An X-Band Linear-to-Circular Polarizer With High Refractive-Index Metamaterials

This paper presents a novel wideband low-profile linear-to-circular polarizer based on high refractive-index metamaterials in the X band. The proposed design is a multilayer structure that is built from two kinds of metamaterial unit cells consisting of 4-H-shaped strips and 1-H-shaped strips. Combining the two unit cells realizes the flexible and independent control of the refractive index difference between two orthogonal E-field components and thus achieves a consistent 90-degree phase delay over a wide bandwidth. Moreover, the proposed unit cell shows a high refractive index, which contributes to lowering the height of the LP-CP polarizer. In order to improve the axial ratio (AR) bandwidth, two impedance-matching layers are introduced to combat the reflection loss on the metamaterial-air boundary. Nevertheless, the proposed design can provide a wide 3 dB AR bandwidth of 37.7% and a low insertion loss of 0.5 dB with a thin profile of $0.21~{\lambda }_{0}$ . The prototype was fabricated and measured, while the experimental results agree well with the simulated ones, thus validating the working mechanism and its practical feasibility.


I. INTRODUCTION
C IRCULARLY-POLARIZED (CP) wave possesses the distinguished advantages of strong anti-interference ability, and low polarization mismatching from multipath fading compared with linearly-polarized (LP) wave.They make CP waves widely used in satellite communication systems, navigation systems [1], imaging systems [2], polarization diversity of radar systems [3], and so on.For radar systems, CP waves can be used to distinguish the transmitted and received waves.For example. the left-hand circularly polarized (LHCP) incident wave reflects right-hand circularly polarized (RHCP) from the target.For the earth stations, CP antennas can allow for receiving the incoming LP wave with all angles of the polarization vector.
LP-CP polarizer is the commonly used method to generate CP wave from LP incident wave.The basic working principle is to decompose the LP incident wave into two orthogonal components with nearly equal magnitude and 90 • phase difference such that a CP wave can be generated.In the last decades, various approaches have been reported to realize and analyze circular polarizers.Among them, the dielectric polarizer utilizing the anisotropy of the complex material has been reported in the literature [4], [5].The dielectric material approach yields good radiation performance in the aspect of the wide axial ratio and stable radiation pattern across the bandwidth, but it lacks independent manipulation on the orthogonal components of the LP incident wave and results in a high profile of the polarizer itself.
Recently, approaches of 3D metamaterial unit cells have been found effective in controlling two orthogonal components of incident wave [6], [7], [8].The proposed work in [7] combines the dielectric-air slabs with the split ring patterns to realize the 90-degree electrical path delay of the transmitting wave.In [8], a metamaterial-based partially reflected surface c 2023 The Authors.This work is licensed under a Creative Commons Attribution 4.0 License.
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(PRS) and high-impedance surface in a Fabry-Perot (FP) cavity antenna allows the independent magnitude and phase control of the transmissive wave and the reflective wave.Although the 3D unit cell-based polarizer can achieve a wide AR bandwidth with a low insertion loss, it brings a high-profile (almost one-half wavelength) structure and complexity to the antenna.
Alternatively, the 2D metasurfaces employed with the anisotropic characteristics have been investigated as polarizers extensively [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21].Meander-line polarizer report in [9], [10], [11], [12], [13], [14] is composed of dielectric slabs with metallic meandering lines printed on both sides of the slabs.Design equations of the meanderline polarizers [10], [11], [12], [13] and circuit analysis [14] have been reported.Although the design procedure of this type of polarizer is simple, the structure of the polarizer requires multiple dielectric slabs to build which leads to a very high thickness for the overall antenna body.Another approach to realizing a circular polarizer is to use frequency selective surfaces (FSS) [11], [15], [16], [17], [18], [19], [20], [21].In [15], a single-layer FSS polarizer composed of split-slot-rings (SSRs) unit cells was proposed to convert the 45 • LP wave to a CP wave.The gaps of the outer and inner SSR are placed in two orthogonal axes and optimized to generate the CP wave.It can realize a reasonable axial ratio (AR) bandwidth of 11.75% with an insertion loss of 3 dB.A dual-layer polarizer composed of a modified Jerusalem cross (MJC) FSS unit was proposed to reduce the insertion loss within the AR bandwidth [16].By inserting a one-quarter wavelength spacer into the two metallic MJC patterns, the matching for the wave between the polarizer and the air can be improved.However, the high Q-factor structure results in a narrow AR bandwidth of 10% for the polarizer, which limits applications to recent wideband wireless systems.Furthermore, cascading layers with a spacer is a simple way to improve the AR bandwidth by reducing the Q-factor of the polarizers [11], [18], [19], [20], [21].As reported in [20], a four-layer FSS polarizer based on a cross-shaped composition structure can realize a wide 3-dB AR bandwidth of 64%.However, the peak insertion loss reaches 2 dB.This indicates that a tradeoff among AR bandwidth, structural simplicity, polarization purity, and insertion loss exists in designing the multilayered polarizers.
In this paper, we are going to present a new structure of a low-profile polarizer for linear-to-circular polarized wave (LP-CP) conversion based on high refractive-index metamaterials.The proposed polarizer is composed of a new metamaterial combining of orthogonal H-shaped metallic strip pattern, which offers more degree of freedom than [22] in the manipulation of refractive index variations for two orthogonally polarized incident waves.By simply tuning the length of the metallic strip line pattern along two orthogonal polarizations, a 90 • phase difference can be achieved.On the other hand, two impedance matching layers (IMLs) are introduced to make the amplitudes of the two orthogonal-polarization waves nearly equal, which meets the requirement of generating a CP wave.The proposed design is an attempt to design the polarizer by extracting the refractive index of the subwavelength anisotropic metamaterial.To validate the design, the proposed meta unit cell is analyzed with a full-wave simulator in periodic boundary conditions.The results show that the proposed polarizer exhibits a 3-dB AR bandwidth of 42% and an insertion loss of 1 dB across the AR bandwidth for its thin structure of 0.21 λ 0 .
This paper is organized as follows.Section II introduces the design methodology of the high refractive index metamaterial-based polarizer.In Section III, the design and performance of the polarizer will be presented.The refractive indexes for the core layer and IMLs in x-and y-directions are simulated and extracted to analyze the working mechanism of the proposed polarizer.A prototype was fabricated and measured to validate the performance in Section IV.A conclusion is drawn in Section V.  with orthogonal polarizations E x and E y .As shown in Fig. 1 (b), since the anisotropic metamaterial possesses different refractive indexes in two orthogonal polarizations, phase difference occurs when the wave goes through the anisotropic material.The phase difference ϕ between the two orthogonal polarizations is calculated below,

II. DESIGN METHODOLOGY OF WIDEBAND LP-CP POLARIZER A. PRINCIPLE OF THE WIDEBAND LP-CP TRANSFORMATION
where n x and n y represent the effective refractive indexes for the two components Ex and Ey propagating through the metamaterial respectively, n is the difference of n x and n y , h is the height of the metamaterial, and λ 0 is the wavelength at working frequency.In order to fulfill the requirement for the generation of a CP wave, the phase difference between the two polarization ϕ is set as 90 • .The calculated refractive index differences at 8 GHz-12 GHz are derived from (1) as shown in Fig. 2 by fixing the phase difference ϕ as 90 • .As can be seen, for a given height of the polarizer, the refractive index difference n = n yn x is inversely proportional to the height of the polarizer h/λ 0 , while it varies decreasingly over the increasing frequency range.The relationships retrieved from Fig. 2 implied that the wideband and low-profile polarizer design required a high refractive index difference and rapid variation, and thus to maintain the phase difference between orthogonal polarizations ϕ = 90 • over the frequency range.The calculated results will give a guideline for establishing a wideband polarizer.

B. INDEPENDENT PHASE CONTROL OF ORTHOGONAL POLARIZED COMPONENTS
To realize the independent path delay control of the orthogonal components, the proposed anisotropic meta unit cell employed one H-shaped strip pattern (UC-X) and four H-shaped strip patterns (UC-Y) along the x-and y-axis respectively, as shown in Fig. 3. Succeeding the design proposed in [22], the unit cell design in four H-shaped strips (UC-Y) provides a large variation of the refractive index along the y-axis by varying the length of the horizontal arm length as depicted in Fig. 3 (a).However, it limited  variation of the x-axis refractive index due to the insufficient changes in the vertical arm's length.Thus, the unit cell design with an additional vertical H-shaped strip (UC-X) is introduced to offer another degree of freedom in varying the x-axis refractive index as shown in Fig. 3 (b), and thus to realize the CP wave with independent control of the orthogonally polarized components.Considering UC-X for x-polarized wave, one H-shaped metallic strip is sandwiched with two substrates Rogers 4003C with ε r sub = 3.38, h m1 = 1.016mm.Meanwhile, for UC-Y in y-polarization, four Hshaped metallic strips are symmetrically attached on both sides of a dielectric substrate Rogers 4003C with ε r sub = 3.38, h c1 = 0.508mm that is sandwiched between two substrates Rogers 4003C with ε r sub = 3.38, h c2 = 0.254mm.Detailed parameters are provided in Table 1.
The relationship between the number of H-shaped pattern repetitions and the refractive index difference of the orthogonal polarizations is investigated through the simulation of the meta unit cell with different H-shaped pattern repetitions as depicted in Fig. 4. Throughout the ascending repetitions of the H-shaped pattern, the refractive index on y-polarizations Re(n y ) is gradually increased from 4.38 to 6.99 at the operating frequency of 10 GHz as shown in Fig. 5(a), while the increment of that on x-polarizations Re(n x ) is insignificant ranging from 1.95 to 2.25 as shown in Fig. 5(b).Such a phenomenon implies the refractive index difference   of the orthogonal polarization is in direct proportion to the repetitions of the H-shaped pattern.To achieve the largest refractive index difference between the orthogonal polarizations, the four H-shaped strips (UC-Y) and one H-shaped strip (UC-X) are chosen to provide the maximum and minimum refractive index on y-and x-polarization respectively.
The equivalent analysis of the UC-Y and UC-X are illustrated in Fig. 6, when the unit cells are excited with the incident wave propagating along the z-axis, the incident electric field induces currents on the parallel arm of the H-shaped strip pattern, which is represented by the inductance L p .As the electrical charges are accumulated on the vertical arms between the H-shaped strip and its neighbor unit cells in the y-axis, the capacitance C p is considered.Especially for UC-Y, the inter-layer inductance L s and capacitance C s are also introduced due to the coupling within the substrate.
As a result, the equivalent circuit model of UC-Y and UC-X are evaluated in the Advanced Design System (ADS) as shown in Fig. 7.In the proposed work, the equivalent circuits for UC-Y and UC-X are listed with their corresponding values in Table 2. To validate the equivalent circuit, the scattering responses obtained from ADS, S 11x_EC , S 11y_EC , S 21x_EC , S 21y_EC , are compared with that from full wave    simulator HFSS, S 11x_EM , S 11y_EM , S 21x_EM , S 21y_EM as illustrated in Fig. 8, and thus the results made a good agreement with each other.The equivalent circuit analysis of the UC-Y in the variation of L c1 is demonstrated in Fig. 9 as the coupling of currents to the adjacent cell and the interlayer induction is mainly contributed by the parameters L c1 , thus providing significant changes in the equivalent circuit values among the dimensions of the structure.The effect of the physical dimension L c1 on the equivalent circuit is summarized in Table 3.As the horizontal arm length L c1 of the H-shaped strip is reduced, there is a significant reduction of the capacitance C p due to the distance between the vertical arms of the H-shaped strip and its adjacent cells is increased, and thus the coupling effect between the unit cells weakens.Meanwhile, regarding the four H-shaped strips printed on both sides of the substrate as 4 pairs of parallel wires, the inductance L p and L s are reduced as the decrease of wire length.The capacitance C s remains the same over the variation of L c1 as the substrate thickness is unchanged.
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C. REDUCTION OF INSERTION LOSS
Consider the wave impedance of the proposed non-magnetic metamaterial with the refractive index of n in this work where μ r is the relative permeability, ε r is the relative permittivity, η 0 = √ μ 0 /ε 0 is the wave impedance in the free space.As observed in (2), the wave impedance decreases as the refractive index increases.The different wave impedances between the metamaterial and the free space can result in the mismatching issue.Hence, it can increase the insertion loss and unbalanced magnitudes for two polarizations which will degrade the performance of CP radiation.
Therefore, impedance matching layers (IMLs) between the polarizer and the air are introduced to compensate for the loss due to the mismatching as shown in Fig. 10.To achieve the impedance matching, h 2 is selected as a quarter wavelength in IML.The wave impedance of IML is meant to be the geometrical mean of the air and the core layer, which leads to where n 1x , n 1y are the core layer refractive indexes and n 2x , n 2y are the IML refractive indexes Following the schematic diagram of the proposed polarizer as shown in Fig. 10, the relationship between the thickness of the core layer and that of the IML (denoted as h 1 and h 2 ) in terms of the corresponding refractive indexes n 1x , n 1y , n 2x , and n 2y is derived as follows,

FIGURE 11. Configuration of the proposed multilayered wideband LP-CP polarizer.
where h is the overall thickness and n y , n x the refractive indexes of the polarizer in orthogonal polarization.
According to (4), the height of the core layer h 1 and IML layer h 2 can be obtained with the corresponding refractive indexes n 1x , n 1y , n 2x , and n 2y to achieve the phase difference as 90 • .

III. DESIGN OF HIGH REFRACTIVE INDEX-BASED LP-CP POLARIZER
The meta unit cell for the polarizer is shown in Fig. 11.The unit cell is composed of ten layers of substrates printed on four kinds of patterns.Layers 3 to 8 form the core layer.Layer 1, 2, and air gaps compose an IML while Layer 9, 10, and air gaps compose another IML.The substrates are chosen as Rogers RO4003C (ε r = 3.55, tanδ = 0.0027) with thicknesses of 0.254 mm and 0.508 mm for consolidating the multilayer structure.The detailed design and performance of the layers will be discussed in the following sections.

A. CORE LAYER DESIGN AND ANALYSIS
Combining the UC-X and UC-Y introduced in Section III-B, the anisotropic meta unit cell structure UC1 is proposed for constructing the core layer of the polarizer, as shown in Fig. 12.The core layer unit cell is composed of 6 layers  of PCBs.Four horizontal H-shaped metallic strips along the y-axis are printed on the bottom of Layers 3 and 6, and top of Layers 5 and 8.A single vertical H-shaped metallic strip along the x-axis is printed between Layers 5 and 6.Fig. 13 shows the of UC1 for both x-and y-It be observed that because the variation of the refractive indexes in the x-polarization is larger than of indexes the y-polarization, the difference of the refractive indexes for two polarizations decreases as the frequency increases, which fulfills the requirement of index variation for establishing a wideband polarizer as shown in Fig. 2. The refractive indexes in the x-and y-polarizations at 10 GHz with different L m1 and L c1 are simulated as shown in Fig. 14.When L m1 and L c1 are changed, other parameters are fixed.It is indicated that the refractive indexes in two polarizations can be tuned independently.The index in the x-polarization (n x ) varies from 2.1 to 3.8 by changing L m1 from 0.1 to 0.3 mm, while that in the y-polarization (n y ) varies from 2.1 to 6.88 by tuning L c1 from 0.1 to 2.95 mm.The independent control of refractive indexes in two polarizations enables tuning the index difference as required.

B. IMPEDANCE MATCHING LAYER DESIGN AND ANALYSIS
According to (3), the required refractive index for IML is lower than that for the core layer so the height of IML is higher than the core layer.A dual-layer unit cell sandwiched with air layers is proposed to form the IMLs as shown in Fig. 15.Four horizontal H-shaped metallic strips along the y-axis are printed on top of Layer 1 and bottom of Layer 2. A single vertical H-shaped metallic strip along the x-axis is printed between Layers 1 and 2. Fig. 16 depicts the simulated refractive indexes versus frequencies for UC2 in both x-and y-polarizations.The refractive indexes in both polarizations are lower than those for the core layer, which meets the requirement for IML.The refractive indexes in the x-and y-polarizations for different values of L a1 and L n1 are carried out to demonstrate the characteristics of the proposed meta unit cell.As shown in Fig. 17     L a1 is varied from 0.1 to 3 mm, the n y changes from 1.2 to 3.8.Fig. 17 reveals that the refractive indexes in the x-and y-polarizations can be tuned by changing the parameters of L n1 and L a1 independently.This enables us to optimize the key parameters for achieving a wideband and low-loss polarizer.

C. OVERALL STRUCTURE DESIGN AND ANALYSIS
Based on the unit cell designs for the core layer and IML, a multilayered LP-CP polarizer is constructed by sandwiching UC1 with two UC2 as depicted in Fig. 18.All the parameters are optimized and provided in Table 4 in order to meet the requirements of composing a wideband LP-CP polarizer based on (1) to (4).To extract the reflection coefficient and transmission coefficient from the polarizer, the unit cell is simulated in the periodic boundary condition with the aid of the full-wave simulator HFSS.
The simulated reflection and transmission coefficients are shown in Fig. 19, which indicates that the reflection coefficients are below -10 dB, and the transmission coefficients are above -0.5 dB in frequencies ranging from 8.13 to 11.68 GHz for both polarizations.Furthermore, the effects of the IML on the transmission magnitude and phase of the proposed polarizer unit cell are depicted in Fig. 20.As shown in Fig. 20 (a), the polarizer utilizing the core layer only demonstrated the phase difference ϕ = 90 • at a single frequency point of 10 GHz and resulted in an unequal magnitude between the two orthogonal components over the bandwidth.While inserting the IML as a multilayer design, the consistent phase difference of 90 degrees and magnitude difference of 0 dB has been achieved for a wider bandwidth as illustrated in Fig. 20 (b).It can be claimed that the impedance matching layer is a crucial part of the design that affects and contributes to the AR bandwidth and impedance matching, and thus the proposed unit cell can meet the requirement for generating a CP wave.
Through the extracted transmission coefficients (T x , T y ), the corresponding axial ratio and cross-polarization level can be derived from the scattering parameters.Accordingly, the cross-polarization levels (XPL) can be expressed with the commonly used formula, The axial ratio (AR) can be calculated as, where L and R are the left-handed and right-handed circular polarization phasors.Based on ( 5) to ( 6), the XPL and AR are calculated and depicted in Fig. 21.It can be found that the XPL is below -15 dB within frequencies ranging from 8.06 to 11.89 GHz while the 3-dB AR bandwidth is 8.11-11.88GHz.The overlapped bandwidth of R x , R y ≤ −10 dB, and AR ≤ 3 dB leads to the operating bandwidth of 8.13 GHz-11.68GHz (35.8%).Moreover, the AR ≤ 1 dB bandwidth is 25%.The total thickness of the polarizer is 6.448 mm, which is 0.21 λ 0 where λ 0 is the wavelength of the center frequency at 10 GHz.The effect of the incident angle on AR bandwidth for each elevation plane is revealed in Fig. 22.The AR bandwidth is in descending order as the incident angle increases, while the polarizer works improperly when the incident angle exceeds θ = 20 o .

IV. PERFORMANCE OF THE LP-CP POLARIZER A. FABRICATED PROTOTYPE
To validate the performance of the proposed design, the LP-CP polarizer consisting of 41*41 elements was fabricated and measured as shown in Fig. 23.Bonding films between the substrate layers and plastic screws on the substrate margin are added for ensuring seamless contact of the substrates and enhancing the robustness of the fabricated prototype.The selected bonding film, Rogers 3001, featured a low dielectric constant of ε r = 2.28 and low loss tangent of tanδ = 0.03 at the X-band frequency spectrum, and thus had minimum interference with the electrical performance of the multilayer construction.The effect of the bonding film on the AR and insertion loss (IL) are shown in Fig. 24.The 3dB AR bandwidth is slightly shifted to a higher frequency which may be related to the slight reduction of the proposed polarizer height, while there is no significant difference between the insertion losses of the polarizer with and without bonding film due to the comparable dielectric constant of the bonding film with air.Considering the alignment between the air gap and the substrate layers, the 3D-printed spacer with a precision of ±0.03mm is utilized on the margin of the substrate layer.Owing to the reinforced strength of the chosen substrate material, Rogers 4003C, the flatness of the fabricated prototype is promising with the consolidated multi-layered structure fastening with a significant amount of plastic screws., and 135 • according to the equation [23], [24], [25],

Fig
, and E 4 = S 21 (ϕ 2 = 135 • ).The measured 3-dB AR bandwidth is 42% (7.9 -12.2 GHz) as shown in Fig. 26 (b), which agrees well with the simulated one (8.1 -11.8 GHz, 37%).The discrepancy between the simulated and measured AR may be caused by the misalignment between the transmitting and receiving horns at different rotation angles hence resulting in the inconsistency and variation of measured S 21 at different rotation angles for AR calculation.However, the measured insertion loss ≤ 1 dB is within 8.2-11.8GHz while the simulation value is lower than 0.5 dB.It is indicated that the working frequency band of the proposed design should be the overlapped bandwidth of 8.2 -11.8 GHz.
Table 5 lists the performance of the proposed work compared with the polarizers proposed in the previous  literature.Although the profile of the proposed design is higher than that in [13], [17], [18], and [20], the insertion loss of our design is much lower.For the low-insertion loss polarizers [6], [14], [21], [26], and [27], the proposed design takes advantage of low profile and wide bandwidth.

V. CONCLUSION
In this paper, a novel low-profile LP-CP polarizer based on anisotropic metamaterial has been investigated.It proposed a polarizer with independently controllable refractive indexes for two electric field components in orthogonal polarizations.Anisotropic meta unit cells are introduced to realize the Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.
different refractive indexes in both polarizations and broaden the AR bandwidth.By inserting two impedance-matching layers, the impedance matching between the air and polarizer can be improved.As seen from the results, the polarizer exhibits a broadband characteristic with a wide 3-dB AR bandwidth of 42% ranging from 7.9 to 12.2 GHz.Moreover, the measured insertion loss within the operating bandwidth is smaller than 1 dB which is higher than that in simulation.Furthermore, the total thickness of the proposed polarizer is only 0.21 λ 0 , which makes it a compact component for generating CP waves in high-speed communication, microwave imaging, and tracking systems.

FIGURE 1 .
FIGURE 1.(a) General behavior of the LP-CP polarizer excited by the 45 • LP incident plane wave.(b) Diagram of the anisotropic metamaterial excited with Ex and Ey .

Fig. 1 (
Fig.1 (a)  shows the working principle of the polarizer based on the meta unit cell.The incident wave E i with 45 • polarization can be decomposed into two components

FIGURE 2 .
FIGURE 2. Calculated refractive index differences versus frequencies for different heights of the converter.

FIGURE 3 .
FIGURE 3. Configuration of (a) four H-shaped strip unit cell (UC-Y) and (b) one H-shaped strip unit cell (UC-X).

FIGURE 4 .
FIGURE 4. Configuration of the proposed core layer unit cell with different H-shaped pattern repetitions.

FIGURE 5 .
FIGURE 5. Refractive indexes of the meta unit cell with different H-shaped pattern repetitions in (a) y-pol.arizationsand (b) x-polarization.

FIGURE 6 .
FIGURE 6. Equivalent analysis of the H-shaped strip unit cell.

FIGURE 7 .
FIGURE 7. Equivalent model of the proposed meta unit cell (a) UC-Y (b) UC-X.

FIGURE 8 .
FIGURE 8. Scattering response of (a) four H-shaped strip unit cell (UC-Y) and (b) one H-shaped strip unit cell (UC-X) under circuit and full wave simulator.
(a), when L n1 is tuned from 0.1 to 3 mm, the n x varies from 1.2 to 2.25.Other parameters except for L n1 are fixed.Similarly, when Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

FIGURE 18 .
FIGURE 18. Geometry of the proposed multilayered wideband LP-CP polarizer.

FIGURE 19 .
FIGURE 19.Simulated reflection coefficient (Rx and Ry ) and transmission coefficient (Tx and Ty ) of the proposed LP-CP polarizer.

FIGURE 20 .FIGURE 21 .
FIGURE 20.Simulated results of the transmission magnitude and phase for the proposed polarizer with (a) core layer only (b) core layer and IMLs.

FIGURE 23 .FIGURE 24 .
FIGURE 23.Fabricated prototype of the LP-CP polarizer measurement and actual measurement setup for prototype.(a) perspective view (b) side view.
Fig.25(a) displays the schematic diagram of the AR measurement setup while Fig.25 (b)  shows the actual measurement setup with the prototype of the proposed LP-CP polarizer.The proposed polarizer is fixed in the middle of the anechoic chamber with no mobility.For the transmission side, the Narda model 640 standard gain horn with the aperture size D 2 of 78.49mm x 59.44mm is selected as the feeding horn and placed at the far-field distance r = 2D 2 /λ = 404mm at 13 GHz.Meanwhile, an NSI-RF-RGP40 double-ridged broadband horn antenna with an aperture size of 52mm x 44mm is deployed as the receiving horn and

FIGURE 25 .
FIGURE 25.Schematic diagram of the LP-CP polarizer measurement and actual measurement setup for prototype.(a) Schematic diagram of the measurement.(b) Actual measurement setup with the photograph of the proposed prototype.

FIGURE 26 .
FIGURE 26.Measured results for the proposed LP-CP polarizer.(a) Measured S21 with the receiving antenna at ϕ2=0 • , 45 • , 90 • , and 135 • .(b) Comparison of the insertion loss and AR between the simulation and measurement.