A High Angular Stability, Single-Layer Transmission Linear-to-Circular Polarization Converter for Dual ISM-Band Operation

This paper presents a planar metasurface structure for linear to circular polarization (CP) conversion at the industrial, scientific, and medical bands of 2.4 GHz and 5.8 GHz. The new unit-cell structure consists of two loaded symmetric cross-shaped stubs with a size of <inline-formula> <tex-math notation="LaTeX">$0.26\,\,\lambda \times 0.26\,\,\lambda $ </tex-math></inline-formula> in a single thin substrate with a thickness of <inline-formula> <tex-math notation="LaTeX">$0.0064 \lambda $ </tex-math></inline-formula>. The design process and parametric study are presented to clarify the operating mechanism for each frequency. The unit-cell element is analyzed and simulated, which exhibits an axial ratio (AR) bandwidth of 15.8% and 12.6% at a 3-dB criterion with the same polarization mode at 2.4 GHz and 5.8 GHz, respectively. The proposed LP-to-CP converter achieves a stable AR bandwidth with an oblique incident angle up to 45°. Moreover, a dual-band linear polarization (LP) antenna is combined with an array of <inline-formula> <tex-math notation="LaTeX">$4\times $ </tex-math></inline-formula> 4 unit cells to validate the polarization converting ability of the proposed metasurface. The LP-to-CP converter and the antenna are fabricated and measured. The measurement results show that the antenna obtains a good impedance matching and CP waves radiation at 2.33–2.51 GHz and 5.55–5.89 GHz. The integrated antenna with dual-band CP waves radiation is suitable for improving the transmission efficiency in communication and wireless power transfer applications.


I. INTRODUCTION
Electromagnetic waves with circular polarization (CP) have been widely used in RF signal transceivers since their advantageous properties [1], [2]. They overcome the weaknesses of linear polarization (LP) waves, such as multipath fading and polarization mismatch loss, which is necessary for communication applications. Several antenna structures were proposed for the CP wave achievement, such as coplanar waveguide(CPW)-fed planar antennas [3], crossdipole antenna [4], substrate integrated waveguide (SIW) cavity [5], antenna array with sequential-phase (SP) [6]. However, designing an antenna with dual-band CP waves remains a challenge by the complex structure of the feeding network [7], [8]. The metasurface as a polarization converter The associate editor coordinating the review of this manuscript and approving it for publication was Bilal Khawaja . has emerged as an efficient technique for obtaining CP characteristics from the LP incident wave. The dual-band LP antenna integrated with an LP-to-CP converter can generate dual-band CP waves radiation without the feeding network structures.
Recently, many studies about the LP-to-CP converter at a single or dual-band of Ku-/Ka-band for millimeter-wave and satellite communication applications have been reported in [9], [10], [11], [16]. The polarization converter is divided into two main categories: reflection polarization converter and transmission polarization converter. The transmission LP-to-CP converter is based on meander lines (ML) [17], Jerusalem cross (JC) [18], split-ring-based (SPR) [19] structures with single band operation. In contrast, the dual-band LP-to-CP converter is designed in multi-substrates [20], [21], making a bulky structure, increasing the dimensions and fabrication difficulty. The simple transmission polarization converters have been proposed in [12], [13], and [22]. The unit-cell structure consists of a split ring or squares arranged inside a square ring to obtain dual/tri-band operation in an ultra-thin substrate. On the other hand, the reflection polarization converters with a similar structure and a ground layer for reflecting incident waves can achieve high angular stability and a larger axial ratio (AR) bandwidth than the transmission polarization converters, as reported in [23] and [24]. However, the transmission polarization converters are usually integrated with the antenna using a periodic structure to generate the CP wave radiation, which makes it more convenient to design the low-profile CP integrated antenna.
Wireless power transfer (WPT) is a potential technique for charging devices based on the energy of the electromagnetic wave. With the CP wave, the efficiency of the WPT system is significantly improved in case of misalignment between transmitter and receiver, ensuring energy-supplying stability for systems [25]. The WPT system mainly operates in the ISM bands, typically at 2.4 GHz and 5.8 GHz, for communication and power transfer. In the WPT systems deployed simultaneously for data transmission and power transfer at these bands, the transmitter antenna with the CP waves will enhance the transmission efficiency for both applications. In [26], the antenna combined with an LP-to-CP converter metasurface achieves CP radiation at a single band of 2.45 GHz. The unit-cell structure is a rectangular loop with a diagonal microstrip. Especially in [27], the low-profile CP metasurface-based bowtie slot antenna has AR broadband characteristic of 31.7% (4.38-5.98 GHz), but insufficient for both the ISM bands at 2.4 GHz and 5.8 GHz due to their large band gap.
In this paper, a transmission LP-to-CP converter is designed and analyzed. The unit-cell structure is based on two symmetric cross-shaped stubs loaded on a single thin substrate. The designing steps and parametric results explain the operating mechanism of the structure for each frequency. The polarization converter operates at the dual ISM bands of 2.4 GHz and 5.8 GHz with AR bandwidths of 15.8% and 12.6%, respectively. Table 1 compares the proposed polarization converter with the recent related studies on dimensions, operating frequency, number of layers, AR bandwidth, and angular stability. It can be seen that the proposed unit-cell structure has advantages in simple structure, just a single metallic layer in the substrate, it is easy for fabrication. Besides the small size of 0.26λ, the high angular stability of 45 • and the thickness of the substrate (calculated in λ) is much thinner than other structures, it easily integrates with the antenna to obtain CP wave radiation as a low-profile antenna. The AR bandwidths are over 10% at both bands, which is not broad AR bandwidth as [15] and [13], but wide enough according to the frequency allocations standard for ISM bands of 2.4-2.5 GHz and 5.725-5.825 GHz. In addition, a simple dual-band LP antenna with the same frequency is used to verify the polarization-converting ability of the converter. As a result, the antenna obtains the simultaneous CP radiation waves at 2.33-2.51 GHz and 5.55-5.89 GHz. The simulation and measurement results exhibit a good agreement. This design is a potential candidate for efficiency enhancement in dual-band WPT and communication systems.

II. DUAL-BAND LINEAR TO CIRCULAR POLARIZATION CONVERTER DESIGN A. BRIEF REVIEW OF POLARIZATION CONVERTER PRINCIPLE
An incident electric field E i consisting of two components in E i x and E i y , is expressed as follows: When the incident wave goes through the metasurface, transmission matrix T represents the relation between complex VOLUME 11, 2023 amplitudes of incident and transmission field.
where the cross-transmission coefficients T xy and T yx are nearly zero due to the weak coupling of two orthogonal components [10]. Therefore, T xx and T yy are shown as the polarization transmission coefficients and are derived as: Furthermore, the phase difference between the two components is given by: φ = φ yy − φ xx . To achieve the CP wave, the magnitudes and phases of the transmission coefficient should be satisfied the final condition: Consequently, the AR parameter given by [10]: , which is derived based on the transmission coefficient amplitudes and phases difference. As a result, the AR parameter value below the 3-dB should be obtained at the desired operating frequency to achieve the CP characteristics. The phase difference of π 2 or − π 2 defines the polarization mode of the CP wave are right-handed circular polarization (RHCP) or left-handed circular polarization (LHCP), respectively.  33.2 mm × 32 mm × 0.8 mm (0.26λ × 0.26λ × 0.0064λ, λ is the wavelength at 2.4 GHz). In the design of unit cells, the impedance manipulates the phase or group velocity to control surface waves. The unit-cell size smaller than the wavelength can be easily expressed in terms of homogeneity boundary conditions of impedance; it is related to the tangential components of the average electric and magnetic fields [28]. The symmetric geometry comprises two cross-shaped stubs loaded with an I-shape in the center. The symmetric geometry to prevent cross-coupling between the electric field components along the x-and y-direction [9]. The cross-shaped stubs generate resonances at dual-band, and I-shape in the center adjusts the phase difference to improve the AR. The properties of the unit cell are simulated in Ansys HFSS with floquet port and periodic boundary master-slave conditions. The incident wave is transmitted into two directions corresponding x and y-axis. Based on the transmission coefficient and phase difference results, the AR value is calculated following the equation (5).

C. DESIGN EVOLUTION AND PARAMETRIC ANALYSIS
Figs. 1(b)-(d) demonstrate the evolution of the unit cell by modifying the cross-shaped stubs loaded structure from single-band resonance at step 1, dual-band resonance at step 2, and increasing the phase difference to satisfy CP conditions in the final step. The transmission coefficient, phase difference, and AR value of the unit cell in each step are shown in Figs. 2(a)-(c). In more detail, in step 1, the unit cell operates at a single-band of 2.4 GHz with a transmission coefficient higher than -1.5 dB for both x and y-directions and the phase difference of around 40 • .
Step 2, adding the T-shape, generates one more resonant frequency at 5.8 GHz, as shown in the transmission coefficient in Fig. 2(a). However, the AR values at dual-band are still higher than 3 dB because the phase difference is insufficient 90 • . By adding the I-shape in step 3, the phase difference in both bands is increased at around 90 • as shown in Fig. 2(b). Besides, the transmission coefficient at 5.8 GHz is improved higher than -1 dB. Herein, the AR values below 3 dB as shown in Fig. 2(c). The transmission magnitudes of T xx and T yy at 2.4 GHz and 5.8 GHz are nearly same, the phase difference is approximately 90 degrees, and it satisfies the CP conditions as the equation (4).
The surface current distribution on the metallic surface in Fig. 3 verifies the results of the design steps. At 2.4 GHz, the surface current concentrates on the outer line corresponding to the structure in step 1. In contrast, the surface current at VOLUME 11, 2023

5.8
GHz is mainly on the T-shape added in step 2, as shown in Fig. 3(b). It is good evidence to show that the unit cell can operate at dual-band by cross-shaped stubs. Additionally, the orientation of the surface current at dual-band with different phases (0 • , 90 • , 180 • , and 270 • ) are shown in Fig. 3 to demonstrate the CP operation mechanism. It can be observed that the surface current with anti-clockwise rotation at both resonance frequencies creates RHCP wave radiation. The similarity in polarization mode at dual-band is also shown by the same phase difference 90 • in Fig. 2(b).
The Lx 1 and Ly 3 are the lengths in part of the structure where the surface distribution currents are concentrated. Fig. 4 shows the AR value results according to the variation of these parameters. The AR value has significantly been affected by changing Lx 1 , even is vanished the CP at 2.4 GHz with a length of 9.5 mm, as shown in Fig. 4(a). However, it does not affect the AR value at 5.8 GHz frequency much. In contrast, the AR value is stable at 2.4 GHz by varying the parameter Ly 3 , but it is shifted to the higher frequency around the 5.8 GHz band if the length of Ly 3 is reduced, as shown in Fig. 4(b). It can be seen that the adjusting Lx 1 and Ly 3 individual affect AR values at 2.4 GHz and 5.8 GHz, respectively. Table 2 presents the final optimized parameters, ensuring that the unit cell operates with AR less than 3 dB at the dual The incident wave from the radiation antenna is an oblique incident wave. Therefore, the AR values are investigated with different incident angles from 0 • to 45 • . The angular stability of the proposed converter is shown in Fig. 5. The AR value is observed to be stable with incident angle up to 45 • , below 3 dB at both center frequencies, and the AR bandwidth slightly changes at the higher frequency band. As a result, the proposed LP-to-CP converter achieves good angular stability.

III. FABRICATION AND MEASUREMENT A. EXPERIMENTAL VERIFICATION OF THE PROPOSED DUAL-BAND LP-TO-CP CONVERTER
The fabricated prototype of the proposed LP-to-CP converter metasurface is shown in Fig. 6(a). The array of 10 × 10 unit cell elements is used to validate the polarization converter's properties by direct measurement. The array of 10 × 10 elements of the unit cell is measured with two broadband horn antennas connected to the vector network analyzer (VNA Protek A338), and the measurement setup is shown in Fig. 6(b). The horn antennas as transmitting and receiving antennas have been placed on both sides of the metasurface to investigate the transmission coefficient and phase difference corresponding to the x and y-directions. The polarization converter is placed in the far-field region of the antenna to obtain the exact measurement results [23]. The measurement results of the array of 10 × 10 elements LP-to-CP converter in Figs. 7(a)-(c) show the transmission coefficients, phase difference, and AR value. A good agreement between the measured and simulated transmission coefficients, whereas the phase difference has a slight difference of 10 • -30 • with the simulation result. However,   the AR values at the center of the two desired frequencies are 1.39 dB and 0.7 dB, acceptable by less than 3 dB, and shown in more detail in Fig. 7(c). The difference between the measurement and simulation results by the simulation results assume an infinite number of unit-cell elements. In contrast, the number of the unit-cell array is fixed in measurement. VOLUME 11, 2023

B. THE INTEGRATED DUAL-BAND ANTENNA WITH LP-TO-CP CONVERTER
In order to verify the polarization converting ability in cases integrated with the antenna, an LP dual-band antenna has been implemented. The dual-band antenna is designed on the TLY-5 substrate (ε r = 2.2 and tanδ = 0.0009) with a thickness of 1.2 mm. The dimensions and the configuration of the polarization converter with the antenna are shown in Fig. 8. The polarization converter metasurface is placed in the radiation region of the antenna with a distance greater than λ/4. The simulated result for the different arrays of the unit cell with optimal distance is illustrated in Table 3. It can be seen that, the array of 4 × 4 elements of the unit cell with a distance of 31.5 mm yields the best results of AR value, which are 1.6 dB and 1.4 dB at 2.4 GHz and 5.8 GHz, respectively. Besides, the antenna also achieves an AR value of less than 3 dB at dual-band in case the array of unit cell with a larger number of elements. The simulation results of the reflection coefficient and the AR value of the dual-band antenna are illustrated in Fig.9. The antenna operates well at the dual ISM bands of 2.4 GHz and 5.8 GHz with a reflection coefficient of less than -10 dB at the frequency bands 2.38-2.43 GHz and 5.72-5.87 GHz. The AR value of 26.3 dB and 34.4 dB at two center frequencies. With the proposed LP-to-CP converter, the reflection coefficient changes negligibly, and the CP wave radiation is achieved at both bands with the AR bandwidth of 2.38-2.46 GHz and 5.75-5.85 GHz.
The LP antenna with the 4 × 4 metasurface polarization converter array is measured in a microwave chamber, shown in Fig. 10. Fig. 11 illustrates the antenna's reflection coefficient and polarization properties. It can be seen that the antenna operates exactly at dual-band. The difference between the measurement and simulation results of the  reflection coefficient is negligible. Fig. 12 shows the radiation patterns of the antenna without the polarization converter. With the polarization converter, the antenna achieves the CP wave radiation at the ISM bands of 2.4 GHz and 5.8 GHz, as demonstrated in the AR values of 1.39 dB and 0.68 dB with the AR bandwidth of 2.33-2.51 GHz (7.5%) and 5.55-5.89 GHz (5.9%), respectively. Fig. 13 illustrates the measured and simulated radiation patterns at two center frequencies in the xoz and yoz-planes. The radiation patterns are broadside in each plane, and the RHCP radiation is greater than the LHCP radiation by about 13 dB at 2.4 GHz and about 20 dB at 5.8 GHz. Hence, the integrated antenna with the proposed LP-to-CP converter easily generates dual-band CP waves radiation without the complex feeding network. With both experimental verification methods and the results are shown in Fig. 7 and Fig. 11, it can seen that the proposed LPto-CP converter operates accurately and can convert the LP wave to CP wave radiation at dual ISM bands. The CP wave radiation at dual ISM bands can improve the transmission efficiency in communication and WPT applications.

IV. CONCLUSION
The dual-band transmission LP-to-CP converter is proposed in this paper, which consists of two symmetric cross-shaped stubs loaded in a layer with unit-cell dimensions of 0.26λ × 0.26λ × 0.0064λ (λ is the wavelength at 2.4 GHz). The designing steps and parametric study are analyzed in detail to clarify the operating mechanism for each frequency. The polarization converter operates at the dual ISM bands of 2.4 GHz and 5.8 GHz with the similarity in polarization mode (RHCP). The angular stability are 45 • and the AR bandwidths are 15.8% (2.25-2.63 GHz) and 12.6% (5.67-6.4 GHz). The proposed converter has advantages in simple structure, and it is easy for fabrication. The LP dual-band antenna combined with the metasurface achieves the CP radiation wave at dual-band, and the measured AR bandwidth at below 3 dB is 2.33-2.51 GHz and 5.55-5.89 GHz. This polarization converter is useful for communication and WPT applications.