Circular polarizer for cosecant square beam forming and design method

Abstract

The invention belongs to the technical field of microwaves, and particularly relates to a cosecant square beam forming circular polarizer and a design method. The cosecant square beamforming circular polarizer comprises a waveguide slot antenna array, a cosecant square beamforming metasurface and a polarization conversion metasurface which are concentrically and sequentially arranged from bottom to top along a central axis. According to the circular polarizer for cosecant square beam forming, a cosecant square reflecting surface in the prior art is removed, cosecant square beam forming and circular polarization processing are carried out at the same time, the integration level of the antenna is improved, the size of the antenna is reduced, and the machining difficulty is lowered. In addition, by replacing the dielectric substrate and the metal layer, other beam forming such as broadened beams and tapered beams can be realized, and the product expansion capability is improved.

Description

Technical Field

[0001] The present invention belongs to the field of microwave technology, and particularly relates to a cosecant-squared beamforming circular polarizer and a design method thereof. Background Art

[0002] In ground-based field surveillance radars, it is usually desired to use an antenna with a narrow beam in the horizontal plane and a fan-shaped beam in the vertical plane. However, since the power density of electromagnetic waves is inversely proportional to the square of the distance during propagation, the intensity of the received echo signal often varies with the distance. To make the intensity of the received echo signal the same, in modern search radar technology, a dual-curved reflector beam cosecant-squared beam antenna is usually used to achieve beamforming. That is, the beam in the vertical plane is in the shape of a cosecant square, while the horizontal plane is in the shape of a narrow beam. The current mainstream method is to radiate electromagnetic waves generated by a linear array onto a single-curved cosecant-squared reflector. Since its focus is on a straight line, a cylindrical reflector with a cross-sectional curvature capable of radiating energy to a specified height is required. The linear array feed and the cosecant-squared reflector are integrally fabricated. The antenna beam generated by the linear array feed is reflected by the shaped reflector to a specified area, and then the radiated linearly polarized electromagnetic wave is converted into a circularly polarized electromagnetic wave through a circular polarizer. This method will face some problems: the combination of the antenna and the cosecant-squared reflector is integrally machined, with high overall processing difficulty and precision requirements, and the overall volume is relatively large, the weight is also relatively heavy, and the requirements for servo will also increase. In addition, the traditional cosecant-squared reflector cannot perform other expected beamforming. Summary of the Invention

[0003] In order to solve the problems of the prior art, reduce the processing difficulty and the requirements for servo, and perform other expected beamforming, the present application provides a cosecant-squared beamforming circular polarizer and a design method thereof.

[0004] In a first aspect, a cosecant-squared beamforming circular polarizer is provided, and the following technical solutions are adopted:

[0005] The cosecant-squared beamforming circular polarizer includes: a waveguide slot antenna array, a cosecant-squared beamforming metasurface, and a polarization conversion metasurface;

[0006] Among them, the waveguide slot antenna array, the cosecant-squared beamforming metasurface, and the polarization conversion metasurface are concentrically arranged along the central axis and are arranged in sequence from bottom to top.

[0007] Further, the waveguide slot antenna array couples energy from the waveguide feed in the lower layer to the upper waveguide through slots, and radiates electromagnetic waves through the slots in the upper waveguide.

[0008] Furthermore, the cosecant-squared beamforming metasurface includes a dielectric substrate and a beamforming metal layer; wherein, the beamforming metal layer covers both the front and back sides of the dielectric substrate, and the structures of the beamforming metal layers on the front and back sides are the same; the beamforming metal layer uses cross-shaped hollowed-out patches and circular patches as periodic amplitude-phase modulation units.

[0009] Furthermore, the cosecant-squared beamforming metasurface includes two dielectric substrates, and the two dielectric substrates have the same dielectric constant.

[0010] Furthermore, the dielectric constant of the two dielectric substrates is 3.0.

[0011] Furthermore, the size of the periodic amplitude-phase modulation unit is half of the wavelength of the working frequency band, and the thickness is 1.575 mm.

[0012] Furthermore, the polarization conversion metasurface includes a dielectric substrate and a polarization conversion metal layer; wherein, the polarization conversion metal layer covers both the front and back sides of the dielectric substrate; the polarization conversion metal layer covering the front side of the dielectric substrate uses periodically arranged long strip-shaped polarization gratings; the polarization conversion metal layer covering the back side of the dielectric substrate uses a king-character-like metal patch; the polarization conversion metal layer uses long strip-shaped polarization gratings and king-character-like metal patches as polarizer units, and the polarizer units are arranged periodically.

[0013] Furthermore, the polarization conversion metasurface includes one dielectric substrate.

[0014] In a second aspect, a design method for a cosecant-squared beamforming metasurface is provided, which is used to implement the cosecant-squared beamforming metasurface described in the first aspect, and the following technical solutions are adopted:

[0015] Establish a cosecant-squared shape constraint line on the circular polarizer of the cosecant-squared beamforming.

[0016] According to the values of the target pattern and the cosecant-squared shape constraint line, establish a fitness function to optimize the target pattern.

[0017] Use the genetic algorithm method to adjust the amplitude and phase of the periodic amplitude-phase modulation unit of the cosecant-squared beamforming metasurface to obtain the final optimization result.

[0018] Derive the amplitude and phase of the final optimization result, and in the simulation software, establish a cosecant-squared beamforming metasurface model for simulation verification.

[0019] The beneficial effects of the present invention are as follows:

[0020] In the prior art, to achieve cosecant-squared beamforming, electromagnetic waves generated by a linear array are radiated onto a single-curved cosecant-squared reflector. The linear array feed and the cosecant-squared reflector are integrally fabricated, which results in high processing difficulty, high precision requirements, a relatively large overall volume, heavy weight, and high requirements for servo systems.

[0021] The circular polarizer with cosecant-squared beamforming of the present invention consists of an electromagnetic metasurface formed by arranging sub-wavelength units according to a certain pattern, and through the design of the unit structure, precise control over the amplitude, phase, polarization, and other states of electromagnetic waves is achieved. The cosecant-squared reflector in the prior art is removed, and at the same time, cosecant-squared beamforming and circular polarization processing are carried out, improving the integration of the circular polarizer, reducing the volume, making it lightweight, having low processing difficulty, good maintainability, and a simple assembly method, being suitable for engineering applications.

[0022] By replacing the dielectric substrate and the metal layer, other beamforming can be achieved, such as beam broadening, conical beam, etc., and the product has strong expansion ability.

[0023] The circular polarizer with cosecant-squared beamforming is fabricated using PCB boards, with a simple preparation process and low cost, being suitable for engineering applications. Brief Description of the Drawings

[0024] Figure 1 is a schematic structural diagram of the circular polarizer with cosecant-squared beamforming according to an embodiment of the present invention;

[0025] Among them, 1. waveguide slot antenna array; 2. cosecant-squared beamforming metasurface; 3. polarization conversion metasurface;

[0026] Figure 2 is a side view of the waveguide slot antenna array, cosecant-squared beamformer, and polarization conversion metasurface according to an embodiment of the present invention;

[0027] Figure 3 is a schematic structural diagram of the cosecant-squared beamforming metasurface according to an embodiment of the present invention;

[0028] Figure 4 is a schematic structural diagram of the polarization conversion metasurface according to an embodiment of the present invention;

[0029] Figure 5 is the normalized radiation pattern of the circular polarizer with cosecant-squared beamforming according to an embodiment of the present invention;

[0030] Figure 6 is the axial ratio characteristic of the circular polarizer with cosecant-squared beamforming according to an embodiment of the present invention;

[0031] Figure 7 is the return loss of the circular polarizer with cosecant-squared beamforming according to an embodiment of the present invention. Detailed Implementation Manner

[0032] The present invention will be further described below.

[0033] The present invention provides a circular polarizer with cosecant-squared beamforming, including a waveguide slot antenna array 1, a cosecant-squared beamforming metasurface 2, and a polarization conversion metasurface 3; wherein, the waveguide slot antenna array 1, the cosecant-squared beamforming metasurface 2, and the polarization conversion metasurface 3 are concentrically arranged along the central axis and are arranged in sequence from bottom to top. The overall structural schematic diagram is as shown in Figure 1 shown, and the side view is as shown in Figure 2 shown.

[0034] The specific structure and functions are as follows:

[0035] Waveguide slot antenna array 1: The energy is coupled from the lower-layer waveguide to the upper-layer waveguide through the slots, and the electromagnetic wave is radiated through the slots on the upper-layer waveguide; the waveguide slot antenna array 1 can adopt an equal-amplitude and co-directional design, or the amplitude can be weighted by functions such as Taylor distribution; the central axis of the waveguide slot antenna is the z-axis upward, and the direction parallel to the antenna and the metasurface is the xoy plane;

[0036] In this embodiment, the length of the antenna slot is 15.35 mm, the width is 1.62 mm, the spacing between the slots is 21 mm, and the slot offset is 4.09 mm.

[0037] The cosecant-squared beamforming metasurface 2 and the polarization conversion metasurface 3 are electromagnetic metasurfaces composed of sub-wavelength units arranged according to a certain rule, and the precise control of the amplitude, phase, polarization and other states of the electromagnetic wave can be realized through the design of the unit structure. As a two-dimensional planar structure of metamaterials, it has the advantages of thin thickness, low loss, and easy fabrication. Therefore, the emergence of metasurfaces provides a simple way to control electromagnetic waves.

[0038] The overall structural schematic diagram of the cosecant-squared beamforming metasurface 2 is as shown in Figure 3 shown, including a dielectric substrate and a beamforming metal layer. The function of the dielectric substrate is to support the metal structure and realize the PCB board process. The beamforming metal layer covers both the front and back sides of the dielectric substrate, and the structures of the beamforming metal layers on the front and back sides are the same; the beamforming metal layer uses cross-shaped hollowed-out patches and circular patches as periodic amplitude-phase modulation units, and the size of the periodic amplitude-phase modulation units is half of the wavelength of the working frequency band. The diameter of the circular patch and the length of the cross-shaped hollowing determine the amplitude and phase of the periodic amplitude-phase modulation unit of the cosecant-squared beamforming metasurface 2. The cosecant-squared beamforming of the electromagnetic wave is realized through the cosecant-squared beamforming metasurface 2.

[0039] In this embodiment, the cosecant-squared beamforming metasurface 2 includes two dielectric substrates and four layers of beamforming metal layers. The two dielectric substrates have the same dielectric constant of 3.0 and a loss tangent of 0.002. The size of the periodic amplitude-phase modulation unit is half of the wavelength of the operating frequency band, and the thickness is 1.575 mm. The diameter of the circular patch is 3 mm to 3.6 mm, and the length of the cross-shaped cutout is 2 mm to 3 mm.

[0040] This application does not limit the number of dielectric substrates of the cosecant-squared beamforming metasurface 2 and the number of layers of the beamforming metal layers, as long as the electromagnetic characteristics can be satisfied.

[0041] The overall structural schematic diagram of the polarization conversion metasurface 3 is as Figure 4 shown, including a dielectric substrate and a polarization conversion metal layer; wherein, the polarization conversion metal layer covers both the front and back sides of the dielectric substrate; the polarization conversion metal layer covering the front of the dielectric substrate of the polarization conversion metasurface 3 uses periodically arranged long-strip polarization gratings; the polarization conversion metal layer covering the back of the dielectric substrate of the polarization conversion metasurface 3 uses a king-character-like metal patch; the polarization conversion metal layer uses long-strip polarization gratings and king-character-like metal patches as polarization unit, and the polarization units are arranged periodically. By arranging the polarization units periodically, cross-coupling of electrical resonance and magnetic resonance is generated for the incident wave to achieve circular polarization, and the beam is converted from linear polarization to circular polarization through the polarization conversion metasurface 3. When the linearly polarized electromagnetic wave in the air is incident on the polarization conversion metasurface 3, the polarization unit generates resonance, so that the transmitted electromagnetic wave has equal amplitudes in the horizontal and vertical directions and a phase difference of 90°. Thus, the conversion from linear polarization to circular polarization is completed.

[0042] In this embodiment, the dielectric substrate of the polarization conversion metasurface 3 has a dielectric constant of 2.2 F / m, a loss tangent of 0.0009, and a thickness of 1.575 mm; the period of the polarization unit is 4 mm. The length of the long-strip polarization grating is 3.4 mm, the width is 0.3 mm, and the interval is 0.3 mm. The length of the king-character metal patch is 2.2 mm and the width is 0.3 mm.

[0043] The metal described in this embodiment is all copper foil with a thickness of 0.035 mm.

[0044] This application does not limit the number of dielectric substrates of the polarization conversion metasurface 3 and the number of layers of the polarization conversion metal layers, as long as the electromagnetic characteristics can be satisfied.

[0045] Based on this embodiment, the polarization conversion metasurface 3 and the cosecant-squared beamforming metasurface 2 can also be fixed on the waveguide slot antenna array 1 through plastic parts or support parts to ensure the stable connection between the metasurfaces (including the polarization conversion metasurface 3 and the cosecant-squared beamforming metasurface 2) and the antenna array, avoid displacement or deformation caused by vibration or temperature changes, and ensure long-term stable performance.

[0046] Embodiment 2:

[0047] A design method of a cosecant-squared beamforming metasurface for implementing the cosecant-squared beamforming metasurface 2 in Embodiment 1 includes the following steps:

[0048] Step 1: Establish a cosecant-squared shape constraint line on the target pattern of the cosecant-squared beamforming circular polarizer; specifically as follows:

[0049] Determine the coverage range, gain distribution, etc. of the target pattern, find the maximum gain point in the pattern, usually located in the main lobe direction of the antenna, calculate the cosecant-squared gain according to the cosecant-squared formula, and draw the constraint line of the gain distribution on the pattern.

[0050] Step 2: Obtain the specific values of the current target pattern and the cosecant-squared shape constraint line, establish a fitness function, and use this function as the basis for optimizing the current target pattern, specifically as follows:

[0051] The fitness function fit(m) consists of two parts: fit_1 and fit_2; where, fit_1 is used to measure the performance of the main beam; fit_2 is used to measure the performance of the side lobes; the final fitness value fit(m) is the normalized value of the weighted sum of these two parts. Specifically as follows:

[0052] fit_1(i) = 0.6*sum(sqrt((S_theta(m,i)-D_theta(i))^2))

[0053] fit_2(i)=0.4*abs(secondMaxVal-D_theta(i))

[0054] fit(m)=1 / norm(fit_1+fit_2)

[0055] Where, D_theta is the cosecant-squared target pattern; S_theta is the antenna excitation synthesis pattern;

[0056] fit_1(i) is used to measure the matching degree between the antenna excitation synthesis pattern S_theta and the cosecant-squared target pattern D_theta in the main beam region; fit_2(i) is used to measure the performance of the side lobe region, and the goal is to make the side lobe level as low as possible.

[0057] $S_{\theta}(m,i)$ is the value of the antenna excitation synthesis pattern at angle $i$ in the $m$-th optimization iteration;

[0058] $D_{\theta}(i)$ is the value of the cosecant-squared target pattern at angle $i$;

[0059] $\sqrt{(S_{\theta}(m,i)-D_{\theta}(i))^2}$ calculates the error between the antenna excitation synthesis pattern and the cosecant-squared target pattern at angle $i$;

[0060] $\sum$ sums the errors for all angles $i$;

[0061] $0.6$ is the weight coefficient, representing the importance of the main beam matching degree in the total fitness. It is an empirical value and can be adjusted according to actual needs;

[0062] $secondMaxVal$ is the maximum sidelobe level in the antenna excitation synthesis pattern excluding the main beam;

[0063] $|secondMaxVal - D_{\theta}(i)|$ calculates the deviation between the sidelobe level and the target pattern;

[0064] $0.4$ is the weight coefficient, representing the importance of sidelobe suppression in the total fitness;

[0065] $fit\_1+fit\_2$ is the weighted sum of the main beam matching degree and sidelobe suppression;

[0066] $norm(fit\_1 + fit\_2)$ normalizes the weighted sum;

[0067] $1 / norm(fit\_1 + fit\_2)$ indicates that the smaller the fitness value, the better the pattern performance.

[0068] Step 3: Use the genetic algorithm method to adjust the amplitude and phase of the periodic amplitude-phase modulation units of the cosecant-squared beamforming metasurface 2, compare the target pattern and the cosecant-squared shape constraint line under the new amplitude and phase. Through multiple iterations, when the fitness value $fit(m)$ is the smallest (i.e., the antenna excitation synthesis pattern $S_{\theta}$ is as close as possible to the cosecant-squared target pattern $D_{\theta}$), and the pattern shape meets the cosecant-squared requirements, it is the optimal result of the final beam.

[0069] Step 4: Export the current optimal amplitude and phase results, and in the electromagnetic simulation software Ansys HFSS, establish a cosecant-squared beamforming metasurface model for simulation verification;

[0070] In the genetic algorithm adopted by the present invention, population selection evolves generation by generation according to the principles of survival of the fittest and elimination of the unfit, generating better and better approximate solutions. In each generation, individuals are selected according to the fitness of individuals in the problem domain, and are combined, crossed and mutated by means of genetic operators in natural genetics to generate a population representing a new solution set. The fitness value is obtained by subtracting the cosecant-squared target pattern from the antenna excitation synthesis pattern and weighting. The functions of population roulette and survival of the fittest are added to obtain a faster convergence rate. The preferred number of iterations G = 1000, the crossover rate Pc = 0.3, and the mutation rate Pm = 0.02. After final convergence, the array amplitude and phase values are used as the input for the design of the periodic amplitude-phase modulation unit of the cosecant-squared beamforming metasurface 2, and then the diameter of the circular patch and the length of the cross-shaped cutout are designed.

[0071] It has been verified that the normalized pattern of the circular polarizer with cosecant-squared beamforming in this embodiment is as Figure 5 shown; the axial ratio characteristic of the circular polarizer with cosecant-squared beamforming is as Figure 6 shown. The return loss of the circular polarizer with cosecant-squared beamforming is as Figure 7 shown, where S11 represents the reflection coefficient. It can be seen that the circular polarizer with cosecant-squared beamforming can achieve the cosecant-squared beamforming function of the beam and the polarization conversion function within the working frequency band.

[0072] Although the present invention has been disclosed above with preferred embodiments, the embodiments are not used to limit the present invention. Any equivalent changes or modifications made without departing from the spirit and scope of the present invention also belong to the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the content defined by the claims of this application.

Claims

1. A circular polarizer with cosecant square beamforming, characterized in that: include: Waveguide slot antenna array (1), cosecant square beamforming metasurface (2), and polarization conversion metasurface (3); The waveguide slot antenna array (1), the cosecant square beamforming metasurface (2), and the polarization conversion metasurface (3) are arranged concentrically along the central axis, in order from bottom to top.

2. The circular polarizer with cosecant square beamforming according to claim 1, characterized in that: The waveguide slot antenna array (1) is fed by the waveguide of the lower layer, couples energy into the waveguide of the upper layer through the slots, and radiates electromagnetic waves through the slots of the waveguide of the upper layer.

3. The circular polarizer with cosecant square beamforming according to claim 1, characterized in that: The cosecant square beamforming metasurface (2) comprises a dielectric substrate and a beamforming metal layer; wherein the beamforming metal layer covers the front and back sides of the dielectric substrate, and the beamforming metal layer structures on the front and back sides are consistent; and the beamforming metal layer uses a cross-shaped hollow patch and a circular patch as a periodic amplitude phase control unit.

4. The circular polarizer with cosecant square beamforming according to claim 3, characterized in that: The cosecant square beamforming metasurface (2) includes two dielectric substrates, and the two dielectric substrates have the same dielectric constant.

5. The circular polarizer with cosecant square beamforming according to claim 4, characterized in that: The dielectric constant used for the two dielectric substrates is 3.

0.

6. The circular polarizer with cosecant square beamforming according to claim 5, characterized in that: The size of the periodic amplitude and phase control unit is half the wavelength of the working frequency band, and the thickness is 1.575 mm.

7. The cosecant square beamforming circular polarizer according to claim 1, characterized in that: The polarization conversion metasurface (3) comprises a dielectric substrate and a polarization conversion metal layer; wherein the polarization conversion metal layer covers both sides of the dielectric substrate; the polarization conversion metal layer covering the front side of the dielectric substrate adopts a periodically arranged long strip polarization grid; the polarization conversion metal layer covering the back side of the dielectric substrate adopts a W-shaped metal patch; the polarization conversion metal layer adopts a long strip polarization grid and a W-shaped metal patch as polarizer units, and the polarizer units are periodically arranged.

8. The circular polarizer with cosecant square beamforming according to claim 7, characterized in that: The polarization conversion metasurface (3) includes a dielectric substrate.

9. A design method for a cosecant square beamforming metasurface, characterized in that: The method for realizing the cosecant square beamforming metasurface (2) as claimed in claim 1 comprises the following steps: Establishing a cosecant square shape constraint line on a cosecant square beamforming circular polarizer; According to the target directional diagram and the value of the cosecant square shape constraint line, a fitness function is established to optimize the target directional diagram; The genetic algorithm method is used to adjust the amplitude and phase of the periodic amplitude and phase control unit of the cosecant square beamforming metasurface (2) to obtain the final optimization result; The amplitude and phase of the final optimization result are exported, and a cosecant square beamforming metasurface model is established in the simulation software for simulation verification.

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