Structural design: Transmissive polarisation converters usually consist of a three-layer structure of a resonant structure array and two layers of orthogonal metal wire grids above and below it [14]. This structure is often called a quasi Fabry–Pérot-like cavity resonant unit structure. The F-P cavity resonant unit structure can cause electromagnetic waves to be reflected multiple times between multi-layer dielectric plates and ultimately increase the amplitude of the transmission coefficient. The metal wire grid can select electromagnetic waves with different polarisation directions. Because the current direction on the surface of the wire grid is distributed along its direction, and the direction of the current on the metal surface is perpendicular to the electric field in the space near it, only when the polarisation direction of the planar electromagnetic wave is perpendicular to the metal wire grid can it pass through the metal wire grid.
The unit structure and specific structural parameters of the PRS in this paper are shown in Figure 1. The unit is divided into three layers. The front and rear are composed of orthogonal metal wire grids. In the middle are two open resonant rings. It is worth noting that the two resonant rings have different sizes and the opening directions are orthogonal to each other. The three layers of metal are separated by two dielectric layers. The metal patch is copper with a thickness of 0.018mm, the conductivity is 5.8×10-7 S/m, the material of the dielectric layer is F4BM-2, the relative dielectric constant is 2.2, and the loss tangent is 0.0015. Among them, the period of the unit is a, the metal wire grid spacing is d1, the metal wire grid width is d2, the outer diameters of the two middle open resonant rings are r1 and r2, the ring widths are w1 and w2, and the opening sizes are t1 and t2, the thickness of both dielectric layers is h.
Simulation results and analysis: When the polarisation direction of the incident electromagnetic wave (y-polarisation) is perpendicular to the metal wire grid, periodic boundary conditions are used to simulate the unit of the polarized rotating surface. The S parameters of the polarisation rotation unit are shown in Figure 2. When there is only a larger double-opening resonant ring in the middle layer of the polarisation rotating surface, the operating bandwidth is 8GHz ~15GHz, and the relative bandwidth is 60.87%. When there is only a smaller double-opening resonant ring in the middle layer of the polarized rotating surface, the operating bandwidth is 16GHz ~ 18GHz, and the relative bandwidth is 11.76%. But when the two sizes of double-open resonant rings are placed orthogonally, as shown in the figure, the operating bandwidth is 4.8GHz ~18.2GHz, and the relative bandwidth is 116.5%. Through the combination of double-open resonant rings of different sizes, the structure can finally work effectively in an ultra-wide frequency band. It is worth noting that in Figure 2 (c), the two sizes of rings must be placed orthogonal to reduce coupling between the two rings.
The surface current distribution on the open resonant ring is shown in Figure 3. At 6GHz, the current is fully coupled to the large loop, indicating that at low frequencies, the polarisation conversion function is mainly carried out by the large loop, and the small loop is almost ineffective; At 12GHz, the surface current is mainly concentrated on the large ring, and there is also partial coupling on the small ring. At this time, the large and small rings work together, but rely more on the large ring to complete polarisation conversion; At 18GHz, the surface current on the small ring is higher, indicating that at high frequencies, the polarisation conversion work is mainly carried out by the small ring. The comparison of surface currents on these three double open resonant rings confirms the reason why the polarized rotating surface designed in this paper can operate in an ultra-wide frequency band.
The efficiency of polarisation rotation can provide an intuitive description of the performance of polarisation converters. Polarisation rotation efficiency (PRE), also known as polarisation conversion rate (PCR), is defined as the ratio of the transmitted cross-polarized electromagnetic wave energy to the total transmitted energy [15]. When the incident electromagnetic wave is y-polarized, the calculation formula for PCR is:
In the formula, ‘T’ represents the transmitted wave, and the subscript ‘xy’ represents that the electromagnetic wave is incident by the y-polarized wave and transmitted by the x-polarized wave.
The advantages of the dual-size combination ring designed in this article can also be demonstrated through PCR. From Figure 4, it can be intuitively seen that the performance effect of the polarized rotating surface can convert incident electromagnetic waves into their cross-polarized waves in the range of 4.8GHz~18.2GHz, with a conversion efficiency higher than 99%. At the same time, when only a small or large ring exists, only a portion of that frequency band can be covered. It is interesting that the two PCR curves in the figure coincidentally intersect and converge at the 16GHz frequency point, which makes people marvel at the ingenuity of the dual size combination ring design.
Comparison of the proposed PRS with the existing PRS is given in Table 2. The superiority of the proposed PRS is strongly demonstrated by its relative bandwidth, PCR, and relative maximum wavelength thickness.