Panel phased-array antenna near-field calibration method

Abstract

The invention provides a near-field calibration method for a flat phased-array antenna. The method comprises the following steps: configuring normal radiation initial amplitude and phase by using a beam controller, measuring the amplitude and phase of a full-array near-field region under the condition that an array element is fully open, switching a near-field electric field to an aperture field based on a plane wave expansion method, and performing probe compensation. And for the compensated amplitude and phase data of the aperture field, extracting the edge of the aperture field and searching the aperture area based on an image processing mode by combining the actual size and position arrangement mode of the array element of the array area so as to quickly extract the amplitude and phase of each array element. The calibration feasibility is diagnosed according to aperture field data, different calibration strategies are adopted for a transmitting array and a receiving array, amplitude and phase calibration values of all frequency points are generated, the calibration values are loaded into a beam controller, and amplitude calibration, phase calibration and amplitude and phase iteration calibration are completed in sequence. The method provided by the invention is particularly suitable for a large-scale flat phased-array antenna, and high calibration precision is kept on the basis of shortening the calibration time and not obviously reducing the array performance.

Description

Technical Field

[0001] The present invention relates to the field of phased array antenna calibration. Specifically, it is a calibration method for large-scale planar phased arrays. Background Art

[0002] Phased array antennas are a key technology that uses phase control technology to achieve beamforming and beam scanning, and are widely used in fields such as radar, satellite communication, and electronic countermeasures. Compared with traditional mechanically scanned antennas, phased array antennas have the advantages of fast response speed, flexible and controllable beams, and no inertia, and can meet the high-precision detection and communication requirements in complex environments.

[0003] In a phased array antenna system, the beam direction and performance of the antenna depend on the amplitude and phase consistency of each element in the array. However, due to manufacturing errors, device characteristic differences, electromagnetic coupling between elements, and environmental factors, there may be deviations in the amplitude-phase characteristics of the elements. These errors will cause problems such as beam direction drift, increased sidelobes, and decreased gain, which will affect the detection or communication capabilities of the system. Therefore, amplitude-phase calibration of phased array antennas to improve amplitude-phase consistency is a key link to ensure their performance.

[0004] The calibration methods of phased array antennas are mainly divided into far-field calibration and near-field calibration. Far-field calibration usually requires a large test space, has high test costs, and is difficult to adjust the amplitude and phase of elements in real time. In contrast, near-field calibration methods can be carried out in a controlled environment, and the aperture field and far-field characteristics can be deduced through near-field-aperture field and near-field-far-field transformations, so they are widely used in the amplitude-phase error correction of phased array systems.

[0005] Common near-field calibration methods include single-channel calibration and rotation vector method calibration. Single-channel calibration uses a single receiving link to measure the responses of each element in turn. The advantage is that the method is intuitive and simple to implement, and is suitable for small-scale arrays. However, this method has a long test time in large-scale array systems and is difficult to effectively compensate for the coupling effect between elements, affecting the calibration accuracy. Rotation vector method calibration calculates the amplitude-phase error information of elements quickly by changing the phase relationship of the excitation signals, and can compensate for the electromagnetic coupling effect between elements to a certain extent. However, in large-scale arrays, the change in the measured data at the receiving end when changing the excitation of some elements is not obvious, and the amount of data surges, posing higher requirements for computing resources and the accuracy of calibration algorithms.

[0006] With the development of phased array antennas towards large-scale and high-precision directions, how to improve calibration efficiency, reduce calibration errors, and optimize the conversion accuracy from near-field to far-field has become an important research direction. Therefore, the development of efficient and accurate near-field calibration methods is crucial for improving the overall performance of phased array antenna systems. Summary of the Invention

[0007] The object of the present invention is to provide a near-field calibration method for a planar phased array antenna. The method of the present invention is particularly applicable to large-scale planar phased array antennas, and maintains a high calibration accuracy on the basis of shortening the calibration time and not significantly reducing the array performance.

[0008] To achieve the above object of the invention, the technical solution of the present invention is as follows:

[0009] The method is implemented based on an XYZP four-dimensional plane near-field test system.

[0010] In the method, when setting up the test system and the array, the array is in the coordinate system X’Y’Z’, and the test probe is in the XYZ coordinate system. Both the coordinate system X’Y’Z’ and the coordinate system XYZ are right-handed coordinate systems. The X’ axis of the array is in the same direction as the X axis of the test system, and the Y' axis and Z' axis of the array are in the opposite directions to the Y axis and Z axis of the test system, so that the coordinate systems of the test system and the array are opposite to each other, ensuring that the obtained amplitude and phase values of the array elements correspond one by one to the physical array elements. The distance between the probe and the array surface is set to d, and d takes 3-5λ max , λ max is the operating wavelength, to avoid the strong coupling effect between the probe and the array elements and ensure the accuracy of the measurement data. During the near-field test, for each near-field sampling point, data of N frequency points in the operating frequency band from f 1 to f n are measured. f 1 is the first measured frequency point, and f n is the Nth measured frequency point. In the near-field test system, a single-polarization open waveguide is often used as the test probe. During the test, the data of a certain polarization (such as X polarization) should be measured first, and then the data of the other polarization (Y polarization) is measured by rotating the P axis of the probe. The P axis is the polarization axis.

[0011] In the method, when configuring the initial amplitude and phase, the beam controller is used to adjust the array surface to the normal radiation mode, that is, all array elements radiate with the same amplitude and phase. Considering that the element layouts of different planar phased arrays are not the same, for different array layouts, phase compensation is performed: taking the mirror layout elements as an example, the phase shifters compensate 0° and 180° respectively according to the feed settings; taking the rotating layout elements as an example, the phase shifters compensate 0°, 90°, 180° and 270° respectively according to the rotating feed settings; ensuring that the amplitude and phase of the array elements are theoretically the same during radiation. This is used as a benchmark, which is not only convenient for subsequent observation of the amplitude-phase consistency of the aperture field and aperture extraction, but also relatively consistent with the initial amplitude and phase states of the array during actual operation.

[0012] In the method described above, one of the core steps in calibration during near-field to aperture-field transformation and probe compensation is to obtain the amplitude-phase distribution on the aperture field. The near-field to aperture-field transformation method based on the Plane Wave Spectrum (PWS) theory has been relatively mature. Its core idea is as follows: In a homogeneous medium, the phase wavefront is planar, the amplitude is constant, and the plane wave propagating in a single direction is a set of solutions to the Helmholtz equation. Based on the superposition principle, any complex field distribution can be regarded as the superposition of an infinite number of plane waves.

[0013] For the field distribution E(x, y, z 0 ) on a certain plane in space, its spatial Fourier transform is:

[0014]

[0015] Here, A(k x , k y ) represents the plane wave spectrum, that is, the contributions of different plane waves in the wavenumber space (k x , k y ). In free space, the wave vector k of any electromagnetic wave satisfies the dispersion relation:

[0016]

[0017] Therefore, the propagation component of the wave along the z direction can be expressed as:

[0018] The field at any position can be obtained through the inverse Fourier transform:

[0019]

[0020] By operating on the plane wave spectrum in the wavenumber space, the propagation problem in free space can be processed more conveniently, which is very beneficial in the near-field to aperture-field transformation.

[0021] The transformed aperture field data is affected by the near-field number, and the near-field data is affected by the test error. The influence brought by the test probe is the most direct, mainly reflected in: The probe is not an ideal probe. When interacting with the antenna during near-field measurement, the probe itself will affect the test result, and its output signal is the result of the convolution of the probe pattern and the antenna pattern; The frequency response of the probe is not uniform, and parameters such as the pattern and standing wave at different frequencies will change, introducing errors when measuring broadband signals.

[0022] When performing near-field testing, considering the probe scanning and sampling on the plane of z = d, the coupling equation between the probe and the antenna under test is established according to the Lorentz reciprocity theorem:

[0023]

[0024] P 0 is for receiving energy, and S(-k) is the plane wave spectrum of the probe. When is an imaginary number, the field decays exponentially. After attenuation of d≈3 - 5λ max , the energy on the scanning plane decays extremely little and can almost be ignored.

[0025] Considering performing Fourier transform on the above equation gives:

[0026]

[0027] where is in direct proportion to . is the probe pattern. Omitting the constant term, for a test system using an open waveguide as the probe, expand the far - field patterns of the probe measured twice into the wavenumber space:

[0028]

[0029] Relate the probe pattern and the plane wave spectrum according to the coordinate correspondence:

[0030]

[0031] Convert the spherical coordinate system to the rectangular coordinate system:

[0032]

[0033] where I 1 , I 2 are the plane wave spectra corresponding to the two polarization tests in the near - field respectively.

[0034]

[0035] For the E - plane and H - plane patterns of a standard rectangular open - waveguide probe, they can be approximated by the following equations:

[0036]

[0037] where Γ is the reflection coefficient of the probe port, β is the phase constant, and ɑ and b are the lengths of the long side and short side of the waveguide respectively.

[0038] During near - field measurement, if a total of M×N points are collected on the sampling plane, according to the Nyquist sampling theorem, the following relationship exists between the discrete interval in the spectral domain and the range of the sampling plane area:

[0039] Sampling space Wavenumber space

[0040] In the visible space of, there is

[0041] By solving the above formulas simultaneously, the plane wave spectrum A x,y (m,n) of the aperture field after probe compensation is obtained. Through inverse Fourier transform, the aperture field E x,y (m,n) can be solved.

[0042] In the above method, during the calibration feasibility judgment, it is necessary to preliminarily judge whether the array surface is normal based on the converted aperture field. The test system settings and the working state of the array are judged from aspects such as data integrity, noise level, number of damaged array elements, amplitude difference of the array surface, amplitude-phase distribution, etc., to determine the calibration feasibility and troubleshoot the reasons. If the data is incomplete, it is due to incorrect sampling settings and data loss, and it needs to be reset; if the noise level is too high, it is due to incorrect settings of the array or system gain or attenuation, and the input power of the array is too low; if the amplitude of an array element is 20 dB smaller than the expected threshold, it is determined as a damaged array element, and too many damaged array elements will not be calibrated; if the relative difference in polarization amplitude in some areas of the array surface exceeds the adjustment range of the attenuator and variable amplifier, calibration cannot be performed; if there is a regular distribution in the amplitude-phase of the aperture field, such as stripe distribution, checkerboard distribution, gradient distribution, it is suspected that the initial value setting of the beam controller is incorrect and the initial value needs to be readjusted.

[0043] When extracting the amplitude and phase of the array elements, the range of the solved aperture field is consistent with the range of the near-field scan. The actual array aperture is smaller than the scan range. Therefore, it is necessary to find out the area where the actual array aperture is located. The method of the present invention is based on the idea of image processing, and the two-dimensional aperture field data matrix after solution is interpolated to improve the image resolution. When the array elements radiate with equal amplitude and in-phase, the amplitude in the aperture plane is relatively uniform and there are no zeros (edges). The amplitude in the area where the physical aperture is located is much higher than that outside the aperture. The edge detection method of the Sobel operator is used to extract the aperture edge to determine the approximate area where the physical aperture is located. The Sobel operator is a gradient-based method that uses the gray-scale change of the image to detect edges. It determines the position and direction of the edges by calculating the gradients of the image in the horizontal direction (x direction) and the vertical direction (y direction). Its basic idea is to calculate the gradient magnitudes of each pixel point in the horizontal and vertical directions. To reduce the calculation amount, the gradient calculation is completed through a convolution kernel:

[0044] Horizontal convolution kernel Vertical convolution kernel

[0045] Image gradient component Edge intensity

[0046] In addition, a threshold needs to be specified for edge judgment. If the threshold is too small, a large amount of noise and weak edges will be detected, resulting in many unnecessary details in the result; if the threshold is too large, some real edges will be ignored, resulting in incomplete edge detection results. The threshold is specified using a statistical method:

[0047] th = μ + k·σ

[0048] μ is the mean of the gradient magnitude, σ is the standard deviation of the gradient magnitude, and k is a constant from 1 to 3. Set the size of the selection box based on the known array size information in the approximate area of the aperture. Theoretically, the closer the selection box is to the position of the physical aperture, the greater the received energy. Use the maximum area search algorithm of sliding window + prefix sum to search for the position with the maximum energy sum within the selection box, and use this position as the true physical aperture position. Sliding window + prefix sum is an algorithm suitable for processing continuous sub-intervals in large-scale data. Its core idea is to define a window on a two-dimensional plane and then slide the window on the matrix to efficiently calculate the local data sum. For the actual physical aperture area, it is essentially a continuous sub-region window with a fixed size. For the two-dimensional matrix E after edge extraction, with a size of p×q, the window size is a×b, and the sliding step is s. The sliding window starts from the upper left corner and slides from left to right and top to bottom on the matrix, moving s units each time.

[0049] If the upper left corner position of the window is marked as (i,j), then the window coverage area is: E[i:i+a,j:j+b].

[0050] Within the window coverage range, define the prefix sum matrix:

[0051] There is P[i][j] = P[i-1][j] + P[i][j-1] - P[i-1][j-1] + E[i][j], thus converting the two-dimensional summation problem into a one-dimensional addition and subtraction problem. Use the prefix sum matrix to calculate the sum between the matrix regions (x 1 ,y 1 ) to (x 2 ,y 2 ):

[0052] S(x 1 ,y 1 ,x 2 ,y 2 ) = P[x 2 [y 2 - P[x 1 - 1][y 2 - P[x 2 [y 1 - 1] + P[x 1 - 1][y 1 - 1]

[0053] During the calculation process, continuously record the sub-regions and positions, and use this position as the true physical aperture position. The computational complexity of the prefix sum matrix is O(p×q), the traversal complexity of the sliding window is O((p - a + 1)×(q - b + 1)), and the overall complexity is O(p×q), which is suitable for large matrices. After extracting the physical aperture position, according to the known array element arrangement, divide the physical aperture into regions where each array element is located, and use the amplitude and phase means within this region as the amplitude and phase of the array element.

[0054] Generate calibration values based on the obtained amplitude and phase values of the array elements. For large-scale arrays, the amplitude and phase differences between different array elements may be large. The adjustment of the array element amplitude can only be achieved by reducing the channel gain or increasing the attenuator value, that is, the gain can only be adjusted downward and cannot be increased further. If the array amplitudes are fully calibrated to be consistent, then only the gain of other array elements can be reduced based on the array element with the minimum amplitude, which results in an unacceptable gain loss for the entire array. Phase adjustment has a periodicity of 0 to 360°, and theoretically can be adjusted arbitrarily. Since there is additional attenuation during the phase shifter adjustment, which will cause gain changes, large-scale phase changes should be minimized as much as possible.

[0055] For a transmitting array, its equivalent isotropic radiated power (EIRP) is a very important indicator, which is greatly affected by the array gain. The array gain should not be reduced unless necessary; for a receiving array, the figure of merit (G / T) is relatively important, which is affected by the gain and the system noise temperature. The system noise temperature should not be increased unless necessary. Therefore, the method described in the present invention designs the calibration strategies for the transmitting and receiving arrays as follows:

[0056]

[0057] In addition, for the case where there are obvious differences in the polarization performance of different regions of the array, the global mean cannot be simply calculated. Instead, the polarization means should be calculated region by region, and amplitude and phase compensation should be performed separately to make the global mean consistent, so as to ensure the performance consistency after calibration.

[0058] Theoretically, the amplitude and phase characteristics of the array elements are independent of each other and do not affect each other. However, due to the additional attenuation in the control of the phase shifter in the channel, and the additional phase shift in the control of the attenuator and variable gain amplifier, when a certain parameter is modified alone, the other parameter will change; there is mutual coupling between the array elements. Changing the amplitude and phase characteristics of some array elements will affect the amplitude and phase values radiated by its adjacent array elements. As a result, when calibrating the array amplitude and phase at one time, after the amplitude and phase calibration values are written, the operating characteristics of the array elements change greatly, and the calibration result is far from the expectation. Therefore, the calibration is divided into three steps in the method:

[0059]

[0060] Calibrate the amplitude and phase independently to reduce the errors caused by additional phase shift and attenuation, and adopt iterative calibration to reduce the errors caused by element coupling.

[0061] The innovation of the present invention lies in:

[0062] 1) Based on the image processing method, determine the aperture position from the solved aperture field range, extract the aperture edge to narrow the search area, which improves the processing efficiency and ensures the consistency of each selection result during large-scale calibration.

[0063] 2) Compared with reducing the gain of other elements based on the element with the minimum amplitude, if the array amplitude is completely calibrated to be consistent, only the gain of other elements can be reduced based on the element with the minimum amplitude. The method described in the present invention adopts different calibration strategies for the transmitting array and the receiving array, reduces the influence of calibration on key performance indicators, compensates for regional and polarization differences, and improves the consistency of array surface parameters.

[0064] 3) Considering that when modifying only one of the amplitude or phase parameters, it will have an impact on the other parameter. Calibrate the amplitude and phase separately in sequence, and calibrate the additional attenuation existing in the control of the phase shifter in the channel and the additional phase shift existing in the control of the attenuator or variable gain amplifier during the step-by-step calibration process to reduce the errors caused by additional phase shift and attenuation.

[0065] 4) Considering the change in the amplitude-phase characteristics of some elements caused by element coupling, adopt the iterative calibration method. Test the array after amplitude-phase calibration to generate calibration values and write the calibration values again to reduce the errors caused by element coupling.

[0066] Compared with the existing calibration scheme, the significant advantages of the present invention are:

[0067] 1) Adopt the near-field to aperture field scheme; scan the near field once to extract the amplitude-phase of the aperture of the phased array to be measured at each frequency point, without the need for channel-by-channel, regional testing or frequent adjustment of the initial amplitude-phase, which greatly shortens the measurement time itself.

[0068] 2) The traditional method uses visual inspection to find the array range from the solved aperture field range. The error at the position of the array elements in the array is small, but at the edge position, due to the relatively drastic change of the aperture field, the amplitude-phase error of the edge array elements is large. Moreover, manual operation will significantly increase the calibration time. The accuracy of array aperture selection depends on the operator's experience, and it is impossible to guarantee the consistency of each selection result during mass calibration. The method of the present invention is based on the idea of image processing, and interpolates the solved two-dimensional aperture field data matrix to improve the image resolution. When the array elements radiate with equal amplitude and in-phase, the amplitude in the aperture plane is relatively uniform and there are no zeros (edges). The amplitude in the region where the physical aperture is located is much higher than that outside the aperture. By using the edge detection method of the Sobel operator to extract the aperture edge, the approximate region where the physical aperture is located can be determined, reducing the data volume and shortening the processing time. The region maximum sum method is used to search to improve the accuracy of aperture position search;

[0069] 3) Different calibration strategies are adopted for the transmitting array and the receiving array to reduce the overall gain loss of the array and the gain change caused by the additional attenuation existing during the adjustment of the phase shifter, reduce the impact of calibration on key performance indicators, compensate for regional and polarization differences, and make the global mean consistent to ensure the performance consistency after calibration;

[0070] 4) Three-step calibration is adopted to reduce the influence of additional attenuation, phase shift and element coupling, and improve the calibration accuracy.

[0071] The following further describes the present invention in detail with reference to the accompanying drawings. Description of the Drawings

[0072] Figure 1 It is a schematic diagram of the calibration process of the method of the present invention;

[0073] Figure 2 It is a schematic diagram of the near-field test setup, describing the relative relationship between the array and the probe coordinate system;

[0074] Figure 3 It is the near-field initial amplitude and phase images at the center frequency point collected in the near field. p1 is horizontally polarized and p2 is vertically polarized;

[0075] Figure 4 It is the initial aperture field amplitude-phase after near-field aperture field transformation and probe compensation, including a relatively clear array aperture. There are obvious differences in the left and right regions of the array surface to be measured, and the amplitude of the right half array surface is significantly lower than that of the left half array elements;

[0076] Figure 5 It is the result after edge detection and aperture region search. The white dotted line is the detected edge region, and the black dotted line is the searched aperture region;

[0077] Figure 6For the amplitude-phase of the aperture field extracted after amplitude calibration based on the emission calibration strategy, the amplitude of the aperture field is relatively flat after amplitude calibration, and the phase of the aperture plane also changes to a certain extent;

[0078] Figure 7 For the amplitude-phase of the aperture field extracted after phase calibration, the phase plane of the aperture field is flatter after phase calibration, and the aperture amplitude changes slightly;

[0079] Figure 8 For the effect after iterative calibration, both the amplitude and phase planes are relatively flat;

[0080] Figure 9 For the amplitude comparison results of different polarizations of the aperture field in the horizontal section, the amplitude difference between the left and right of the initial array surface is relatively large, and as the calibration steps proceed, the amplitude difference gradually decreases;

[0081] Figure 10 For the phase comparison results of different polarizations of the aperture field in the horizontal section, the phase fluctuation of the initial array surface is relatively large, and as the calibration steps proceed, the phase difference gradually decreases;

[0082] Figure 11 For the changing trend of the standard deviation of the amplitude and phase of different polarizations of the entire 1024-element aperture field with calibration, the standard deviation of the aperture amplitude is less than 1 dB and the standard deviation of the phase is less than 6° after calibration. Specific implementation mode

[0083] The following uses specific specific examples to illustrate the implementation mode of the present invention. In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings. The present invention can also be implemented or applied through other different specific implementation modes, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0084] Figure 1 This is a flowchart of a near-field calibration method for a planar phased array antenna proposed by the present invention. Based on this flowchart, an embodiment is given. This embodiment can perform amplitude-phase calibration on a large-scale phased array.

[0085] In this embodiment, an XYZP four-dimensional planar near-field test system is adopted, which includes a three-dimensional scanning robotic arm, a vector network analyzer, a single-polarization open waveguide probe (WR28 standard waveguide horn with a size of 7.112×3.556 mm and a working frequency band of 26.25 - 40 GHz), and a data acquisition module. A planar transmitting phased array with a working frequency band of 27.5 - 31 GHz is calibrated. This array is equipped with a beam controller to control the amplitude and phase values of each channel. The array surface is a square with a size of 161 mm×161 mm. The array elements are arranged in a rectangular pattern and use rotational feeding to radiate circular polarization, with a total of 32×32 = 1024 elements.

[0086] Fix the flat-panel phased array antenna to be measured within the XYZ coordinate system of the test system, ensuring that the line where the X’Y’ axis of the array is located is strictly parallel to the line where the XY axis of the test system is located, and the line where the Z’ axis is located is parallel to the line where the Z axis of the test system is located. Adjust the position of the array through a laser calibrator, and control the alignment error within ±0.1 mm.

[0087] The distance d between the probe and the array surface is approximately 4λ max (λ max corresponding to the lowest frequency of 27.5 GHz) to suppress the electromagnetic coupling between the probe and the elements; fix the probe in the X polarization (P1) direction and perform point-by-point scanning along the XY plane. Measure the data of 15 frequency points within the working frequency band f 1 ~f 15 for each point. The sampling intervals Δx and Δy are slightly less than λ min / 2 (λ min corresponding to the highest frequency of 31 GHz), and collect M×N = 68×68 near-field data points. After the collection is completed, rotate the probe to the Y polarization (P2) direction and repeat the scanning and data collection.

[0088] Through the beam controller, set the amplitude of all elements to the same maximum value, and set the phase to 0°, 90°, 180°, and 270° respectively according to rotational feeding, so that the radiation amplitude and phase of the array surface are theoretically consistent, and the radiation direction is perpendicular to the array surface.

[0089] Figure 3 is the near-field initial amplitude and phase image at the center frequency point collected in the near field, where Figure (a) is the horizontal polarization amplitude image, Figure (b) is the vertical polarization amplitude image, Figure (c) is the horizontal polarization phase image, and Figure (d) is the vertical polarization phase image.

[0090] Convert the collected electric field amplitude to a linear value: Amp = 10 ^ (Amp dB / 20), and convert the phase to radian system: Synthesize the near-field electric field complex vector: E nf = Amp·e j·Pha .

[0091] Define the wavenumber space:

[0092] Define the propagation function as Modify k in the invisible space Z To be 0, to avoid the amplified evanescent wave by the propagation function, perform a two-dimensional Fourier transform on the electric field vector to convert it to the spectral domain, multiply it by the propagation function, and convert it to the aperture field. Solve the far-field pattern of the probe through an approximate equation based on the actually measured reflection coefficient curve of the WR28 waveguide horn port, and solve the intermediate process factor:

[0093]

[0094] For this embodiment, it operates in the circular polarization mode and is decomposed into two linear polarization components during calibration, corresponding to the two measured linear polarization data. For the plane wave spectrum A x Adopt M 11 , M 21 Compensation calculation, A y Adopt M 22 , M 12 Compensation calculation to obtain two single-polarization plane wave spectra. Perform an inverse two-dimensional Fourier transform to synthesize the actual electric field vector, and obtain the complex electric field of the aperture field after probe compensation. Interpolate the calculated (M×N) aperture field data to improve the image resolution, and the dimension of the interpolated data is (Mi×Ni = 301×301).

[0095] Figure 4 Is the initial aperture field amplitude image after near-field aperture field transformation and probe compensation, where Figure (a) is the horizontal polarization amplitude image, Figure (b) is the vertical polarization amplitude image, Figure (c) is the horizontal polarization phase image, and Figure (d) is the vertical polarization phase image.

[0096] The phased array to be measured operates in the circular polarization mode. When performing image processing, the main polarization mode should be used as the standard. Process the complex electric field data of the aperture field into amplitude data in logarithmic scale, normalize the data so that the maximum value is 0 dB, and modify the value of the data less than -60 dB to -60 dB to avoid the influence of the minimum value outside the aperture area on edge extraction. Use the Sobel operator to calculate the gradient values G x , G y in the XY direction of the amplitude image, calculate the edge intensity G(x,y), and calculate the mean value of the edge intensity based on statistical methods Standard deviation of edge intensity Calculate the threshold th = μ + k·σ, where k is taken as 1. Binarize the image according to the threshold, and extract the area where the edge matrix is located EDGE[x min , y min , x max , y max, with dimensions (p×q).

[0097] Its pseudocode is as follows:

[0098]

[0099] According to the actual size of the array to be measured, combined with the number of interpolation points, determine the size of the array selection box (a×b), ensuring that p≥a and q≥b. If the requirements are not met, increase the values of p and q. Use the sliding window + prefix sum method to search for the area where the aperture field is located. The size of the sliding window is fixed at (a×b), and the search area is the edge matrix EDGE[x min ,y min ,x max ,y max . Construct the prefix sum matrix Step the sliding window forward in sequence, record the maximum value of the prefix sum. If the maximum value increases, update the window position; otherwise, keep the window.

[0100] Its pseudocode is as follows:

[0101]

[0102]

[0103] In this embodiment, the energy of the finally searched aperture field area accounts for more than 99% of the total energy of the entire aperture surface. It can be considered that the searched area is the physical aperture.

[0104] Figure 5 Is the result of edge detection and aperture area search. The white dashed line is the detected edge area, and the black dashed line is the searched aperture area.

[0105] For the phased array to be measured, divide it into 32×32 rectangular grids according to its physical size and element arrangement. Based on this grid, divide the aperture into 32×32 sub-regions, and use the amplitude and phase mean values within each sub-region as the actual element amplitude and phase.

[0106] This array to be measured is a transmitting array. Adopt the calibration strategy for the transmitting array. Set the element damage determination threshold to -20dB. Mark the elements higher than this threshold as normal elements, and calculate the amplitude and phase mean values of the normal elements as the calibration reference. For normal elements, the amplitude calibration value of elements with amplitudes higher than the mean is the element amplitude minus the amplitude mean, and the amplitude calibration value of elements with amplitudes lower than the mean is 0; the phase calibration value is the element phase minus the phase mean. For damaged elements, the amplitude calibration value is 0, and the phase calibration value is the element phase minus the phase mean.

[0107] In this embodiment, there are significant differences between the left and right front surfaces of the array to be measured. The average values of the amplitudes and phases of the left and right half array elements are calculated respectively, with the right half as the reference, and the average difference is compensated for the other half of the array elements. There are certain differences between the X and Y polarizations of the array to be measured. The average values of the amplitudes and phases of the X and Y polarization array elements are calculated respectively, with the X polarization as the reference, and the average difference is compensated for the Y polarization array elements. First, the amplitude of the front surface of the array to be measured is calibrated, the amplitude calibration value is written, the phase value remains unchanged, and the amplitude and phase of the aperture field are measured and extracted. Then, based on the amplitude calibration, phase calibration is performed. According to the measured amplitude and phase of the aperture field, the calibration value is extracted and the phase calibration value is written, while the amplitude value remains unchanged. The amplitude and phase of the aperture field are measured and extracted again. Then, the calibration value is generated based on the extracted amplitude and phase of the aperture field, and this calibration value is superimposed with the amplitude calibration and phase calibration values to generate an iterative calibration value, which is written into the phased array for testing to verify the final result.

[0108] Figure 6 For the amplitude and phase images of the aperture field extracted after amplitude calibration based on the emission calibration strategy, where Figure (a) is the horizontal polarization amplitude image, Figure (b) is the vertical polarization amplitude image, Figure (c) is the horizontal polarization phase image, and Figure (d) is the vertical polarization phase image. After amplitude calibration, the amplitude of the aperture field is relatively flat, and the phase of the aperture surface also changes to a certain extent.

[0109] Figure 7 For the amplitude and phase of the aperture field extracted after phase calibration, where Figure (a) is the horizontal polarization amplitude image, Figure (b) is the vertical polarization amplitude image, Figure (c) is the horizontal polarization phase image, and Figure (d) is the vertical polarization phase image. After phase calibration, the phase surface of the aperture field is flatter, and there are slight changes in the aperture amplitude.

[0110] Figure 8 For the effect after iterative calibration, where Figure (a) is the horizontal polarization amplitude image, Figure (b) is the vertical polarization amplitude image, Figure (c) is the horizontal polarization phase image, and Figure (d) is the vertical polarization phase image. Both the amplitude and phase surfaces are relatively flat.

[0111] Figure 9 For the amplitude comparison results of different polarizations of the aperture field on the horizontal section, where Figure (a) is the amplitude image of the horizontal polarization on the horizontal section and Figure (b) is the amplitude image of the vertical polarization on the horizontal section. There are significant differences in the amplitudes between the left and right of the initial front surface, and as the calibration steps progress, the amplitude differences gradually decrease.

[0112] Figure 10 For the phase comparison results of different polarizations of the aperture field on the horizontal section, where Figure (a) is the phase image of the horizontal polarization on the horizontal section and Figure (b) is the phase image of the vertical polarization on the horizontal section. The phase fluctuations of the initial front surface are large, and as the calibration steps progress, the phase differences gradually decrease.

[0113] Figure 11The standard deviation of the different polarization amplitudes and phases of the entire 1024-element aperture field as a function of calibration progress, where Figure (a) is the image of the amplitude standard deviation and Figure (b) is the image of the phase standard deviation. After calibration, the aperture amplitude standard deviation is less than 1 dB and the phase standard deviation is less than 6°.

[0114] The above embodiments are only illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A near-field calibration method for a flat-panel phased array antenna comprises the following steps: (1) Test system setup: An XYZP four-dimensional planar near-field test system is used. When setting up the test system and the flat-panel array to be calibrated, the array is in the coordinate system X'Y'Z', and the test probe is in the XYZ coordinate system. Both the coordinate system X'Y'Z' and the coordinate system XYZ are right-handed coordinate systems. The X' axis of the array is in the same direction as the X axis of the test system, and the Y' axis and Z' axis of the array are in the opposite direction to the Y axis and Z axis of the test system, so that the coordinate systems of the test system and the flat-panel array to be calibrated are opposite. The single-polarized open waveguide probe is 3 to 5 times the maximum working wavelength away from the array. The near field is scanned at multiple frequencies by the single-polarized open waveguide probe to collect near-field data under X polarization and Y polarization respectively. (2) Initial amplitude and phase configuration: All array elements are set to the normal radiation mode through the beam controller, and the initial phase compensation of the phase shifter is performed according to the array layout; (3) Near field-aperture field transformation and probe compensation: Based on the plane wave spectrum theory, the near field data is Fourier transformed to obtain the plane wave spectrum. Combined with the probe pattern compensation model, the probe coupling error is eliminated and the aperture field distribution after probe compensation is calculated. (4) Calibration feasibility judgment: Based on the converted aperture field data, the array working status is evaluated through data integrity, noise level, number of array element damage, amplitude difference and phase distribution, and the calibration feasibility is preliminarily judged; (5) Extracting the amplitude and phase of array elements: The aperture field data is interpolated to enhance the resolution, the Sobel operator is used for edge detection, and the sliding window + prefix sum algorithm is used to search for the maximum energy area to determine the physical aperture position. The array element area is divided according to the array element arrangement, and the amplitude and phase mean of each array element are extracted. (6) Calibration strategy: The gain-preserving and noise-preserving calibration strategies are used for the transmitting array and the receiving array, respectively, and the amplitude and phase are compensated by partition or polarization. (7) Calibration steps: Through the three steps of amplitude calibration, phase calibration and iterative calibration, the array parameters are adjusted in sequence to reduce the additional errors of the phase shifter and attenuator and the coupling effect between array elements.

2. A near-field calibration method for a flat-panel phased array antenna according to claim 1, characterized in that: The method supports multi-frequency point calibration, completes full-band data acquisition through one near-field scan, and performs near-field-aperture field transformation and calibration on each frequency point respectively.

3. The near-field calibration method for a flat-panel phased array antenna according to claim 2, characterized in that: In (3), the probe pattern compensation model establishes the coupling equation between the probe and the antenna to be measured through the Lorentz reciprocity theorem, and combines the E-plane and H-plane patterns of the probe to convert the plane wave spectra of the two polarization measurements into aperture field components in a rectangular coordinate system.

4. The near-field calibration method for a flat-panel phased array antenna according to claim 3, characterized in that: In (5), the threshold of edge detection is determined by a statistical method, specifically a linear combination of the mean and standard deviation of the gradient amplitude: th=μ+k·σ Among them, μ is the mean value of the gradient amplitude, σ is the standard deviation, and k is a constant of 1 to 3. The size of the selection box is set in the approximate area of ​​the aperture based on the known array size information. In theory, the closer the selection box is to the location of the physical aperture, the greater the energy received. The maximum area search algorithm of sliding window + prefix sum is used to search for the position that maximizes the energy sum in the selection box, and this position is taken as the actual physical aperture position.

5. The near-field calibration method for a flat-panel phased array antenna according to claim 4, characterized in that: In (5), the sliding window + prefix sum algorithm is an algorithm suitable for processing continuous sub-intervals in large-size data, and is used to efficiently search for the maximum energy sub-region of a fixed size in two-dimensional data. Its complexity is o(p×q), where p and q are the dimensions of the edge matrix.

6. A near-field calibration method for a flat-panel phased array antenna according to claim 5, characterized in that: In (5), the method of dividing the array element region according to the array element arrangement is not only applicable to rectangular array elements, but also to triangular array elements and diamond array elements.

7. A near-field calibration method for a flat-panel phased array antenna according to claim 6, characterized in that: (6), the transmit array calibration strategy is: Normal array element channel: Take the average of the amplitude and phase of the normal channel, and attenuate the amplitude of the array element with an amplitude higher than the average to the average, and calibrate the phase to the average; for the array element with an amplitude lower than the average, do not calibrate the amplitude, and calibrate the phase to the average; Damaged array element channel: The amplitude is not calibrated, and the phase is calibrated to the average value of the normal channel. The receiving array calibration strategy is: Normal array element channel: Take the average of the amplitude and phase of the normal channel, and attenuate the amplitude of the array element with an amplitude higher than the average to the average, and calibrate the phase to the average; for the array element with an amplitude lower than the average, do not calibrate the amplitude, and calibrate the phase to the average; Damaged array element channel: closed. In addition, when there are obvious differences in polarization performance in different areas of the array, the global mean cannot be simply calculated. Instead, the mean should be calculated by region and polarization, and the calibration values ​​should be compensated for the regional and polarization differences respectively to make the global mean consistent to ensure performance consistency after calibration.

8. The near-field calibration method for a flat-panel phased array antenna according to claim 7, characterized in that: (7), the calibration step comprises: Step 1: Amplitude calibration. Based on the calibration value generated in the first test, only the amplitude calibration value is written, and the phase calibration value remains unchanged; Step 2: Phase calibration. Test the array after amplitude calibration, generate calibration values, and calibrate again, writing only the phase calibration value, leaving the amplitude calibration value unchanged; Step 3: Iterative calibration. Test the array after amplitude and phase calibration to generate calibration values. Perform iterative calibration once, write both amplitude and phase calibration values, and make final iterative adjustments.

9. A near-field calibration method for a flat-panel phased array antenna according to claim 8, characterized in that: After calibration of the 1024-scale array, the aperture amplitude standard deviation is less than 1 dB, and the phase standard deviation is less than 6°.

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