. 7, pp. 425–428, 2008. [15] M. A. Antoniades and G. V. Eleftheriades, “A broadband dual-mode monopole antenna using NRI-TL metamaterial loading,” IEEE Antennas Wireless Propag. Lett., vol. 8, pp. 258–261, 2009. [16] N. Engheta, “An idea for thin subwavelength cavity resonators using metamaterials with negative permittivity and permeability,” IEEE Antennas Wireless Propag. Lett., vol. 1, pp. 10–13, 2002. [17] F. Bilotti, A. Alù, N. Engheta, and L. Vegni, “Anomalous properties of scattering from cavities partially loaded with double-negative or singlenegative metamaterials,” Progr. Electromagn. Res., vol. 51, pp. 49–63, 2005. [18] A. Alù, F. Bilotti, N. Engheta, and L. Vegni, “Sub-wavelength planar leaky-wave components with metamaterial bilayers,” IEEE Trans. Antennas Propag., vol. 55, pp. 882–891, 2007. [19] F. Bilotti, L. Scorrano, E. Ozbay, and L. Vegni, “Enhanced transmission through a sub-wavelength aperture: Resonant approaches employing metamaterials,” J. Opt. A: Pure Appl. Opt., vol. 11, p. 114029, 2009. [20] G. V. Eleftheriades, A. K. Iyer, and P. C. Kremer, “Planar negative refractive index media using periodically L-C loaded transmission lines,” IEEE Trans. Microwave Theory Tech., vol. 50, pp. 2702–2712, Dec. 2002.

[21] G. V. Eleftheriades, O. Siddiqui, and A. K. Iyer, “Transmission line models for negative refractive index media and associated implementations without excess resonators,” IEEE Microwave Wireless Compon. Lett., vol. 13, pp. 51–53, 2003. [22] C. Caloz and T. Itoh, “Positive/negative refractive index anisotropic 2-D metamaterials,” IEEE Microwave Wireless Compon. Lett., vol. 13, pp. 547–549, 2003. [23] A. Sanada, C. Caloz, and T. Itoh, “Planar distributed structures with negative refractive properties,” IEEE Trans. Microwave Theory Tech., vol. 52, pp. 1252–1263, 2004. [24] F. Bilotti, A. Toscano, L. Vegni, K. B. Alici, K. Aydin, and E. Ozbay, “Equivalent circuit models for the design of metamaterials based on artificial magnetic inclusions,” IEEE Trans. Microw. Theory Tech., vol. 55, pp. 2865–2873, 2007. [25] F. Bilotti, A. Toscano, and L. Vegni, “Design of spiral and multiple splitring resonators for the realization of miniaturized metamaterial samples,” IEEE Trans. Antennas Propag., vol. 55, pp. 2258–2267, 2007.

Giorgio Bertin (M’82) was born in Aosta, Italy, in 1956. He received the Doctor degree in electronic engineering from the Polytechnic of Turin, Italy, in 1982. In 1983, he joined the Microwave Department of CSELT (now Telecom Italia Lab), where he was first engaged in dielectric oscillator and dielectric-loaded cavity design. His activities then focused on the modelling of microwave discontinuities and the computer-aided design of guiding structures, with particular attention devoted to discontinuities between nonstandard waveguides, such those involving a dielectric loading or the presence of ridges. Then he moved to the area of microwave filters, where he studied new type of technologies such as resonant cavities dielectrically loaded and superconducting filters. Currently he is involved in antenna design for wireless application.

Filiberto Bilotti (S’97–M’03–SM’06) was born in Rome, Italy, on April 25, 1974. He received the Laurea (summa cum laude) and Ph.D. degrees both in electronic engineering from “Roma Tre” University, Rome, Italy, in 1998 and 2002, respectively. Since 2002, he has been with the Department of Applied Electronics, “Roma Tre” University, where he works as an Associate Professor of electromagnetic field theory. His main research interests are in the microwave and optical applications of complex media, metamaterials and metasurfaces; in the analysis and synthesis of planar and conformal integrated components and phased antenna arrays; in the development of improved numerical algorithms for an efficient analysis of printed antennas and circuits. He is the author of more than 300 papers in international journals, conference proceedings, and book chapters. Since 1999, he is a national expert of the European actions on antenna technology and design. Since 2007, he is also an expert member of COST MP0702: “Towards Functional Sub-Wavelength Photonic Structures.” From 2004 to 2008, he was a member of the governing bodies of METAMORPHOSE, the European Network of Excellence on Metamaterials, where he acted as the coordinator of spreading activities. Dr. Bilotti is a Member of the Optical Society of America. He was the recipient of the Raj Mittra Travel Grant Senior Researcher Award in 2007. He is a member of the Steering Committee of the European Doctoral School on Metamaterials and the organizer of several international school events and international workshops and conferences in the field of metamaterials. He has been the local organizer of the First Congress on Advanced Electromagnetic Materials and Metamaterials in Microwaves and Optics—Metamaterials 2007 (Rome, Italy, October 2007) and served as the Chairman of the Steering Committee of the same conference for the period 2008–2012. He served also as a member of the Technical Program, the Steering Committee, and the 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 a member of the Editorial Board of the journals Metamaterials and International Journal on RF and Microwave Computer-Aided Engineering, and as a Technical Reviewer of the major international journals related to electromagnetic field theory and metamaterials. Since 2007, he is a member of the Board of Directors of the Virtual Institute for Artificial Electromagnetic Materials and Metamaterials (the European Society on Metamaterials), where he acts as the Director of the dissemination activities.


Bruno Piovano was born in Turin, Italy. He received the Doctor degree in electronic engineering from the Polytechnic School of Turin, Italy, in 1978. In 1980, he joined the Centro Studi e Laboratori Telecomunicazioni S.p.A. (now named Telecom Italia Lab, TILAB), Turin, Italy. In the past he was responsible for the design of waveguide networks in activities as the INTEL-717 “exible/reconfigurable antenna” and the European Space Agency “contoured beam reconfigurable Antenna” studies. His current job regards the design of radio mobile antenna subsystems, with particular attention to high-performance printed switched-beam antennas.

Roberto Vallauri (M’07) was born in Cuneo, Italy in 1965. He received the degree in electronic engineering in 1992 (Polytechnic of Turin) and bachelor’s degree in mathematics in 2007. In 1992 he joined, as Researcher, the Antenna Department of CSELT (now Telecom Italia Lab), in Turin. His interests and activities include electromagnetic modelling, code development and design of waveguide components (horns, tracking mode couplers, ortho-mode transducers) and shaped reflector antennas. Moreover he is experienced in antenna measurement (far-field and spherical near-field test range) and passive wave guide devices measurement. He has been involved in the X/Ka band upgrade of ESA VIL-4 antenna and in the SPOrt project (Sky Polarisation Observatory), for the development of 22, 32 and 90 GHz feed system. Recently he has developed and calibrated a reverberation chamber, for mobile terminal test.

3593

Lucio Vegni (M’73–LM’12) was born in Castiglion Fiorentino, Italy, on June 20, 1943 and received the degree in electronic engineering from the University of Rome, Rome, Italy. After a period of work at Standard Elektrik Lorenz in Stuttgart (Germany), as an Antenna Designer, he joined the Istituto di Elettronica of the University of Rome, where he was Researcher a in applied electronics. From 1976 to 1980, he was Research Professor of applied electronics at the University of L’Aquila. From 1980 to 1985, he became Research Professor of applied electronics and, from 1985 to 1992, he was Associate Professor of electromagnetic compatibility at “La Sapienza” University of Rome. Since 1992, he was at the “Roma Tre” University, Rome, Italy, where he is, currently, Full Professor of electromagnetic field theory, the President of the undergraduate and graduate Courses in Electronic Engineering, the President of the Doctoral Courses in Electromagnetisms, TLC, and Bio-engineering, the Head of the Applied Electromagnetic Laboratory. His research interests are in the areas of microwave and millimetre wave circuits and antennas with particular emphasis to the EMC problems. Up to 1977, he developed active research in partial coherence radio links and multipath propagation effects. Then he moved to the area of integrated microwave circuits, where he studied microstrip planar circuits and antennas modeling. In 1985-1990, in cooperation with industry, he developed integrated antennas for satellite applications and studied electromagnetic compatibility problems. Since 1990, he is actively working on theoretical and numerical aspects of new planar antennas modeling involving unconventional materials. In these recent studies he gave new contributions to equivalent circuit representations of planar microwave components and new variational formulations for their numerical simulations. Finally, in the area of unconventional materials he gave noteworthy contributions to the study of chiral and omega grounded dielectric slab antennas. All his contributions appeared in more than 500 international papers and conferences. His recent research activities are in the field of metamaterials. Prof. Vegni has been the organizer and the Chairman of the second and third edition of the International Workshop on Metamaterials and Special Materials for Electromagnetic Applications and TLC, held in Rome, Italy in 2004 and 2006, respectively. He has been also the Local Chairman of Metamaterials 2007—The First International Congress on Advanced Electromagnetic Materials in Microwaves and Optics (Rome, 2007). He is a member of the European Chiral Group, the METAMORPHOSE VI, and the Italian Electric and Electronic Society (AEI).