Plane near-field rapid measurement method for large-aperture antenna

Abstract

The invention provides a plane near-field rapid measurement method for a large-aperture antenna, and relates to the technical field of antennas. According to the method, sparse field amplitude and phase information of a plurality of sampling surfaces of a radiation near-field region of an antenna to be measured is obtained through large-step uniform sampling; meanwhile, sampling finite strip accurate field amplitude phase information for each sampling surface, and calculating the fluctuation degree of the amplitude of a sampling point to obtain field distribution characteristics; based on sparse field amplitude and phase information and field distribution characteristics of the sampling surfaces, obtaining an initial electric field of each sampling surface by using a partition interpolation and field distribution calibration method; and performing numerical iteration on the initial electric field based on a plane spectrum theory, recovering near-field distribution of each sampling surface, and finally obtaining far-field radiation characteristics of the antenna through near-far field transformation. According to the method disclosed by the invention, the key indexes in the far-field characteristics of the antenna can be obtained while the test time is greatly shortened, wherein the key indexes comprise a main lobe and a first side lobe of a normalized directional diagram, 3dB beam width, side lobe level, axial ratio and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of antennas, and particularly to a method for rapid planar near-field measurement of large-aperture antennas. Background Technique

[0002] In recent years, with the rapid development of fields such as deep space exploration, satellite communication, and radar remote sensing, the application demand for large-aperture antennas has been increasing day by day. Its characteristics such as high gain, narrow beam, and high resolution make it have irreplaceable advantages in long-distance communication and high-precision imaging. The performance of the antenna directly determines the overall efficiency of the system, so antenna testing has become a key link. Existing testing methods are mainly divided into two categories: one is the direct method, including the far-field method, the focusing method, and the compact range method; the other is the indirect method, that is, near-field measurement. Compared with the direct method, near-field measurement has the advantages of small space requirements, high precision, strong environmental adaptability, and high testing efficiency, and has become the mainstream technology. According to the form of the measurement surface, near-field measurement can be divided into planar, cylindrical, and spherical measurements. Among them, planar measurement is the most widely used because of its simple structure, high precision, wide applicability, and low cost.

[0003] The traditional planar near-field measurement method obtains the amplitude-phase information of the electric field in the near-field region according to the Nyquist sampling theorem, and obtains the far-field characteristics through near-field to far-field transformation. This method requires the sampling interval not to exceed half a wavelength, and as the operating frequency of the antenna increases, the number of sampling points surges, resulting in a significant increase in the testing time; long-term testing is likely to cause amplitude-phase fluctuations in the output field of the transmitting system, affecting the measurement accuracy.

[0004] Currently, the following two non-phase measurement methods are mainly used for planar near-field measurement of millimeter-wave antennas:

[0005] The first method is to recover the near-field based on the iterative Fourier transform algorithm. For example, in the application with the application number 201910632593.6, a non-phase near-field antenna measurement method based on the iterative Fourier transform algorithm is disclosed. This method constructs an initial field with random phases and test amplitudes, and optimizes to obtain the phase information by iterating between two planes and applying amplitude constraints; however, the iterative process is prone to falling into a local optimum. For this reason, in the application with the application number 202310171476.0, a non-phase planar near-field measurement method based on the conjugate gradient method is disclosed. By introducing the conjugate gradient method, a global cost function is constructed to optimize the iterative direction and enhance the global convergence; however, this method still needs to meet the Nyquist sampling condition, which requires a huge time cost, and the randomly generated initial phase will make the amount of data to be optimized for large-aperture antennas quite large, occupying a large amount of computing resources while reducing the testing efficiency.

[0006] The second method optimizes the initial phase by using the uniqueness principle and then combines the iterative Fourier algorithm to restore the near field [Wang Yuan. Research on Non-phase Near-field Antenna Measurement Based on Interpolation Algorithm and Source Reconstruction Method [D]. Shaanxi: Xidian University, 2018.]; this method constructs a discrete equivalent surface current source and combines the particle swarm optimization algorithm to reconstruct a current distribution similar to the actual field distribution on the antenna aperture surface to obtain the initial phase. However, the optimization problem is still non-convex and prone to falling into local optima; in addition, there are errors in approximating the continuous field distribution with discrete current sources, and the number of current sources will also affect the optimization effect. Summary of the Invention

[0007] Aiming at the problems existing in the prior art, such as the cumbersome measurement process of large-aperture array antennas, high time cost, and the phase optimization algorithm in traditional non-phase testing being prone to falling into local optima, the present invention proposes a method for rapid planar near-field measurement of large-aperture antennas. This method can obtain key indicators in the far-field characteristics of the antenna while significantly shortening the test time, including the main lobe and the first side lobe of the normalized radiation pattern, 3dB beam width, side lobe level, axial ratio, etc.

[0008] The technical solution adopted by the present invention is as follows:

[0009] A method for rapid planar near-field measurement of large-aperture antennas, characterized in that sparse field amplitude-phase information of several sampling surfaces in the radiation near-field region of the antenna to be measured is obtained through large-step uniform sampling; at the same time, a limited number of accurate field amplitude-phase information is sampled on each sampling surface, and the field distribution characteristics are obtained by calculating the fluctuation degree of the sampling point amplitudes; based on the sparse field amplitude-phase information and field distribution characteristics of the sampling surface, the initial electric field of each sampling surface is obtained by using the partition interpolation and field distribution calibration methods; the near-field distribution of each sampling surface is restored by numerical iteration based on the plane wave spectrum theory, and finally the far-field radiation characteristics of the antenna are obtained through the near-field to far-field transformation.

[0010] Specifically, it includes the following steps:

[0011] Step 1. In the near-field region of the antenna to be measured, a number of surface data and a number of line data are uniformly sampled.

[0012] Step 1.1. At several planes at different distances from the aperture surface, the amplitude and phase data of the near field are uniformly sampled with a large step size.

[0013] Specifically, by increasing the proportion of the central effective field region in the total sampling surface, the sparse sampling data can provide more effective field data and less distortion, and a better iterative initial value can be obtained by using the interpolation strategy of the present invention. When the proportion of the central effective field region in the total sampling surface is greater than 70%, the sampling step size is selected as 5λ or more; when the proportion of the central effective field region in the total sampling surface is less than 70%, the sampling step size is selected as 2 - 5λ.

[0014] The continuity of the effective field distribution, the degree of data distortion, and the number of sampling points will all affect the recovery effect. If the effective field distribution is continuous and the data distortion is small, two sampling planes are selected; otherwise, the sampling step size is reduced and the number of sampling planes is increased;

[0015] To avoid spectral aliasing, the distance between adjacent sampling planes is greater than 1λ;

[0016] Step 1.2. According to the Nyquist sampling theorem, a number of line data passing through the antenna elements are uniformly sampled in the x and y directions on each sampling plane respectively;

[0017] The field distribution characteristics of the sampling plane are obtained by analyzing the sampled line data; if the central effective field is continuous and the amplitude change is less than 5 dB, 3 - 5 symmetric line data are sampled in the x and y directions respectively; otherwise, the number of sampled line data is increased to obtain the global characteristics of the electric field;

[0018] Obtaining multiple sampling planes is beneficial to improving the recovery accuracy, but it will increase the sampling time. Considering the recovery efficiency and accuracy, usually two planes of the electric field are sparsely sampled, and 4 symmetric line data in the x and y directions are accurately sampled;

[0019] Step 2. By calculating the fluctuation degree of the amplitudes of adjacent sampling points from the line data sampled in Step 1.2, the range of the effective field is determined, and the framed effective field range will be used as a reference to correct the propagated field;

[0020] Step 3. Different interpolation strategies are selected according to the field distribution characteristics of different regions to obtain the field data that meet the plane wave spectrum propagation conditions;

[0021] Step 3.1. Bilinear interpolation is used for the sampling data within the effective field range of each sampling plane. After interpolation, the interval meets the same sampling step size as the line data, and the plane range covered by the data remains unchanged;

[0022] Step 3.2. In the cut-off region outside the central effective field, the field within the rectangular ring region surrounding the central field changes continuously, while there is a jump in the radial phase information. Therefore, in order to make the field within the ring region smooth and continuous, the line sampling data are first expanded into column vectors according to their positions within each rectangular ring, and then cubic spline interpolation is used to correct the data with missing phase characteristics caused by sparse sampling; the data outside the rectangular ring are supplemented by non-uniform interpolation;

[0023] Step 4. For the electric field data of each sampling plane obtained in Step 3.2, the range of the effective field is calculated every ten line data in the x and y directions according to the method shown in Step 2, and the correction is made with reference to the effective field range; non-uniform interpolation is used to supplement the error data with field offset caused by the interpolation method; the test data are replaced at the corresponding positions, and at this time, the initial electric field to be recovered for each sampling plane is obtained;

[0024] Step 5. Based on the plane wave spectrum theory of fast Fourier transform, perform iterative operations on the initial electric fields of each sampling plane to recover the electric fields of each sampling plane:

[0025] Step 5.1. Set the sampling plane closer to the antenna aperture plane among two adjacent sampling planes as the first sampling plane, and the other sampling plane as the second sampling plane; propagate the initial electric field Ep1 of the first sampling plane to obtain the iterative electric field Em2_1 of the second sampling plane;

[0026] Step 5.2. Calculate the range of the central effective field for the iterative electric field Em2_1 according to the method shown in Step 4, and correct the error data; use the test data of the second sampling plane in Step 1 to replace at the corresponding position; at this time, the electric field Em2 of the second sampling plane after iterative correction is obtained;

[0027] Step 5.3. Propagate the corrected electric field Em2 of the second sampling plane to obtain the iterative electric field Em1_1 of the first sampling plane;

[0028] Step 5.4. Calculate the range of the central effective field for the iterative electric field Em1_1 according to the method shown in Step 4, correct the error data; use the test data of the first sampling plane in Step 1 to replace at the corresponding position; then, continue to propagate to the second sampling plane for iteration.

[0029] Step 5.5. The two propagation processes in Steps 5.1 - 5.4 are regarded as one cycle, and the difference ΔFX between the vector correlation coefficients of the iterative electric fields of the first sampling plane and the initial electric field in two cycles is used as the judgment condition for whether to terminate the iterative operation:

[0030] If the number of iterations reaches the set maximum number of iterations Q or the difference ΔFX between the vector correlation coefficients is less than 1%, then jump out of the loop and output the electric fields of the two sampling planes in the last iteration, which are the final recovered electric fields;

[0031] Otherwise, continue to iterate until the termination condition is met.

[0032] Step 6. Use the plane near - far field transformation to obtain the far - field radiation characteristics of the antenna under test from the recovered electric field of the first sampling plane.

[0033] The working principle and test time of the present invention:

[0034] This method avoids falling into the local optimum problem: by sampling with a large step size, the test time is significantly shortened, and the influence of the output field fluctuation caused by the long-time test of the emission system is reduced. Numerical iteration is carried out based on the test field data, avoiding the problem of relying on random initial phase optimization in traditional phase-free testing. The sparse multi-plane - exact multi-line sampling strategy is adopted: combining large-step plane sampling and line sampling under the Nyquist criterion, effectively reducing the number of sampling points while retaining the key field distribution characteristics. The method for identifying and calibrating the central effective field region is introduced: the effective region is automatically extracted by analyzing the line data fluctuation, which is used for interpolation region division and calibration of the propagated field, improving the adaptability of the algorithm. Different interpolation strategies are adopted for the field characteristics of different regions: enhancing the reduction accuracy of sparse sampling data and reducing the interpolation error.

[0035] Compared with the sampling time of the traditional sampling plane near-field test with a 0.5λ step size, according to the method shown in the present invention, when sparsely sampling the plane data with a 5λ step size, it takes about 1% of the time, and then precisely sampling the limited number of line data with a 0.5λ step size can complete the sampling of the electric field of a single sampling plane, thus significantly shortening the overall test duration.

[0036] The present invention has the following advantages compared with the current phase-free sampling plane near-field test method:

[0037] First, the present invention overcomes the problems of relying on the initial value and being prone to falling into the local optimum in the phase-free planar near-field test. By using large-step uniform sampling, the amplitude-phase data of the electric field of several sampling planes are quickly obtained, significantly shortening the test time, reducing the influence of the output field fluctuation caused by the long-time test of the emission system, and improving the measurement accuracy of the electric field of the sampling plane. By analyzing the distribution characteristics of the field from the limited number of precise data, combined with methods such as partition interpolation and field distribution calibration, a relatively accurate iterative initial value can be provided, avoiding falling into the local optimum and saving a large amount of computing resources.

[0038] Second, the present invention can achieve fast and efficient measurement of large-aperture antennas. Based on the field distribution characteristics analyzed from the precise sampling line data, different interpolation strategies are adopted for different regions, and a relatively reliable iterative initial value can be obtained. And in each iteration process, the propagated field of each plane is calibrated with the reference effective field region, which can effectively correct the data with missing field characteristics caused by sparse sampling and the error data with field offset caused by the interpolation method, accelerating the iterative convergence speed.

[0039] Third, the present invention has good engineering practicability. This method has been successfully applied to the test of Ka-band large-aperture strong-feed array antennas, and its recovery effect on Gaussian feeds has been verified. Under low-frequency conditions, the phase is less affected by the test time and test environment, so high- and low-frequency antennas can be shared. The data is uniformly sampled by plane near-field, and most anechoic chambers have the test conditions. Description of the Drawings

[0040] Figure 1 is the overall flowchart for the implementation of the present invention;

[0041] Figure 2 is the sub - flowchart for determining the boundary of the central field in the present invention;

[0042] Figure 3 is the implementation flowchart for fast measurement based on the plane - wave spectrum theory of the present invention;

[0043] Figure 4 is the schematic diagram of the experimental test of the present invention;

[0044] Figure 5 is the recovery effect diagram of the vertically polarized 5λ sparse sampling data;

[0045] Figure 6 is the recovery effect diagram of the horizontally polarized 5λ sparse sampling data;

[0046] Figure 7 is the recovery effect diagram of the right - hand circularly polarized 5λ sparse sampling data. Detailed implementation manners

[0047] In order to more clearly express the purpose and technical solution of the present invention, the following will further elaborate on the present invention in combination with embodiments and drawings. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0048] Refer to Figure 1 , the method for fast planar near - field measurement of a large - aperture antenna provided in this example includes the following steps:

[0049] Step 1. In the near - field region of the antenna to be measured, uniformly sample a number of surface data and a number of line data, and the sampling environment refers to Figure 4 .

[0050] (1.1) At two planes at different distances from the aperture plane, uniformly sample the amplitude and phase data of the near - field with a large step size.

[0051] The antenna is a large - aperture strong - feed array antenna, and the aperture plane is 2.2 * 2.2 m 2 , the sampling frequency band is 27 - 33 GHz, the sampling plane is the xoy plane, and the sampling plane is parallel to the antenna aperture plane and has the same sampling range.

[0052] The sampling ranges Lx and Ly both satisfy being greater than or equal to 2D * tanθ + a, where D is the maximum distance between the sampling plane and the antenna aperture plane, θ is the maximum credible far - field angle, and a is the width of the aperture plane; pre - scanning needs to be performed before the test, and the ranges of Lx and Ly are determined according to the sampling level difference between the effective field boundary and the sampling plane boundary being less than - 10 dB. In this embodiment, both Lx and Ly are 2.5 m.

[0053] When the proportion of the central effective field region in the total sampling surface is greater than 70%, the sampling step size is selected as 5λ or more; when the proportion of the central effective field region in the total sampling surface is less than 70%, the sampling step size is selected as 2 - 5λ. The continuity of the effective field distribution, the degree of data distortion, and the number of sampling points will all affect the recovery effect. If the effective field distribution is continuous and the data distortion is small, then two sampling surfaces are selected. Otherwise, reduce the sampling step size and increase the number of sampling surfaces. In this embodiment, two planes are sampled, and the distances from the aperture surface of the antenna under test are 4λ and 5.2λ respectively, and the sampling step sizes dx and dy are both 5λ.

[0054] Sampling obtains the amplitude A1, phase P1 of the first sampling surface and the amplitude A2, phase P2 of the second sampling surface of the antenna under test. According to the electric field E = A·e j.P The electric fields E1 and E2 of the two sampling surfaces are obtained.

[0055] (1.2) According to the Nyquist sampling theorem, on each sampling surface, 4 line data L1_k and L2_k passing through the antenna elements are uniformly sampled along the x and y directions respectively, and the sampling step size is 0.5λ.

[0056] Step 2. By calculating the fluctuation degree of the amplitudes of adjacent sampling points from the sampled line data, determine the range of the effective field. The framed effective field range will be used as a reference to correct the propagated field.

[0057] Refer to Figure 2 Taking two planes as an example, this step is implemented as follows:

[0058] (2.1) Sequentially read the amplitudes of four sampling points from the initial point of the line sampling data, and calculate the fluctuation degree of the amplitudes of adjacent sampling points; if the fluctuation of the sampling data within the effective field range exceeds 5dB, the sampling data needs to be smoothed first.

[0059] Calculate the fluctuation degree of the amplitudes of adjacent sampling points, and the formula is as follows:

[0060]

[0061] Among them, x(n) is the amplitude of the nth sampling point, is the average amplitude of all sampling points.

[0062] (2.2) If the difference between the amplitude change rates of the two adjacent amplitudes before and after this sampling point is greater than 15% and the difference between the amplitude change rates of the two consecutive amplitudes after this point is less than 5%, then output the amplitude and position of the left boundary point, and then sequentially read the subsequent data; if the difference between the amplitude change rates of the two adjacent amplitudes before and after this sampling point is greater than 15% and the difference between the amplitude change rates of the two amplitudes before this point is less than 5%, then output the amplitude and position of the right boundary point.

[0063] (2.3) Based on the boundary points on both sides of each line data determined in step (2.2), the area connected by these boundary points is recognized as the range of the reference effective field.

[0064] Step 3. The electric field after partition interpolation and field distribution calibration is iterated between the sampling planes using the plane wave spectrum theory to recover the electric field of the sampling surface.

[0065] Refer to Figure 3 , taking the two planes sampled in the embodiment as an example, this step is implemented as follows:

[0066] (3.1) Different interpolation strategies are adopted for different regions to obtain field data that meet the plane wave spectrum propagation conditions.

[0067] Bilinear interpolation is performed on the sampling data within the effective field range of each sampling surface. After interpolation, the interval meets the same step size as the line data sampling, and the plane range covered by the data remains unchanged.

[0068] In the cut-off region outside the effective field range, the field changes continuously within the rectangular ring region surrounding the effective field, while there is a jump in the radial phase information. Therefore, in order to make the field within the ring region smooth and continuous, first, the line sampling data within each rectangular ring is expanded into a column vector according to its position, and then cubic spline interpolation is used to correct the data with missing phase characteristics caused by sparse sampling; the data outside the rectangular ring is supplemented by non-uniform interpolation.

[0069] (3.2) Calibrate the error data with an offset effective field caused by the interpolation method according to the reference effective field range.

[0070] For the interpolated electric field of each sampling surface obtained in step (3.1), calculate the range of the effective field every ten line data in the x and y directions according to the method shown in step 2, and correct it with the reference effective field range; use non-uniform interpolation to supplement the error data with an offset effective field caused by the interpolation method; replace the sampling data at the corresponding positions to obtain the initial electric fields Ep1 and Ep2 to be recovered for the two sampling surfaces.

[0071] (3.3) Based on the plane wave spectrum theory of the fast Fourier transform, perform iterative operations on the initial electric field of each sampling surface to obtain the recovered electric field of each sampling surface:

[0072] Based on the plane wave spectrum propagation of the fast Fourier transform, the formula is as follows:

[0073] E(x, y, d2) = IFFT2[FFT2[E(x, y, d1)] · exp(-jk z (d2 - d1))]

[0074] Among them, j is the imaginary unit, k zis the spatial wave number, FFT2 is the two-dimensional Fourier operator, IFFT2 is the two-dimensional inverse Fourier operator, d1 is the distance between the sampling plane before propagation and the aperture plane of the antenna under test, and d2 is the distance between the sampling plane after propagation and the aperture plane of the antenna under test.

[0075] To improve the accuracy of frequency-domain propagation, zero-padding is performed on the initial electric field, and the data volume of a single dimension is expanded to four times the original.

[0076] The iterative electric field Em2_1 of the second sampling plane is propagated from the initial electric field Ep1 of the first sampling plane.

[0077] For the iterative electric field Em2_1, calculate the range of the effective field according to the method shown in step 2, and correct the error data; replace the corresponding positions with the sampling data E2 and L2_k of the second sampling plane. At this time, the electric field Em2 of the second sampling plane after iterative correction is obtained.

[0078] The iterative electric field Em1_1 of the first sampling plane is propagated from the corrected electric field Em2 of the second sampling plane.

[0079] For the iterative electric field Em1, calculate the range of the central field according to the method shown in step 2, and correct the error data; replace the corresponding positions with the test data E1 and L1_k of the first sampling plane. Then, continue to propagate to the second sampling plane for iteration.

[0080] The two propagation processes in step (3.3) are regarded as one cycle, and the difference ΔFX between the vector correlation coefficients of the iterative electric fields of the first sampling plane and the initial electric field in two cycles is used as the judgment condition for whether to terminate the iterative operation:

[0081] Calculate the vector correlation coefficient FX, and the formula is as follows:

[0082]

[0083] Among them, Ed1 and Ed2 are the electric fields after iteration and before iteration of the same sampling plane respectively, represents the conjugate of the complex field, |.| represents taking the magnitude of the vector, and (i, j) represents the position of the sampling point on the plane,

[0084] If the number of iterations reaches the set maximum number of iterations Q or the difference ΔFX of the vector correlation coefficients is less than 1%, then jump out of the loop and output the electric fields of the two sampling planes in the last cycle, which is the final restored electric field;

[0085] Otherwise, return to (3.3) to continue the iteration until the termination condition is met.

[0086] Step 4. Use the plane near-field to far-field transformation theory to obtain the far-field radiation characteristics of the antenna under test from the final restored electric field of the first sampling plane.

[0087] The formulas used for planar near - far field transformation are as follows:

[0088]

[0089]

[0090] Among them, respectively represent the coordinate points of the antenna under test in the spherical coordinate system and the element values in the far - field pattern in the θ and Φ directions. j is the imaginary unit, k is the wave number, and A x and A y respectively represent the planar wave spectrum components of the antenna under test along the x and y directions.

[0091] The effects of the present invention can be further illustrated by the following experimental verification.

[0092] The surface sampling data with a sampling step of 5λ will be recovered using the above - mentioned method, and compared with the main indicators of the standard far - field radiation characteristics obtained with a sampling step of 0.5λ, such as: far - field normalized pattern, sidelobe level, 3dB beamwidth, axial ratio, etc. The recovery error of the normalized pattern is calculated according to the following formula: error = 20log(||Eh| - |Ec||), where |Eh| and |Ec| are the normalized amplitudes of the recovered far - field and the standard far - field obtained by near - far field transformation respectively.

[0093] Verification 1: The near - field is recovered using the sparse sampling data with a vertical polarization and a 5λ step, and the recovery effect diagram as shown in Figure 5 is obtained, and the main indicators of the antenna far - field characteristics are compared. Figure 5 Among them, (a) and (b) are the standard amplitude and the recovered amplitude, (c) and (d) are the standard phase and the recovered phase, and the vector correlation coefficient is 0.9846; (e) is the E - plane normalized pattern and the recovery error, and the recovery errors are all less than - 49.881dB. The standard 3dB beamwidth is 0.2286, the recovered 3dB beamwidth is 0.2321, the standard sidelobe level is - 37.439dB, and the recovered sidelobe level is - 38.115dB; (f) is the H - plane normalized pattern and the recovery error, and the recovery errors are all less than - 58.618dB. The standard 3dB beamwidth is 0.1481, the recovered 3dB beamwidth is 0.1517, the standard sidelobe level is - 64.731dB, and the recovered sidelobe level is - 63.832dB; the main indicators of the recovered far - field are in good agreement with the standard values.

[0094] Verification 2: The near - field is recovered using the sparse sampling data with a horizontal polarization and a 5λ step, and the recovery effect diagram as shown in Figure 6 is obtained, and the main indicators of the antenna far - field characteristics are compared. Figure 6Among them, (a) and (b) are the standard amplitude and the restored amplitude, (c) and (d) are the standard phase and the restored phase, and the vector correlation coefficient is 0.9818; (e) is the E-plane normalized radiation pattern and the restoration error, and the restoration errors are all less than -49.398 dB. The standard 3 dB beamwidth is 0.2277, the restored 3 dB beamwidth is 0.2314, the standard sidelobe level is -37.313 dB, and the restored sidelobe level is -37.915 dB; (f) is the H-plane normalized radiation pattern and the restoration error, and the restoration errors are all less than -56.726 dB. The standard 3 dB beamwidth is 0.1485, the restored 3 dB beamwidth is 0.1523, the standard sidelobe level is -64.414 dB, and the restored sidelobe level is -64.685 dB; The main indicators of the restored far field are in good agreement with the standard values.

[0095] Verification 3: The restored electric fields obtained from Verification 1 and Verification 2 are synthesized to obtain a Figure 7 right-hand circularly polarized wave as shown, and the main indicators of the antenna far-field characteristics are compared. Figure 7 Among them, (a) is the normalized radiation pattern and the restoration error at an azimuth angle of 90 degrees. The standard 3 dB beamwidth is 0.2282, the restored 3 dB beamwidth is 0.2319, the standard sidelobe level is -37.376 dB, the restored sidelobe level is -38.015 dB, and the restoration errors are all less than -48.094 dB; (b) is the normalized radiation pattern and the restoration error at an azimuth angle of 0 degrees. The standard 3 dB beamwidth is 0.1483, the restored 3 dB beamwidth is 0.1519, the standard sidelobe level is -64.581 dB, the restored sidelobe level is -64.318 dB, and the restoration errors are all less than -57.38 dB; (c) is the axial ratio at an azimuth angle of 90 degrees. At the maximum radiation direction of 0 degrees, the standard axial ratio is 0 dB, and the restored axial ratio is 0.02 dB; The main indicators of the restored far field are in good agreement with the standard values.

[0096] This embodiment completed the circular polarization near-field test of a large-aperture strong-feed array antenna in about 7% of the time using the traditional planar near-field test method. Among them, sparse sampling of two planes with a step size of 5λ accounts for about 4.3%; precise sampling of four lines in the x and y directions of the two planes with a step size of 0.5λ accounts for about 2.7%.

[0097] Supplementary description of the restoration effect of this method:

[0098] The restoration results of this embodiment are in good agreement with the standard values. The reasons are analyzed as follows: First, the proportion of the central effective field area in the total sampling surface is 77.5%, and the interpolation strategy of the present invention can obtain better initial iteration values; Second, the beam of this antenna is very narrow and the field distribution is concentrated.

[0099] Based on this method, multiple plane near-field rapid measurements of Ka-band large-aperture array antennas have been completed, and the main indicators of the far field can be restored. This shows that while significantly reducing the test time, this method can reduce the influence of the output field fluctuation caused by long-term testing of the transmitting system, quickly obtain the radiation electric field characteristics of the antenna under test, and effectively restore the far-field characteristics of the antenna based on this data.

Claims

1. A fast planar near-field measurement method for large-aperture antennas, characterized in that, The described planar near-field rapid measurement method includes: obtaining the sparse field amplitude-phase information of several sampling planes in the radiation near-field region of the antenna to be measured through large-step uniform sampling; meanwhile, sampling a limited number of accurate field amplitude-phase information on each sampling plane, and obtaining the field distribution characteristics by calculating the fluctuation degree of the sampling point amplitudes; based on the sparse field amplitude-phase information and the field distribution characteristics of the sampling plane, using the partition interpolation and field distribution calibration methods to obtain the initial electric field of each sampling plane; numerically iterating the initial electric field based on the plane wave spectrum theory to restore the near-field distribution of each sampling plane, and finally obtaining the far-field radiation characteristics of the antenna through the near-far field transformation.

2. The planar near-field rapid measurement method for a large-aperture antenna according to claim 1, wherein Specifically, it includes the following steps: Step 1. In the near-field region of the antenna to be measured, uniformly sample several surface data and several line data; Step 1.

1. At several planes at different distances from the aperture plane, uniformly sample the amplitude and phase data of the near-field with a large step; When the proportion of the central effective field region in the total sampling plane is greater than 70%, the sampling step is selected as 5λ or more; when the proportion of the central effective field region in the total sampling plane is less than 70%, the sampling step is selected as 2 - 5λ; If the effective field distribution is continuous and the data distortion is small, two sampling planes are selected; otherwise, reduce the sampling step and increase the number of sampling planes; Step 1.

2. According to the Nyquist sampling theorem, uniformly sample several line data passing through the antenna elements along the x and y directions on each sampling plane; Obtain the field distribution characteristics of the sampling plane by analyzing the sampled line data; if the central effective field is continuous and the amplitude change is less than 5dB, symmetrically sample 3 - 5 line data in the x and y directions respectively; otherwise, increase the number of sampled line data to obtain the global characteristics of the electric field; Step 2. Calculate the fluctuation degree of the amplitudes of adjacent sampling points for the line data sampled in Step 1.2, determine the range of the effective field, and use it as a reference to correct the propagated field; Step 3. Select different interpolation strategies according to the field distribution characteristics of different regions to obtain the electric field that meets the plane wave spectrum propagation conditions; Step 4. For the electric fields of each sampling plane that meet the plane wave spectrum propagation conditions obtained in Step 3, recalculate the range of the effective field every ten line data in the x and y directions; correct the recalculated range of the effective field according to the reference effective field range, and use non-uniform interpolation to supplement the error data caused by the interpolation method for the effective field offset to obtain the corrected electric field; Replace the sampling data at the corresponding electric field positions to obtain the initial electric field to be restored for each sampling plane; Step 5. Based on the plane wave spectrum theory of the fast Fourier transform, perform iterative operations on the initial electric field of each sampling plane, and obtain the final restored electric field after the iteration is completed; Step 6. Use the planar near-far field transformation to obtain the far-field radiation characteristics of the antenna to be measured from the final restored electric field of the sampling plane.

3. The planar near-field fast measurement method for a large-aperture antenna according to claim 2, wherein, The described Step 3 includes the following steps: Step 3.

1. Use bilinear interpolation for the sampling data within the effective field range of each sampling plane. After interpolation, the spacing meets the same as the sampling step of the line data, and the plane range covered by the data remains unchanged; Step 3.

2. In the cut-off region outside the effective field range, first expand the line sampling data of each rectangular ring region surrounding the effective field range into column vectors according to their positions, and then use cubic spline interpolation to correct the data with missing phase characteristics caused by sparse sampling; for the data outside the rectangular ring, non-uniform interpolation is used for supplementation; after interpolation, the interval of the data in the cut-off region is consistent with the line data sampling step size.

4. A method for rapid planar near-field measurement of a large-aperture antenna according to claim 2 or 3, characterized in that The said Step 5 includes the following steps: Step 5.

1. Set that among two adjacent sampling planes, the sampling plane closer to the antenna aperture plane is the first sampling plane, and the other sampling plane is the second sampling plane; propagate the initial electric field Ep1 of the first sampling plane to obtain the iterative electric field Em2_1 of the second sampling plane. Step 5.

2. Calculate the effective field range and correct the error data for the iterative electric field Em2_1 according to the method shown in Step 4; use the sampling data of the second sampling plane in Step 1 to replace the data at the corresponding positions; at this time, the electric field Em2 of the second sampling plane after iterative correction is obtained. Step 5.

3. Propagate the corrected electric field Em2 of the second sampling plane to obtain the iterative electric field Em1_1 of the first sampling plane. Step 5.

4. Calculate the effective field range and correct the error data for the iterative electric field Em1_1 according to the method shown in Step 4; use the sampling data of the first sampling plane in Step 1 to replace the data at the corresponding positions; then, continue to propagate to the second sampling plane for iteration. Step 5.

5. The two propagation processes in Steps 5.1 - 5.4 are regarded as one cycle, and the difference ΔFX between the vector correlation coefficients of the iterative electric field of the first sampling plane and the initial electric field in the two cycles is used as the judgment condition for whether to terminate the iterative operation: If the number of iterations reaches the set maximum number of iterations Q or the difference ΔFX between the vector correlation coefficients is less than 1%, then jump out of the loop and output the electric fields of the two sampling planes obtained in the last iteration, which are the final restored electric fields. Otherwise, continue the iteration until the termination condition is met.

5. A planar near-field rapid measurement method for a large-aperture antenna according to claim 4, characterized in that, The distance between adjacent sampling planes is greater than 1λ.

Images