A low-side-lobe folded transmitarray antenna based on metasurface with independent amplitude/phase control

In this paper, a low-side-lobe folded transmitarray antenna (LSLFTA) based on metasurface (MS) that can independently control amplitude and phase is proposed. An MS, composed of two vertically placed polarization gates, can realize the efficient reflection of y-polarized waves, efficient transmission of x-polarized waves, and independently control of phase and amplitude is designed for the top surface. A polarization conversion surface (PCS) using an open resonant ring that can efficiently achieve cross polarization conversion is presented. The feed antenna is placed on the center of the top surface to obtain an accurate electromagnetic amplitude distribution. To the best of our knowledge, our work is thus the first one to achieve low-side-lobe FTA. The LSLFTA operating at 14.5 GHz is fabricated and measured to verify the design. After adding amplitude modulation (Taylor distribution), the side lobe level (SLL) is 5.2 dB and 7.5 lower than that of only phase modulation in the yoz plane and xoz plane, respectively. The 3 dB gain bandwidth of the antenna is 19.4% (13.5∼16.4 GHz), and the maximum aperture efficiency is 25.8%. Our work is a promising candidate in the design of high-gain, low SLL, and low-profile antenna.


Introduction
With the increasing complexity of the electromagnetic environment, the antenna performance in a competitive and crowded electromagnetic environment is very important to the whole communication system.In order to ensure the accurate operation of the antenna in a complex environment, it needs to have high gain, low side lobe level (SLL), narrow beam, and other functions [1][2][3].In recent years, due to its advantages of high gain, simple structure and light weight, transmitarray antenna (TA) has become a research hotspot in satellite communication, radar and other fields [4][5][6][7][8].Metasurface (MS), as a kind of artificial metamaterial composed of subwavelength scattering structures arranged periodically, have been developed vigorously [9][10][11][12][13].Because MS can flexibly control the basic characteristics of electromagnetic waves, such as frequency, beam, phase, amplitude and polarization, it provides a new idea and method for the design of high performance SLL TA [14,15].In [15], an MS with independently manipulate the transmissive phase and amplitude was proposed to realizes a 5.7 dB average SLL suppression.Polarization-mismatching transmissive metasurface was proposed in [16], to independent control the amplitude and phase of transmissive circular-polarization (CP) waves.
In the design of traditional TA, the focal to diameter ratio of the transmission array antenna is generally greater than 0.5 to ensure high efficiency.At this time, the height of the antenna is large, which limits the further application of the traditional TA.In order to reduce the profile of the TA antenna, the folded transmitarray antenna (FTA) based on the ray tracing principle has been proposed [2,[17][18][19][20][21].A broadband FTA with ultralowprofile, high-gain is presented in [17].Although it has a very ultralow-profile and broadband performance, its SLL value is only −11.5 dB.In [22], A wideband FTA by combining a TA aperture and a reflective polarizationconversion surface (RPCS)with a low profile is proposed.While its SLL value is −14.9 dB.However, the existing works of FTAs have been mostly studied in terms of efficiency and bandwidth.There are few studies that focus on the SLL FTA.
To overcome these problems, we propose a low-side-lobe FTA (LSLFTA) based on MS with independent amplitude/phase control.It consists of three parts: the top MS, the bottom polarization conversion surface (PCS), and the feeding antenna.The low SLL can be obtained by using the top MS with amplitude modulation and simulated amplitude distribution on the top MS.With the elaborate design, a low SLL, high gain, and low profile FTA is obtained.
The novelty and contributions of this work are summarized as follows:  (1) Different from the traditional FTA, the feed antenna of the proposed LSLTA is located on the top surface.So the amplitude of electromagnetic waves reaching the top surface can be accurately extracted.
(2) Since a polarization conversion structure is adopted for the top MS, the polarization isolation performance of the antenna will not be greatly affected by the amplitude modulation.
(3) When the top MS amplitude is adjusted, the bottom PCS can make the partial x-polarized wave reflected as a y-polarized wave, thus reducing the influence on the top MS amplitude distribution.This paper is organized as follows.Section 2 presents the design of the proposed LSLFTA.Section 3 shows the simulated/measured results of the LSLFTA, as well as comparisons of the antenna performances.Finally, conclusions are drawn in section 4.

Design of the proposed LSLFTA
Figure 1 shows the configuration and operation mechanism of the proposed LSLFTA.The antenna consists of a top MS, a bottom PCS, and an LP feed antenna.The top MS is a transmission array, which can not only convert the x-polarized incident wave into the y-polarized transmitted wave but also realize the independent control of the transmission amplitude and phase.The bottom PCS can convert y(x)-polarized incident waves into x(y)polarized reflected waves.It can be seen from the figure that the incident y-polarized wave from the feed antenna is transformed into an x-polarized wave after being reflected by the PCS.Then the reflected x-polarized wave passes through the top MS and realizes amplitude and phase modulation.Depending on the propagation path of the electromagnetic wave, the height of this antenna can be reduced to half of the focal length F (H = F/2).

The unit cell design of top MS.
In order to realize the efficient reflection of y-polarized waves, efficient transmission of x-polarized waves, and the function of amplitude and phase control, the structure shown in figure 2 is used as the unit cell of top MS.The surface consists of two F4B substrates with a thickness of 2 mm (the loss tangent is 0.001, and the relative dielectric constant is 2.65).The upper and lower surfaces are composed of two vertically placed polarization   It can be seen that the x-polarized wave can be converted to the y-polarized wave with a high transmission amplitude, but only a 180°phase variation range can be obtained.Therefore, the middle patch needs to be mirrored to obtain another 180°phase variation range.This can be explained by the Jones matrix, which can be expressed as:  ( ) where t yx is transmission amplitude under x-polarized excitations, j yx is transmission phase under x-polarized excitations.
It can be seen from equation (1) that the phase of the unit cell is increased by 180°after mirroring the patch.Figures 3(c) and (d) show the simulation results of top MS under oblique incidence.For oblique incidence up to 40°, the transmission amplitude can still maintain high transmittance and the phase variation is less than 28°.
Figure 4 shows the simulation results of the magnitude and phase of the top MS when varying the parameter α at 14.5 GHz.It can be found that the phase variation range can completely cover 360°and transmission amplitude is above 0.91 at 14.5 GHz.
In order to achieve the amplitude control capability, the middle layer patch needed to rotate its polarization.After rotating the patch, the Jones matrix can be expressed:    As can be seen from equation (2), the transmission amplitude will be changed when the parameter β varies, while the transmission phase remains unchanged.The simulation results of the magnitude and phase of the MS element when varying the parameter β at 14.5 GHz are shown in figure 5.It can be found that the transmission amplitude decreases from 1 to 0.14 when β changes from 45°to 5°, and the phase change is less than 23°.From the above results, it can be seen that the designed top MS realizes the polarization conversion function and the independent control of amplitude and phase.

The unit cell design of PCS.
To achieve efficient linear polarization conversion of reflected waves, the PCS structure shown in figure 6 is adopted as the bottom MS in this paper.It is composed of an F4B substrate with a thickness of h = 3 mm (relative dielectric constant 2.2, loss tangent 0.001), an open resonant ring etched on the top layer, and the ground plane.The structural parameters of the element are p = 8 mm, w = 1.5 mm, α = 306°, and r = 3.3 mm.
The working principle of the substructure is first introduced below.The electromagnetic wave emitted by the feed antenna is incident on the bottom PCS along the y polarization direction, which can be decomposed into two orthogonal components in the u direction and the v direction:  Then the reflected electric field can be represented by two incident electric field components:  Since the substructure is symmetric about the u and v directions, the reflection coefficient cross-polarization component is very small and can be neglected.The reflected phase difference in u and v directions can be controlled by adjusting the opening angle of the open resonant ring.When the opening angle of the resonant ring makes the phase difference of the reflection in the two directions equal to 180°and the amplitude equal, the final reflected wave electric field is x-polarized.The orthogonal polarization wave conversion is completed.
Figure 7 shows the structure of the bottom MS under oblique incidence.It can be seen from the figure that the PCS can realize the conversion from y-polarized wave to x-polarized wave in a wide frequency band, and its polarization conversion ratio (PCR) is greater than 90% in the 12.9 GHz-18 GHz.The above results show that the designed PCS can efficiently convert the polarization direction.

The LSLFTA design.
Then the LSLFTA is designed using the above MS.We use the horn antenna as the feed.The top MS consists of 30 × 30 elements with an area of 180 × 180 mm 2 , and the bottom PCS consists of 12 × 12 elements.
According to the method proposed in [23], the relationship between F/D and antenna efficiency is shown in figure 8.In order to obtain the highest aperture efficiency, the focal length F is set to 108 mm, and the corresponding focal diameter ratio F/D = 0.6.According to the principle of MPFTA, the height can be obtained: First, the FTA only with phase modulation is designed.The required phase distribution calculated by the phase compensation principle is shown in figure 9 In this paper, the Taylor distribution, widely used in antenna array design, is used to calculate the desired SLL value.The design targets are −30 dB SLL Taylor distribution in the y direction and −30 dB SLL Taylor distribution in the x direction.Then 2D Taylor-distribution T can be obtained from [15].The calculated Taylor distribution is shown in figure 9(e).Finally, the required amplitude distribution R can be calculated by the following equation:

Results
Finally, the LSLFTA is fabricated to validate the performance.The antenna prototype with a volume of 180 mm × 180 mm × 54 mm is presented in figure 10.The far-field performance can be directly tested in a microwave anechoic chamber.The far-field performance can be directly tested in a microwave anechoic chamber, whose test setup is shown in figure 11.
Figure 12 shows the simulated radiation pattern.The simulated radiation pattern of the FTA with only phase modulation at 14.5 GHz is shown in figure 13.The simulated SLL of xoz and yoz planes are −20.2dB and −16.6 dB, respectively.According to the amplitude distribution shown in figure 9, it can be seen that the amplitude distribution of the yoz plane is closer to the Taylor distribution, so the SLL value of the yoz plane is lower.
The radiation pattern of the LSLFTA at 14.5 GHz is shown in figure 14.After adding amplitude modulation, the SLL of yoz plane is 5.2 dB lower than that of only phase modulation, and the SLL of xoz plane is 7.5 dB lower than that of xoz plane.The measured gains of yoz and xoz planes are 23.92 dBi and 23.94 dBi, respectively.Because the transmission amplitude of the unit cell of top MS under oblique incidence is smaller than that of the normal incident electromagnetic wave and the transmission amplitude of different units is not all 1, the simulated SLL is larger than the target desired SLL value.The measured SLL is larger than the simulation value, which is caused by the machining error.
Figure 15 shows the simulated and measured efficiencies and gains of the LSLFTA versus frequency.It can be seen that the 3 dB gain bandwidth of the antenna is 19.4% (13.5-16.4GHz), and the maximum aperture efficiency is 25.8%.Figure 16 shows the simulated and measured SLL and reflection coefficients.The SLL of the antenna is greatly improved after adding amplitude modulation.
The performance comparison between the proposed LSLFTA and other related works is shown in table 1. [14,15] propose the TA antennas with amplitude and phase control, but they adopts the traditional structure, which leads to a high profile.In addition, in order to reduce the antenna profile, various FTA antennas have been proposed in [17,19,24,25], which achieve good radiation performance while achieving low profile, but all have high SLL values.The proposed antenna simultaneously achieves SLL and low profile.Furthermore, according to the method proposed in [23], a detailed antenna loss budget is provided in table 2.

Conclusion
In this paper, a novel LSLFTA that can simultaneously realize the high gain, low SLL, and low profile is proposed.The antenna consists of a top MS with independent amplitude/phase control, a PCS that can efficiently convert the polarization direction, and a feed antenna on the top surface.Both simulated and measured results demonstrate the design.The SLL of the antenna is greatly improved after adding amplitude modulation, which has potential application value in modern radar, satellite, and other remote communication systems.

Figure 1 .
Figure 1.Configuration and operation mechanism of the proposed LSLFTA.

Figure 2 .
Figure 2. The configuration of the proposed unit cell of top MS (a) perspective view, (b) top view.

Figure 3 .
Figure 3. Simulation polarization-converting transmission results of the top MS under x-polarized excitations (a) transmission amplitude when changing the patch parameter α, (b) transmission phase when changing the patch parameter α, (c) transmission amplitude under oblique incidence, (d) transmission phase under oblique incidence.

Figure 4 .
Figure 4. Simulation results of the magnitude and phase of the MS element when varying the intermediate layer patch parameter α at 14.5 GHz.

Figure 5 .
Figure 5. Simulation results of the magnitude and phase of the MS element when varying the parameter β at 14.5 GHz.

Figure 6 .
Figure 6.The configuration of the proposed unit cell of top MS (a) perspective view, (b) top view.

Figure 7 .
Figure 7. Simulation results of the bottom MS under oblique incidence (a) reflection amplitude, (b) the PCR.

Figure 8 .
Figure 8.The relationship between F/D and antenna efficiency.
gates to realize the polarization conversion function.By changing the parameters of the middle layer metal, the independent control of phase and amplitude can be obtained.Full-wave EM simulations were performed in CST Microwave Studio by setting periodic boundaries in the x and y directions.The parameters of the unit cell are p = 6 mm, β = 45°, w1 = 1 mm, w = 0.3 mm, s = 1 mm, r = 2.9 mm, r1 = 1.5 mm.Simulation polarization-converting transmission results of the top MS under xpolarized excitations are shown in figures 3(a) and (b).

Figure 9 .
Figure 9. (a) Required phase distribution of the top MS, (b) The corresponding β distribution of the top MS (yellow represents −135°a nd blue represents 45°), (c) The corresponding α distribution, (d) The simulated amplitude distribution on the top MS, (e) The calculated Taylor distribution, (f) The corresponding β distribution of the top MS after adding amplitude modulation.

Figure 10 .
Figure 10.(a) The fabricated LSLFTA.(b) top layer, (c) middle layer, and (d) bottom layer of the top TA.(e) front views of the PCS.
where E i is the amplitude of the incident wave,  e u and  e v are the unit vector in the u and v directions.

Figure 13 .
Figure 13.The simulated radiation pattern of the FTA only with phase modulation at 14.5 GHz (a) yoz plane, (b) xoz plane.

Figure 15 .
Figure 15.The measured LSLFTA gain and aperture efficiency change with frequency curve.
(a).Then the parameters of each cell on the top MS obtained by the simulation results of the unit cell are shown in figures 9(b) and (c).

5
where T represents the required Taylor-distribution.I represent the simulated amplitude distribution on the top MS shown in figure 9(d).A is the simulated amplitude of the top MS a shown in figure 4. The corresponding β distribution calculated by equation (3) and figure 5 is shown in figure 9(f).

Figure 16 .
Figure 16.The simulated and measured reflection coefficient and SLL values at different frequencies.

Table 1 .
Comparisons of the proposed LSLFTA and other reported works.

Table 2 .
Loss budge for the proposed LSLFTA.