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Abstract
In this paper, a wideband and reconfigurable polarization converter (RPC) is proposed and realized that combines the concept of a polarization converter with micro-electro-mechanical system (MEMS) techniques. The unit cell of the proposed RPC consists of an L-shaped metallic patch printed on a metal-backed substrate and a metallic via. A MEMS switch is integrated in the surface of the cell. By controlling the working state of the switch, the polarization of the reflected wave can be manipulated dynamically. The experimental and measured results show that, when the switch is ON, it could rotate the linear polarized (LP) incident wave to its orthogonal polarization after reflection from 7.93 GHz to 12.42 GHz. While when the switch is OFF, it could rotate the LP wave to reflected circular polarization wave from 8.07 GHz to 10.77 GHz. The proposed RPC only use one switch each unit but realize double important performance, which may have potential applications for polarization controllable devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With rapid development of radar technology and mobile communications systems, polarization conversion, which can improve system performance through eliminating multipath fading, has received much attention from academia and industry in recent years. Generally, there are two methods to control the polarization of electromagnetic (EM) waves. The first one is to use reconfigurable feed network directly [1–6]. It has the advantage of small size, wide band, and low loss. While the other one is designing polarization converters indirectly [7–10] which can rotate the polarization of EM waves radiated by antenna. Compared with manipulating antenna directly, reconfigurable polarization converters (RPCs) [11–25] are easier to implement and more feasible especially in array applications.
Generally, RPCs can be classified into three types in terms of their implementation methods. One method is altering the relative position between RPC and antenna [11–15]. Another one is replacing conventional substrate with tunable functional materials [16–19]. And the last one is integrating RF-switches [20–25]. Especially, a number of novel RPCs have been reported in the literature which use RF-switches, as a result of convenient for simulating and fabricating, well controllable performance, flexible and diverse structure, etc. In [22], a switchable transmitted polarizer is proposed, while changing the states of double PIN diodes on the unit polarizer, the LP waves radiated by the slot can be converted to either right-hand circularly polarized (RHCP) or left-hand circularly polarized (LHCP) waves. Furthermore, through switching the operating state of four PIN diodes on cell structure, the meta-mirror is expected to achieve three polarization states which are left-handed, right-handed CPs and LP, respectively [23,24]. And in [25], the RPC could be switched between line-to-line RPC with OFF-state diodes and metal with ON-state diodes which loaded two PIN diodes each unit. Even if that’s good enough, RPC with less electronic devices, lower profile, wider band and higher efficiency are still very challenging. In addition, the RPCs between LP and CP have been reported rarely until now.
In this paper, we proposed a wideband and reconfigurable polarization converter (RPC) using micro-electro-mechanical system (MEMS) switches which have low loss and nearly zero DC power consumption. Numerical results show that the proposed RPC cannot only transform a LP incident wave into a CP reflected one, but also rotate the incidence to its orthogonal counterpart after reflection at the same frequencies. Furthermore, the proposed RPC has advantages of multifunction, broad working band, low insertion loss, less electronic devices, and easy fabrication. Reasonable agreement between the experiments and simulations has been obtained.
2. Design and simulation
The unit cell of RPC proposed in this paper is depicted in Fig. 1. It consists of three layers in total. The middle layer is metallic-backed dielectric substrate with permittivity of 2.65 and loss tangent of 0.001. The thickness of the substrate is 3mm(0.094λ, λ is the wavelength at 9.42GHz). An L-shaped metallic patch is printed on the substrate. The MEMS switch (RMSW200HP) connects the patch to the ground plane through a metallic via. The detailed dimensions of the unit cell are: p = 12mm, l = 7.2mm, m = 3.75mm, w = 1mm, d = 1.2mm, r1 = 0.5mm, r2 = 0.8mm. As seen in Fig. 1(b), the locations of the via and the patch are R = (1.8mm,1.8mm) and Q = (7mm,7mm), respectively. Based on our previous work [26,27], the MEMS switch is modeled as a metallic connection bridge to expedite the optimization process in simulations and verify the effectiveness of the proposed designs. To be specific, the MEMS switch is simulated as a metal for ON state, as shown in Fig. 1(b). While for OFF state, as shown in Fig. 1(c), it is simulated as nothing.
Fig. 1 Schematic of the proposed reconfigurable polarization convertor (a) The unit; (b) When the switch is ON; (c) When the switch is OFF. Brassy yellow indicates copper and silver grey indicates dielectric substrate.
When define Ryx and Rxx to represent the reflection ratio of x-to-y and y-to-y polarization conversions, and φyx and φxx are the phases of cross and co-polarized reflections, respectively. Under x-polarized incidences, the infinite array of the proposed unit cell is simulated using Ansoft HFSS. The reflection coefficients and phases are shown in Fig. 2.
Fig. 2 Reflection coefficients (a) when the switch is ON and (b) when the switch is OFF. Phase and phase difference (c) when the switch is ON and (d) when the switch is OFF.
When the switch is ON, the reflected coefficients are shown in Fig. 2(a). It can be seen that Ryx is far larger than Rxx across a relatively wide frequency band. Ryx nearly equals to 1 at frequencies of 8.65GHz and 11.50GHz, implying the reflected EM waves has a polarization conversion effect. When the switch is OFF, the reflected coefficients are shown in Fig. 2(b). The curves of Ryx and Rxx have cross nodes at 8.5GHz and 10.05GHz, and Ryx and Rxx are very close to each other around the cross nodes. The reflected phases and reflected phase difference are shown in Fig. 2(c) and (d), respectively. Whether the switch is ON or OFF, the phase difference between x- and y-polarized reflections maintains Δφ = ± 90° in the frequency band of 6-15GHz. So the state of switch doesn’t influence the reflected phase difference. Taking the reflected coefficients shown in Fig. 2(b) into account, linearly polarized incidences can be converted into circularly polarized incidences at 8.50GHz and 10.05GHz, as shown in Fig. 3(b).
Fig. 3 Schematic of the reconfigurable polarization rotation phenomenon (a) when the switch is ON and (b) when the switch is OFF.
The PCR (Polarization Conversion Ratio) and AR (Axis Ratio) are defined as Eq. (1) and Eq. (2), respectively [28].
When the switch is ON, the PCR of this structure is shown in Fig. 4(a). It shows that two maximum points with value of 1 at 8.65GHz and 11.50GHz are obtained, and the PCR is greater than 80% from 7.93GHz to 12.42GHz, with a relative bandwidth of 44.13%. When the switch is OFF, the structural AR of reflections is illustrated in Fig. 4(b). The AR has two minimum points which are near to 0 at 8.50GHz and 10.05GHz, and the 3dB bandwidth is 8.07-10.77GHz. Considering the reflected phases shown in Fig. 2(d), the reflected EM waves are right-hand CP. According to the previous analysis, this proposed structure can convert the x-polarized incidences into y-polarized and right-hand circular polarized reflections in two frequency bands and the common band is from 8.07 GHz to 10.77GHz.Fig. 4 (a) Polarization conversion ratio of reflected wave when the switch is ON. (b) Axis Ratio of reflected wave when the switch is OFF.
3. Mechanism analysis
In order to make a better understanding of the working mechanism of the proposed structure, we decompose the x-polarized incident wave into two perpendicular components: u and v directions, which are introduced here along 45° direction with respect to x and y direction, as shown in Fig. 1(a). Ruu, Rvu, Ruv, Rvv are the reflected coefficients of u to u-polarizations, v to u-polarizations, u to v-polarizations, and v to v-polarizations, respectively. The reflected coefficients are shown in Fig. 5(a)-(b). It can be seen that Ruu = Rvv≈1, Ruv = Rvu = 0, which indicates that the incidences and reflections are the same polarized waves for both ON and OFF states. The reflected coefficient of co-polarized reflections is nearly equal to 1, and it indicates that there is no energy loss in the polarization conversion process.
Fig. 5 Simulated reflection coefficients under uv coordinate (a) when the switch is ON and (b) when the switch is OFF.
The methods of analyzing elliptical polarization are used to analyze all the polarization characteristics here. The azimuth angle ψxy, which represents the angle between major axis of ellipse and x axis, is introduced in this paper. It can be computed by Eq. (3).
φuu and φvv are the reflected phases of u to u-polarization and v to v-polarizations, respectively, and Δφ' represents the corresponding phase difference. ψuv is the angle between major axis of ellipse and u axis and it can be derived from the following equations. in which N1, N2 and N3 are integers. Since Ruu = Rvv≈1, the right part of Eq. (4) reaches ∞. Using Eq. (5), ψxy can be expressed as Eq. (6).If ψxy in Eq. (3) is substituted with Eq. (6), the left part of Eq. (3) will be equal to 0. If neither Ruu nor Rvv is not 0, the . If either Ruu or Rvv is 0, the reflections will be pure LP. If both Ruu and Rvv are equal to 0, then the reflections will disappear. Under x- or y-polarized incidences, if the reflected magnitudes along u- and v directions are equal, the reflected waves have both x- and y-polarized components. Then the phase difference will be ± 90°, which is consistent with the results of Figs. 2(c)-(d). This is the fundamental theory of CP formation.Figure 6 shows the curves of the reflected phase and phase difference under uv coordinate. When the switch is ON, the phase difference is 180° at 8.65GHz and 11.50GHz. At these two frequencies, the superposition of u- and v-polarized reflections is in y-polarized. When the switch is OFF, the phase difference is 90° at 8.50GHz and 10.05GHz. Considering the magnitudes of the reflected components along u and v axis are the same, the reflections are circularly polarized, and this result is consistent with the result obtained under xy coordinates. As seen from Figs. 6(a)-(b), the curves of φuu in these two figures are the same, while the curves of φvv are distinctively different due to the different switch states. So the switch cannot affect the reflected phase along u-axis but also affect the reflected phase along v-axis. This proposed structure can dynamically convert the LP into orthogonal LP or CP.
Fig. 6 Simulated phase and phase difference under uv coordinate (a) when the switch is ON and (b) when the switch is OFF.
Taking a further step, the top and lateral view of current distributions at resonant frequencies under u- and v-polarized incidences are shown in Fig. 7. Figures 7(a)-(d) are the current distributions when the switch is ON, and Figs. 7(e)-(h) are the current distributions when the switch is OFF. Figures 7(a),(b),(e),(f) are obtained under u-polarized incidences, and Figs. 7(c),(d),(g),(h) are obtained under v-polarized incidences. Under u-polarized incidences, the induced surface currents on the top and bottom sheets are anti-parallel no matter in the case of switch ON or OFF, and thus the proposed unit cell can be equivalent to a current loop and a magnetic dipole is formed. Then current-reversal phenomenon occurs on either side of the metallic via, so the metallic via cannot affect the induced surface current distributions but also can verify the similarity of reflected phase curves under u-polarized incidences as seen in Fig. 6.
Fig. 7 The top view and lateral view of current distributions at resonant frequencies. The first row shows the case that switch is ON, and the last row shows the case that switch is OFF; the 1st and 2nd columns show those for the u-polarized incident wave, and the 3rd and 4th columns show those for the v-polarized incident wave.
Under v-polarized incidences, the state of switch has distinctly effected on current distributions. In the case of ON state, the currents mainly distribute in the metallic via or on the metallic patch near the metallic via. And the induced currents of this part become smaller with increasing frequency, thus the cornered patch also exits some induced surface currents, as shown in Fig. 7(d). In the case of OFF state, the currents distributed in the metallic via is not intensive due to that the switch is mounted on the short circuit point of the top surface, and the loop is formed at 8.50GHz, as shown in Fig. 7(g). The induced currents mainly distribute on the up-left part of the cornered patch, which indicates that the metallic via has little impact on the current distributions. Furthermore, when comparing the side view of Figs. 7(c),(d),(g),(h), the distinct displacement current and two current loops are formed when the switch is ON, thus intensive magnetic resonances are observed. When the switch is OFF, the resonances are not so intensive because of lacking the impact of metallic via.
4. Fabrication and measurement
To validate the proposed structure, the proposed RPC is fabricated using a square prototype consisting of 625(25 × 25) unit cells, as shown in Fig. 8(a). For proof-of-principle, we use a metallic connection bridge pattern in place of a MEMS switch in the experiments.
Fig. 8 (a) The fabricated prototype of the metasurface consisting of 25 × 25 unit cells with a total dimension of 300 mm × 300 mm. (b) Schematic of measured environment.
The sample is measured in a microwave chamber using an Agilent network analyzer. The schematic illustration of the measurement configuration is shown in Fig. 8(b), and the two horn antennas are used to transmit and receive EM waves. What’s more, the distance between the two horn antennas and the prototype is six meters which satisfy the far field condition. The co- and cross-polarization reflections are measured when the receiving horn antenna is rotated by 0 or 90 degrees, respectively.
The PCR and AR curves can be calculated based on the simulated results, and the measured and simulated curves are compared in Fig. 9. When the switch is ON, the measured results indicate that the proposed structure has evident polarization conversion phenomenon with a relative bandwidth of 44.13% from 7.6 GHz to 12.9 GHz, of which the PCR is larger than 80%. Moreover, the measured 3 dB AR bandwidth is 7.75-10.40GHz in the case of OFF state. The measured curve has two resonant frequencies and towards the lower frequency slightly. The discrepancy between simulations and measurements is mainly attributed to the restriction of fabrication and measurement conditions. In general, the simulated and measured results are in good agreement with each other. Thus the effectiveness of this proposed structure is confirmed.
Fig. 9 (a) Simulated and measured PCR when the switch is ON. (b) Simulated and measured AR when the switch is OFF.
5. Conclusion
In summary, we propose a wideband and reconfigurable polarization converter (RPC) which combines the thought of polarization converter and the MEMS technique. Simulations demonstrate that the proposed RPC cannot only transform a LP wave into a CP wave but also rotate the incident wave to its orthogonal counterpart after reflection at the same frequencies. Through decomposing the electromagnetic wave into the u-v directions and investigating the surface current distributions, the working mechanism is analyzed. Experiments and simulations agree well. The proposed RPC has advantages of multi-functions, broad working band, less electronic device, and easy fabrication. Furthermore, this design can also be scaled to higher frequency range such as infrared and optical ranges.
Funding
National Natural Science Foundation of China (61671464, 61701523, 61471389 and 61801508); Postdoctoral Innovative Talents Support Program of China (BX20180375); Natural Science Foundation of Shaanxi Province, China (2018JM6040).
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