† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 61471387, 61271250, and 61571460).
In this paper, a linear-to-circular polarization converter using a three-layer frequency selective surface based on I-shaped circular structure resonant is presented and investigated. Numerical simulations exhibit that when the normal y-polarized waves impinge on this device propagating towards +z direction, the two orthogonal components of the transmitted waves have a 90° phase difference as well as the nearly equal amplitudes at the resonant frequency of 7.04 GHz, which means that the left-hand circular polarization is realized in transmission. For validating the proposed design, a prototype which consists of 25 × 25 elements has been designed, manufactured and measured. The measured results are in good agreement with the simulated ones, showing that the polarization conversion transmission is over −3 dB in the frequency range of 5.22−8.08 GHz and the axial ratio is below 3 dB from 5.86 GHz to 7.34 GHz.
Circularly polarized (CP) wave is preferred in many applications, such as long distance communication and remote sensing systems, in terms of its simplified transmitter–receiver alignment and its robustness against the environment interference. For this purpose, a variety of CP antennas have been carried out in the literature.[1–4] Conventional methods to realize circular polarization consist of introducing radiating systems with a planar array and special feed configurations. An alternative approach of generating CP waves is to insert a LP-to-CP converter on top of linearly polarized (LP) array, which can convert LP waves to the CP ones effectively. Therefore, a lot of efforts have been conducted to manipulate the polarization state of EM waves. Conventional devices for polarization conversion are mainly designed by using the optical gratings and birefringent materials. However, such methods generally lead to the bulky volume, which prevents the polarization converter being applied to particular engineering systems.
Metasurface is a two-dimension equivalent of material, which has been investigated widely for their unique electromagnetic and optical properties over the past decades.[5–9] Recently, metasurface has been considered as a particular case of broadband polarization converter based on open planar structure for high-gain antenna system. This structure can be implemented by using frequency selective surfaces (FSS) which provide different transmission characteristics of two orthogonal components of the incident LP waves. The FSS can be inductive impendence for one component and capacitive impendence for another component. Thus, a phase difference between two components of the transmitted wave can be obtained. If the two components have the same amplitude and the phase difference is equal to 90°, the incident LP wave can be transformed into a CP wave. For this case, different structures of FSS for LP-to-CP converters have been introduced in the literature, including cross-shaped dipoles,[10,11] spilt-ring slots,[12–15] meander lines,[16,17] T-shaped slots,[18] and bisected split-ring.[19] In spite of the advantages tendered by the aforementioned structures, they tend to be low-gain and operate at narrow bandwidth, which have still been the main shackle for particular applications.
In this paper, a novel I-shaped circular patch element is proposed to design the LP-to-CP polarization converter based on three-layer patch–aperture–patch (PAP) FSS. Ansoft HFSS software is used to model the element and investigate the performance of elements. By investigating the transmission characteristics for x- and y polarization, we present that the proposed device is competent to transmit incident LP waves into the CP ones. The proposed polarization converter can maintain a stable performance for the various incidence angles, and the simulation and measurement results are in good agreement. Compared to the previous designs in Refs. [10]–[19], the proposed polarization converter has a simpler geometry but the wider axial ratio bandwidth, higher transmittance and less insertion loss, indicating that it may possess potential applications in the design of CP antennas.
Figure
The unit cell of the proposed LP-to-CP converter is shown in Fig.
To elucidate the operation principle of the polarization converter based on three-layer PAP FSS, the electric field of y-polarized incident LP wave
It can be seen from Fig.
In addition, based on the design in Ref. [23], we introduce the circular structure into the CPW resonator to obtain a second-order band-pass response. The CPW resonator and two patches are coupled at localized coupling points, where the CPW resonator structure can be altered to carry a net magnetic current, and the dimension of CPW resonator determines the coupling degrees. In the circuit model, the two patches are modeled as two series LC resonators, and the circular aperture presents a resonance at the center frequency. Therefore, we combine the design idea of I-shaped circular micro-strip patches and the theory of PAP FSS to design LP-to-CP converter. By adjusting the dimensions of the patches as well as the dimension and position of the aperture, an incident lineal EM wave at the certain frequency can be received effectively by the upper micro-strip patch, and the transmission wave as the radiant wave of the nether micro-strip patch would be the circularly polarized one.
In this design, the repetition period of the unit cell is p = 12 mm, the radius of the circular patch element is r = 5.5 mm, and the radius of the circular aperture is r1 = 3.6 mm. The geometrical parameter w is the width of the slot in the I-shaped circular patch, and l is the length from the center of patch to the slot. The thickness of both two dielectric substrate is d = 1.8 mm, and the relative dielectric constant and loss tangent are εr = 2.65 and tan δ = 0.001, respectively. In addition, the thickness of the metallic groundsheet is t = 0.017 mm.
For investigating the performance of our design, the element is simulated by using the commercial full-wave electromagnetic solver Ansoft HFSS. Periodic boundaries and Floquet mode excitations are introduced to implement the infinite-array approach for element analysis. In order to obtain better performance of the proposed polarization converter, a parameter study is implemented to evaluate the effects of the parameters w and l on the axial ratio (AR) at different frequencies. The AR of the transmitted wave can be calculated by
The calculated AR versus different parameters w and l are shown in Fig.
As shown in Fig.
As for the designed polarization converter, numerical simulation was performed by using Ansoft HFSS. The simulated results under y-polarized EM wave normal incidence from +z direction are presented in Fig.
To assess the performance of the proposed polarization converter at different frequencies, we consider the axial ratio AR and total transmittance T. The axial ratio and total transmittance can be calculated respectively by using Eq. (
Figure
To evaluate the response of the proposed polarization converter to the angle of incidence, simulations have been carried out under different angles of incidence. The calculated results of total transmittance, together with the axial ratio for various angles of incidence from 0° to 60° in x−z plane are shown in Fig.
In order to validate the performance of the proposed polarization converter, a sample containing 25 × 25 unit cells with the size of 300 × 300 mm2 is fabricated by using printed circuit board (PCB) technique, and the photograph of the fabricated prototype is shown in Fig.
In this work, we have proposed a linear-to-circular polarization converter based on three-layer PAP FSS. A novel I-shaped circular patch element has been introduced and optimized to achieve a desirable radiation performance. Numerical simulations exhibit that the polarization conversion transmission is over 3 dB in the frequency range of 5.22−8.08 GHz, while the minimum insertion loss is only 0.007 dB. The axial ratio is below 3 dB from 5.86 GHz to 7.34 GHz. The frequency response of the proposed device is stable for the various incidence angles from 0° to 45°. A prototype of the polarization converter was fabricated and experimentally validated. The measured results and the simulated ones are in good accordance, indicating that the proposed polarization converter possesses the ability to generate CP signals for particular engineering systems.
[1] | |
[2] | |
[3] | |
[4] | |
[5] | |
[6] | |
[7] | |
[8] | |
[9] | |
[10] | |
[11] | |
[12] | |
[13] | |
[14] | |
[15] | |
[16] | |
[17] | |
[18] | |
[19] | |
[20] | |
[21] | |
[22] | |
[23] |