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.