Optik 161 (2018) 12–19
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Optik journal homepage: www.elsevier.de/ijleo
Original research article
Design of metasurface polarization converter from linearly polarized signal to circularly polarized signal Oguzhan Akgol a , Emin Unal a , Olcay Altintas a , Muharrem Karaaslan a , Faruk Karadag b , Cumali Sabah c,d,∗ a
Iskenderun Technical University, Department of Electrical and Electronics Engineering, 31200, Iskenderun, Hatay, Turkey Cukurova University, Department of Physics, Saricam, Adana 01330, Turkey Department of Electrical and Electronics Engineering, Middle East Technical University – Northern Cyprus Campus (METU-NCC), Kalkanli, Guzelyurt, TRNC/Mersin 10, Turkey d Kalkanli Technology Valley (KALTEV), Middle East Technical University – Northern Cyprus Campus (METU-NCC), Kalkanlı, Guzelyurt, TRNC/Mersin 10, Turkey b c
a r t i c l e
i n f o
Article history: Received 28 August 2017 Accepted 8 February 2018 Keywords: Metasurface Polarization conversion Axial ratio
a b s t r a c t In this study, we both numerically and experimentally present a metasurface (MS) polarization converter to transform linearly polarized signal into circularly polarized one. The unit cell consists of two rectangular metallic patches placed at the crossed corners of rectangularly arranged inclusions. The results of a full-wave Electromagnetic (EM) simulator are compared to those of free space measurement using two horn antenna at microwave frequency regime. For a linearly polarized antenna, the s-parameters are obtained for both co-polarized and cross-polarized responses. The polarization quality referred to as axial ratio (AR) is expressed by the ratio of these two responses. It is found that, strong polarization conversion activity is obtained with the proposed MS at the frequency of about 3 GHz. As a result, we can generate polarization converter with a wide bandwidth, of interest for microwave ﬁltering, coating, and especially polarization conversion devices. © 2018 Elsevier GmbH. All rights reserved.
1. Introduction Metamaterials (MTM) are artiﬁcial materials engineered to produce properties that do not occur naturally. It is composed of a set of small scatterers or apertures in a periodical array throughout a region of space, thus obtaining some desirable bulk electromagnetic behavior become possible. The research towards metamaterial started when Veselago ﬁrst described the conditions for a negative index metamaterial. He dreamed about a conceptual material in which dielectric permittivity and magnetic permeability are both simultaneously negative . The concept of negative permittivity and negative permeability came to true by Pendry’s studies in 1999, although it was introduced forty years ago by the general consideration of the electromagnetic properties of the materials with simultaneously negative values of the dielectric permittivity () and magnetic permeability (). Pendry et al. presented their studies in 1996  on the artiﬁcial metallic construction which shows negative permittivity and in 1999  on the split rings which show the negative permeability. In 2000, Smith et al. realized the construction of ﬁrst double negative (DNG) material using the combination of the split rings and wires . Some experimental studies were carried out by Smith et al. to ascertain that the proposed structure show the characteristics of
∗ Corresponding author. E-mail address: [email protected]
(C. Sabah). https://doi.org/10.1016/j.ijleo.2018.02.028 0030-4026/© 2018 Elsevier GmbH. All rights reserved.
O. Akgol et al. / Optik 161 (2018) 12–19
Fig. 1. (a) Linear to circular polarization converter MS, (b) unit cell dimensions of MS, (c) equivalent circuit diagram of MS.
DNG material at microwave frequencies. In 2001, Shelby et al. performed the ﬁrst experimental investigation of negative refraction on DNG materials at microwave frequencies . In recent years, there has been growing interest in metamaterials where the number of research papers published in this area has grown exponentially. Electromagnetic metasurfaces (MS) [6–9] are artiﬁcial sheet material with sub-wavelength thickness, and therefore it is known as a two-dimensional form of a metamaterial [10–14]. MS allow us to control the behaviors of electromagnetic waves through the speciﬁc boundary conditions, rather than the constitutive parameters in three dimensional (3D) space. Metasurfaces have found great potential applications in both microwave and optical frequencies such as electromagnetic absorbers, polarization converters and spectrum ﬁlters etc. Due to low proﬁle and low loss properties, metasurfaces have great advantages, and therefore, for many applications, they also ﬁt well in many application areas of metamaterials [15–22] The goal of this study is to provide a polarization converter to convert linearly polarized signal to circularly polarized one. The axial ratio (AR) is calculated by the ratio of cross-polar response to co-polar response of the antennas, because it is the main parameter in designing a polarization converter. The frequency of 3 GHz is nearly in the middle of the bandwidth, so it is taken as optimal working frequency which is located in the S band. Since it is known that many types of radars and satellites systems are operated in this frequency band. Weather radars operating in S band is affected by bad weather conditions and due to this bad weather conditions it is difﬁcult to operate with linearly polarized waves. Rain and snow cause a microcosm of conditions such as reﬂectivity, absorption, phasing, multi-path and line of sight. This can be serious if linearly polarized antennas are employed. On the other hand, Multi-path is caused when the primary signal and the reﬂected signal reach a receiver at nearly the same time. This creates an out of phase problem which results in dead-spots. Furthermore, the polarization of a linearly polarized wave may be rotated as the signal passes through any anomalies such as Faraday rotation in the ionosphere at lower frequencies. Circular polarization will keep the signal constant regardless of these anomalies [23,24]. Situations such as it is not possible to ascertain the polarization of an incoming wave due to multi-path or reﬂection, it is more reliable to use circular polarization than linear polarization. In this study, we numerically and experimentally designed a MS polarization converter to convert linearly polarized signal to circularly polarized one. The proposed structure has the following advantages: (i) it provides a perfect AR with a bandwidth of about 200 MHz in S band frequency regime. This bandwidth is considered to be sufﬁciently large according to the current polarization converters in literature; (ii) it has a simple geometry consisting of two square metallic patches placed at the crossed corner of a rectangle; (iii) it can be easily fabricated using Roger RT5870 dielectric and copper type metal. These materials are cheap, accessible and exhibit low losses; (iv) it can be effectively used in S-band applications such as weather radar, surface ship radar, communications satellites, space shuttle and so on. 2. Design of the proposed metasurface The proposed MS composed of two rectangular metallic patches are placed at the crossed corners of periodically arranged rectangular patches. The aim of such a designed structure is to realize a conversion in rotation direction of polarization of the incident EM wave. The structure is constructed by using 4 × 4 layout arranged with 16 unit cells. The proposed structure is shown in Fig.1(a), the shaded (yellow) area represents metal plate designed by using copper with a conductivity of 5.80001 × 107 S/m and unshaded (white) area represents ﬂame-Roger RT5870 type substrate with a thickness, loss tangent, and relative permittivity of 1.6 mm, 0.012, and 2.33, respectively. The unit cell of periodic MS is shown in Fig.1(b) with a thickness of strip line connecting metallic patches (w = 0.48 mm), width (x = 14.68 mm), length (y = 13.90 mm), and dimensions of each metallic patch (Rx = 4.90 mm, Ry = 4.64 mm). Beside this, the electrical equivalent circuit representation of the unit cell is extracted as shown in Fig.1c. Since E ﬁeld component of signal is on vertical direction, only left and right side wires shows inductive effect (Lw) and rectangular patches provide higher inductance (Lrp ) with respect to Lw . The capacitive effect arises from two crossed rectangular metallic patches (Ccrp ). 3. Numerical results The results of the proposed structure are obtained numerically with a commercial 3D full-wave EM solver based on ﬁnite integration technique. The periodic boundary conditions with Floquet ports are used in the simulation. First, we examine the
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Fig. 2. Effect of (a) unit cell width (x) (b) unit cell height (y) (c) strip thickness (w) on axial ratio.
effects of the width (x), length (y), strip thickness connecting the metallic patches (w), and the dimensions of the metallic patch (Rx and Ry) of the proposed structure. The axial ratio of any structure is the division of cross-polar transmission response to co-polar one at the same frequency; AR (ω) = T (ω)(cross − polar)/T (ω) (co − polar) where magnitudes of transmission (T) can be represented in terms of scattering parameter (S12 );
T (ω) = S12 Axial ratio is generally represented in dB; AR (dB) = Mag (20 log(AR (ω)) In order to see the effects of width (x) and length (y) of the unit cell of the structure on the AR, a parametric study is realized for various values of x, y as shown in Fig.1(a) and (b). The best results for x and y are 14.68 mm and 13.90 mm, respectively, since the AR (dB) approaches zero with a bandwidth of 200 MHz. Fig. 1(a) and (b) also shows that as the values
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Fig. 3. Effect of (a) width of metallic patch size (Rx) (b) height of metallic patch size (Ry) on axial ratio.
of x or/and y increase the frequency at which the AR approaches zero shift downward and bandwidth increases. Since, the increment of the dimensions results in resonance at higher wavelengths i.e. lower frequencies. It means that the zero-point shift downward is due to the impedance matching of the wavelength of applied EM wave and structure. The best resonance at intermediate value is due to exact matching of capacitive and inductive effects of the structure at this intermediate values (14.68 mm for x dimension and 13.90 mm for y dimension). The higher value of x and y decreases capacitive effect (Ccrp ) between metallic rectangular patches whereas inductive effects (Lw ) remains approximately same due to very thin cooper line. On the other hand, the effect of strip thickness of the metallic patches (w) is shown in Fig. 1(c). The effect of strip thickness connecting the metallic patches is different (in a reverse manner) from those obtained for unit cell width and height. That is, as the thickness increases the frequency at which the AR approaches zero shift upward and bandwidth decreases. Since, the increment of strip thickness decreases the self-inductance of the wire (Lw ) and the resonance frequency shift upward i.e. increases. Greater bandwidth is important since the bandwidth is deﬁned as the frequency range at which the AR is below the 3 dB point. So in order to increase the bandwidth we try to keep the AR below 3 dB. Effects of metallic patch sizes on the axial ratio are demonstrated in Fig. 2(a) and 2(b) size effects of metallic patches can play critical role on operating frequency and bandwidth of axial ratio. Fig. 2(a) shows that, when Rx is increased, the frequency at which the AR approaches zero shift downward and bandwidth decreases. Frequency shift of AR is due to the resonance frequency change of the overall unit cell. The increment of Rx also increases metallic patch surface area and it approaches the crossed patches at the corners of the rectangular unit cells. Whereas the ﬁrst effect increases effective inductive value (Lrp ) of the resonance system, the second effect increases capacitance (Ccrp ) between crossed metallic patches due to come close to each other. Hence the resonance frequency decreases inverse proportionally with both effective capacitance and inductive values. The increment of Ccrp and Lrp decreases resonance frequency and increases quality factor for a parallel L-C circuit, hence the bandwidth becomes narrower as mentioned above. In addition, it is seen that a better axial ratio is observed by increasing Rx (Fig. 3). On the other hand, as shown in Fig. 2(b), the effects of Ry are not the same as those of Rx, Ry has a greater impact on the axial ratio. This is due to the fact that the electric ﬁeld is in vertical direction, pointing upward parallel to the Ry side. As Ry increases bandwidth also increases, whereas the frequency at which the AR approaches zero remains constant. Almost stability of resonance frequency is due to inverse proportionality between Ccrp and Lrp. Whereas increment of Ry increases Ccrp but decreases Lrp. Hence, the resonance frequency stays nearly constant. Unfortunately, AR tends to degrade when Ry increases (Fig. 3).
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Fig. 4. E-ﬁeld distribution of the proposed structure (a) 0◦ , (b) 90◦ , (c) 180◦ , (d) 270◦ .
Fig. 5. (a) Fabricated MS sample (b) experimental setup.
The range of frequencies over which AR < 3 dB is referred to as the operating bandwidths of the antennas, it means the MS provides efﬁcient polarization conversion. Since our aim is to convert linearly polarized signal to circularly polarized one, the performance of the design has been optimized by maximizing the bandwidth of axial ratio using various parameters. In the optimization process, the spacing between the two dips is maximized and it provides AR to be kept below 3 dB between them . In order to enhance the bandwidth, the design parameters play important role. So, we carefully chose the values of those parameters. The proposed structure has a well-developed AR with about 200 MHz of bandwidth. As shown in Fig. 1(a) and (b) there are two lowest dips in AR at around 2.95 GHz and 3.15 GHz. So, the MS based polarization converter achieve conversion from linearly polarized incident wave to circularly polarized one effectively in the frequency range of 2.95 GHz–3.15 GHz. The physical mechanism can be more understandable with ﬁeld distribution of incident wave on the structure. The electric ﬁeld distributions of the MS structure with respect to phase angle are shown in Fig. 4. At 0◦ phase angle, E-ﬁeld reaches the maximum level. This is originated by cross polar form of the horn antennas. So, incident vertical E-ﬁeld is strongly emitted
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Fig. 6. Numerical and measured (a) co-polar transmission results (b) cross-polar transmission results (c) AR results of the proposed MS structure.
from horizontal gaps of proposed MS unit cells at 0◦ and 180◦ phase angles. On the other hand, at 90◦ and 270◦ phase angles, incident horizontal E-ﬁeld is emitted from horizontal gaps of proposed MS unit cells. This is caused by co-polar form of the horn antennas. Thus, both vertically and horizontally linearly polarized signals which are emitted between horizontal and vertical gaps of MS unit cells, respectively, generate circularly polarized signal.
4. Experimental results In order to make a comparison between numerical and measurement results, experimental study is also performed. The fabricated MSs consisting of 16 unit cells in a 4 × 4 arrangement is shown in Fig. 5(a). Fabrication of MS is realized on FR4 substrate with cutting technique by using LPKF E33 Protomat. FR4 is chosen as a dielectric substrate, a high frequency laminate with a thickness of 1.6 mm, relative permittivity of 4.2 and permeability of 1, loss tangent of 0.02, due to their low cost and easy fabrication process. S-parameters of the fabricated MS are given in Fig. 5(a) and measurement setup using R&S ZVL6 vector network analyzer (VNA) with the help of two microwave horn antennas is shown in Fig. 5(b).
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To measure the s parameters using network analyzer, ﬁrst of all, network analyzer is calibrated using free space measurement without MS. Then, proposed structure is placed between two horn antennas whose E-ﬁelds are in the same direction to obtain co-polar measurement and then one of the horn is rotated 90◦ to obtain cross-polar measurement. Both co-polar and cross-polar measurement results of MS are normalized with respect to free space measurements. After performing the measurements, the AR is calculated by dividing cross polar results to co-polar results and the performance of the structure is analyzed. The measurement and simulation results are compared between 2–4 GHz frequency band for co-polar, cross-polar transmissions and axial ratio in dB. The measured co-polar transmission magnitude of the structure is around 0.5 for 2.85 GHz and 3.5 GHz as shown in Fig. 6(a). The mismatch of the measured and simulated values can only arise from fabrication process defects especially along vertical (E ﬁeld polarization direction) direction, since measured cross-polar transmission values are in good agreement with simulated one. The cross-polar transmission value is obtained by measuring horizontal transmission value in case of vertically polarized signal source. As mentioned before, both simulation and measurement values are in good agreement. The transmission response of the proposed structure represents ﬁeld amplitude. The cross-polar transmission level is around 0.5 at 2.85 GHz, 3.6GHz and 3.9 GHz as shown in Fig. 6(b). Measurement and numerical calculations of AR levels are also evaluated depending on co-polar and cross-polar transmission values as shown in Fig. 6(c). There are two band gaps below 3dB which is a critical value to decide the AR efﬁciency of the proposed structure. First frequency band is between 2.82–2.88 GHz with a band width of 60 MHz, and the second band is approximately 200 MHz band width between 2.98–3.16GHz. The AR is below 3 dB at these operating frequency ranges and it can be said that the conversion efﬁciency is highly sufﬁcient for polarization conversion applications. Hence, the proposed structure can transform linear to circular polarization between 2.75–3.25 GHz frequency range with a bandwidth ratio of 1:2. The operating frequency band of proposed MS is carefully selected due to application areas especially for radiolocation in civil engineering and radio navigation for military purposes in radar and navigation systems of European Standards (EN 302 248- EN 302 752). The numerical results are compared to measured result and plotted on the same graph as shown in Fig. 6. There is a good agreement between numeric and measured results. Although the ARs corresponding to numerical and measured results are almost the same level, the bandwidth corresponding to numerical solutions are much wider than that of measured results. Experimental and Simulation studies have showed that MS structure always has two lowest dips in axial ratio at around 2.85 and 3,15 GHz which means that proposed structure can generate circularly polarizes signals. 5. Conclusion A metasurface polarization converter is designed to transform linearly polarized EM signals to circularly polarized EM signals. Numerical and experimental results are compared to each other; it is shown that the results are in good agreement. Perfect conversion is obtained from 2.98 GHz to 3.16 GHz frequency band. Within this frequency band AR is below 3 dB. Considering AR, operating range of the MS is about bandwidth of 200 MHz from 2.98 GHz to 3.16 GHz. This bandwidth is wider than the literature value . 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