Method for measuring P-band RCS in reverberation room

Abstract

The invention relates to the field of RCS measurement, in particular to a P-band RCS measurement method in a reverberation room. The method comprises the following steps: calibrating the field uniformity of a reverberation chamber, and establishing a measurement environment; echo data in the reverberation chamber are measured under the target-free condition; echo data in the reverberation chamber are measured under the condition that a target exists; using the echo data without the target to calibrate the echo data with the target; performing two-dimensional imaging by using the calibrated echo data; and extracting the target RCS according to the two-dimensional image. By means of the field uniformity of the reverberation chamber, a transfer function about wave propagation in the reverberation chamber is regarded as complex noise, the target RCS is extracted through calibration, imaging, calibration, comparison methods and the like, the application field of the reverberation chamber is expanded, and a new thought is provided for P-band RCS measurement. The method is suitable for RCS measurement.

Description

Technical Field

[0001] The invention relates to the field of RCS (Radar Cross Section) measurement, and in particular to a P-band RCS measurement method in a reverberation chamber. Background Art

[0002] Existing stealth and anti-stealth technologies are usually concentrated in specific frequency bands, and P-band radar is an important anti-stealth approach. In the P-band, the longer wavelength makes it difficult for the thickness of the stealth equipment's absorbing materials to reach the engineering application level, resulting in a larger radar scattering cross section of the target, which increases the possibility of detection by anti-stealth radar. The development of anti-stealth forces the design and research of stealth equipment to innovate further in order to achieve more efficient stealth performance in the P-band. Therefore, RCS testing is particularly important as an indispensable part of stealth design.

[0003] Compared with the commonly used RCS test frequency bands, the P-band beam has a wide main lobe and high side lobes, and clutter interference causes a decrease in test accuracy. Microwave darkrooms and compact fields are the main places for RCS testing. When the test frequency is low, the microwave darkroom needs to increase the test site to achieve far-field conditions. The compact field requires the use of an oversized high-precision parabola, and the test environment is expensive. The reverberation chamber is made of a metal cavity, which is low-cost and convenient, and there is no need to consider the performance of the absorbing material. In addition, the reverberation chamber can simulate the complex random electromagnetic fields in the actual combat environment to a certain extent. Therefore, it is of great significance to study how to perform P-band RCS measurements in a reverberation chamber. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a P-band RCS measurement method in a reverberation chamber.

[0005] The present invention adopts the following technical solutions to achieve the above-mentioned purpose. In a first aspect, the present invention provides a P-band RCS measurement method in a reverberation chamber, comprising:

[0006] S1. Calibrate the field uniformity of the reverberation chamber and build the measurement environment;

[0007] S2, measure the echo data in the reverberation chamber without a target;

[0008] S3, measure the echo data in the reverberation chamber when there is a target;

[0009] S4, using the echo data when there is no target to calibrate the echo data when there is a target;

[0010] S5, performing two-dimensional imaging using the calibrated echo data;

[0011] S6. Extract the target RCS according to the two-dimensional image.

[0012] Furthermore, the reverberation chamber measurement environment consists of a metal shielded cavity with a rectangular cavity size of L×W×H, a built-in stirrer and a logarithmic periodic antenna, and the lowest usable frequency of the reverberation chamber is less than 300MHz, and the lowest usable frequency is at least 3 to 4 times the lowest resonant frequency. The frequency of the resonant mode is:

[0013]

[0014] Where l, m, and n are non-negative integers, and only one of them can be 0. l,m,n The unit is MHz, which indicates the frequency of the resonant mode. L, M, and H indicate the length, width, and height of the rectangular cavity, respectively.

[0015] Furthermore, the field uniformity of the calibration reverberation chamber specifically includes:

[0016] Adjust the arrangement of the stirrer and antenna, change the structural design of the stirrer, the stirring method and the antenna design, perform a field strength test every 100MHz within the set frequency range, and calibrate the field uniformity in the reverberation chamber. The specific method is as follows:

[0017] When the stirrer is rotating, select 8 vertices in the working area, test and record the field strength values ​​in 3 directions of each vertex, normalize the maximum field strength recorded in 3 directions of each point relative to the square root of the average input power, and take the standard deviation. The obtained standard deviation must meet the relevant requirements (100-400MHz, the field standard deviation decreases linearly from 4dB to 3dB; 400MHz-1GHz, the field standard deviation is less than 3dB), which means that the field uniformity is met in the working area.

[0018] Furthermore, step S2 specifically includes:

[0019] The state of the stirrer and antenna in the reverberation room remains unchanged. The total height of the foam column and the turntable is H0. The height of the horn antenna is H0. The antenna normal points to the foam column, and the distance from the center of the foam column is L0. L0 ≥ 2D 2 / λ, where D is the target size, λ is the signal wavelength, there is no target on the foam column, and the vector network analyzer, computer and other test devices are set outside the reverberation chamber;

[0020] The vector network analyzer transmits a broadband signal with a center frequency of f0, a step frequency of Δf, and a sweep frequency of 2N+1. The turntable rotates at a constant speed of M×Δθ degrees with a step of Δθ, where M is the number of azimuth points. The echo data S is measured. 0(2N+1)×M , the scattered field in this state comes from the antenna reflection coefficient and the backscattering of the reverberation chamber cavity, then the echo data when the stirrer position is α is:

[0021] S0(f i ,α)=SFS (f i )+(1-|S FS (f i )| 2 )η ant H(f i ,α);

[0022] In the formula, f i =f0+(2N+1)Δf, where i=0,±1,±2,...,±N is the frequency point number, S FS is the reflection coefficient of the antenna in free space, η ant is the antenna radiation efficiency, H(f i ,α) is the transfer function of the interaction between the reverberation chamber wall and the log-periodic antenna. The real and imaginary parts of this function are random variables that obey Gaussian distribution.

[0023] Furthermore, step S3 includes:

[0024] Keep the test environment in the reverberation room unchanged, place the test target on the foam column, and the vector network analyzer transmits a broadband signal with a center frequency f0, a step frequency Δf, and a sweep frequency point number 2N+1. The turntable rotates at a constant speed of M×Δθ degrees with a step of Δθ, and the measured echo data is S (2N+1)×M In this state, the magnetic field environment in the reverberation chamber changes. Only the scattering field changes caused by the interaction between the target and the transmitting antenna are considered, and the scattering between the target reflected wave and the wall of the reverberation chamber is ignored. When the stirrer position is α, the echo data is described by BO approximation as follows:

[0025]

[0026] In the formula, is the echo between the target and the antenna, σ(f i ,θ) is the target at frequency f i , RCS at an incident angle of θ, where

[0027]

[0028] In the formula, c is the propagation speed of electromagnetic waves, R is the distance between the antenna and the target, φ i is the initial phase of signals with different transmission frequencies.

[0029] Furthermore, step S4 includes:

[0030] Calculate the difference between the echo data with and without a target, and perform echo calibration based on the difference. The calibrated echo data is as follows:

[0031]

[0032] n(f i ,α,θ) is a complex noise that obeys Gaussian distribution, then:

[0033]

[0034] Furthermore, step S5 includes:

[0035] The calibrated echo data is windowed in azimuth and range, and the frequency domain is zero-filled in the range of each azimuth to achieve interpolation. The range echo data of each azimuth is read in turn, and inverse Fourier transform is performed respectively to obtain a one-dimensional range image.

[0036] Divide the grid (x p ,y q ), calculate the distance R between the antenna phase center and the grid point p,q The corresponding echo delay τ p,q =2R p,q / c, the pixel values ​​of all grid points are The phase compensation is performed by the compensation amount, and coherent accumulation is performed along the azimuth direction to obtain a two-dimensional image of the target.

[0037] Furthermore, step S6 includes:

[0038] After windowing the two-dimensional image, a two-dimensional inverse Fourier transform is performed, and the transformed spectral domain data is interpolated to obtain the target scattering coefficient γ(f,θ) at different frequencies f and different observation angles θ;

[0039] Select the calibration sphere size, calculate the scattering coefficient γ0(f,θ) of the calibration sphere, repeat steps S1 to S6 for the calibration sphere under the same test environment, and use the comparison method to obtain the target RCS measurement value σ(f,θ).

[0040] The beneficial effects of the present invention are:

[0041] The present invention uses the field uniformity of the reverberation chamber to regard the transfer function of wave propagation in the reverberation chamber as a complex noise, and extracts the target RCS through calibration, imaging, scaling, comparison and other processing, thereby improving the accuracy of RCS measurement, expanding the application field of the reverberation chamber, and providing a new idea for P-band RCS measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is a flowchart of a P-band RCS measurement method in a reverberation chamber provided by an embodiment of the present invention;

[0043] Figure 2 It is a schematic diagram of the deployment of measuring instruments and equipment provided by an embodiment of the present invention;

[0044] Figure 3 is a schematic diagram of wave propagation in a reverberation chamber with a target provided by an embodiment of the present invention;

[0045] Figure 4 is a schematic diagram of a reverberation chamber simulation model provided by an embodiment of the present invention;

[0046] Figure 5 It is a curve diagram of the RCS result of a target under far-field conditions provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0047] To make the purpose, technical solution and advantages of the embodiments of the present invention more clear, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0048] The present invention provides a P-band RCS measurement method in a reverberation chamber. Figure 1 As shown, specifically including:

[0049] S1. Calibrate the field uniformity of the reverberation chamber and build the measurement environment;

[0050] The reverberation chamber measurement environment consists of a metal shielded cavity with a rectangular cavity size of L×W×H (length×width×height), with a built-in stirrer and logarithmic periodic antenna. The lowest available frequency of the reverberation chamber is less than 300MHz, and its lowest available frequency is at least 3 to 4 times the lowest resonant frequency. The frequency of the resonant mode is:

[0051]

[0052] Where l, m, and n are non-negative integers, and only one of them can be 0. l,m,n The unit is MHz.

[0053] Adjust the arrangement of the stirrer and antenna, change the structural design of the stirrer, the stirring method, the antenna design, etc., perform a field strength test every 100MHz in the frequency range of 300MHz to 1GHz, and calibrate the field uniformity in the reverberation chamber. The specific method is to select 8 vertices in the working area when the stirrer rotates, test and record the field strength values ​​of the 3 directions of each vertex, normalize the maximum field strength recorded at each point and each point in the 3 directions relative to the square root of the average input power, take the standard deviation and take dB (according to the provisions of the IEC61000-4-21 standard).

[0054] After the field uniformity in the working area meets the requirements (100-400MHz, the field standard deviation decreases linearly from 4dB to 3dB; 400MHz-1GHz, the field standard deviation is less than 3dB), subsequent testing work is carried out in the reverberation chamber.

[0055] S2, measure the echo data in the reverberation chamber without a target;

[0056] like Figure 2 As shown, during the test, the agitator and antenna in the reverberation chamber remain unchanged, the total height of the foam column and the turntable is H0, the height of the horn antenna is also H0, the antenna normal points to the foam column, and the distance from the center of the foam column is L0, L0 ≥ 2D 2 / λ (meeting far-field conditions), where D is the target size and λ is the signal wavelength. There is no target on the foam column, and the vector network analyzer, computer and other test devices are placed outside the reverberation chamber.

[0057] The vector network analyzer transmits a broadband signal with a center frequency of f0, a step frequency of Δf, and a sweep frequency of 2N+1. The turntable rotates at a constant speed of M×Δθ degrees with a step of Δθ, where M is the number of azimuth points. The measured echo data is S 0(2N+1)×M The scattered field in this state comes from the antenna reflection coefficient and the backscattering of the reverberation chamber cavity. The echo data when the stirrer position is α is:

[0058]

[0059] In the formula, f i =f0+(2N+1)Δf is the frequency, i=0,±1,±2,...,±N is the frequency point number, S FS is the reflection coefficient of the antenna in free space, η ant is the antenna radiation efficiency, H(f i ,α) is the transfer function of the interaction between the reverberation chamber wall and the log-periodic antenna.

[0060] S3, measure the echo data in the reverberation chamber when there is a target;

[0061] like Figure 3 As shown in the figure, the state of the reverberation chamber and the test environment remain unchanged. The test target is placed on the foam column. The vector network analyzer transmits a broadband signal with a center frequency f0, a step frequency Δf, and a sweep frequency point number 2N+1. The turntable rotates at a constant speed of M×Δθ degrees with a step frequency of Δθ. The measured echo data is S (2N+1)×M In this state, the magnetic field environment in the reverberation chamber changes. Since the target size is much smaller than the reverberation chamber size, only the scattering field changes caused by the interaction between the target and the transmitting antenna are considered, and the scattering between the target reflected wave and the reverberation chamber wall is ignored. The stirrer position is α st When , the BO approximation is used to describe the echo data as:

[0062]

[0063] In the formula, is the echo between the target and the antenna, σ(f i,θ T ) is the target at frequency f i , RCS at an incident angle of θ.

[0064] in:

[0065]

[0066] In the formula, c is the propagation speed of electromagnetic waves, R is the distance between the antenna and the target, φ i is the initial phase of signals with different transmission frequencies.

[0067] S4, using the echo data when there is no target to calibrate the echo data when there is a target;

[0068] According to the echo data measured above with and without the target, the difference between the two is calculated to achieve echo calibration, that is,

[0069] E (2N+1)×M =S (2N+1)×M -S 0(2N+1)×M ,Right now:

[0070]

[0071] The reverberation room transfer function has nothing to do with the target RCS and can be regarded as a complex noise that obeys a Gaussian distribution. Therefore, the calibrated echo data can be expressed as:

[0072]

[0073] S5, performing two-dimensional imaging using the calibrated echo data;

[0074] After calibration, the echo data E (2N+1)×M The azimuth and range directions are windowed, and the frequency domain is zero-filled for each azimuth and range direction to achieve interpolation. The interpolated data is Read the range echo data of each direction in turn, perform inverse Fourier transform respectively, and obtain a one-dimensional range image

[0075] Divide the grid (x p ,y q ), calculate the distance R between the antenna phase center and the grid point p,q The corresponding echo delay τ p,q , the pixel values ​​of all grid points are The phase compensation is performed by the compensation amount, and coherent accumulation is performed along the azimuth direction to obtain a two-dimensional image of the target.

[0076] S6, extracting the target RCS according to the two-dimensional image;

[0077] The two-dimensional image is windowed and then subjected to a two-dimensional inverse Fourier transform. The spectral domain data is interpolated to obtain the target scattering coefficient γ(f,θ) at different frequencies and observation angles.

[0078] Under the same test environment, repeat steps S1 to S6 for the calibration sphere. The size of the selected calibration sphere is in the resonance region within the frequency range of 300 MHz to 1 GHz (the relationship between the incident wavelength and the target size satisfies 0.5 ≤ ka ≤ 20). Calculate the scattering coefficient γ0(f, θ) of the calibration sphere, and use the comparison method to obtain the target RCS measurement value σ(f, θ).

[0079] The present invention is further described below in conjunction with a simulation example.

[0080] The simulation parameters are set as follows: Based on the actual reverberation chamber, FEKO is used to establish a 1:1 reverberation chamber simulation model, and the shielding cavity, agitator, and periodic logarithmic antenna are modeled respectively to calibrate the field uniformity. The cavity size of the reverberation chamber is 5.3m×2.9m×3m (length×width×height). The transmitting antenna uses a logarithmic dipole array. The transmitting antenna is aligned with the corner of the wall, and the antenna input power is 100W. The total height of the foam column and the turntable is 1.5m, and the horn antenna is also set at a height of 1.5m. The antenna normal points to the foam column, 3m away from the center of the foam column. The frequency range of the signal transmitted by the vector network analyzer is 300MHz~1GHz. Figure 4 The following is a simulation model of a reverberation room established using FEKO. Figure 5 The figure shows the RCS result of the target under far-field conditions. The horizontal axis represents the frequency and the vertical axis represents the RCS value.

[0081] The above is only a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the form disclosed herein, and should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications and environments, and can be modified within the scope of the concept described herein through the above teachings or the technology or knowledge of the relevant field. The changes and modifications made by those skilled in the art shall not deviate from the spirit and scope of the present invention, and shall be within the scope of protection of the claims attached to the present invention.

Claims

1. A P-band RCS measurement method in a reverberation chamber, characterized in that: include: S1. Calibrate the field uniformity of the reverberation chamber and build the measurement environment; S2, measure the echo data in the reverberation chamber without a target; S3, measure the echo data in the reverberation chamber when there is a target; S4, using the echo data when there is no target to calibrate the echo data when there is a target; S5, performing two-dimensional imaging using the calibrated echo data; S6. Extract the target RCS according to the two-dimensional image.

2. The P-band RCS measurement method in a reverberation chamber according to claim 1, characterized in that: The reverberation chamber measurement environment consists of a metal shielded cavity with a rectangular cavity size of L×W×H, a built-in stirrer and a logarithmic periodic antenna. The lowest available frequency of the reverberation chamber is less than 300MHz, and the lowest available frequency is at least 3 to 4 times the lowest resonant frequency. The frequency of the resonant mode is: In the formula, l, m, and n represent calculation parameters, which are non-negative integers, and only one of them can be 0. l,m,n The unit is MHz, which represents the frequency of the resonant mode. L, M, and H represent the length, width, and height of the rectangular cavity, respectively.

3. The P-band RCS measurement method in a reverberation chamber according to claim 2, characterized in that: Calibrating the field uniformity of the reverberation chamber specifically includes: Adjust the arrangement of the stirrer and antenna, change the structural design of the stirrer, the stirring method and the antenna design, perform a field strength test every 100MHz within the set frequency range, and calibrate the field uniformity in the reverberation chamber. The specific method is as follows: When the stirrer is rotating, eight vertices in the working area are selected, and the field strength values ​​at three positions of each vertex are tested and recorded. The maximum field strength recorded at the three positions of each point is normalized to the square root of the average input power, and the standard deviation is taken.

4. The P-band RCS measurement method in a reverberation chamber according to claim 1, characterized in that: Step S2 specifically includes: The state of the stirrer and antenna in the reverberation room remains unchanged. The total height of the foam column and the turntable is H0. The height of the horn antenna is H0. The antenna normal points to the foam column, and the distance from the center of the foam column is L0. L0 ≥ 2D 2 / λ, where D is the target size, λ is the signal wavelength, there is no target on the foam column, and the vector network analyzer, computer and other test equipment are set outside the reverberation chamber; The vector network analyzer transmits a broadband signal with a center frequency of f0, a step frequency of Δf, and a frequency sweep point number of 2N+1. The turntable rotates at a constant speed of M×Δθ degrees with a step frequency of Δθ, where M is the number of azimuth points. The echo data S is measured. 0(2N+1)×M , the scattered field in this state comes from the antenna reflection coefficient and the backscattering of the reverberation chamber cavity, then the echo data when the stirrer position is α is: S0(f i ,α)=S FS (f i )+(1-|S FS (f i )| 2 )η ant H(f i ,α); Where S0(f i , α)tt represents the echo data when the stirrer position is α, f i =f0+(2N+1)Δf, is the frequency, i=0,±1,±,2...,±N is the frequency point number, S FS is the reflection coefficient of the antenna in free space, η ant is the antenna radiation efficiency, H(f i ,α) is the transfer function of the interaction between the reverberation chamber wall and the log-periodic antenna. The real and imaginary parts of this function are random variables that obey Gaussian distribution.

5. The P-band RCS measurement method in a reverberation chamber according to claim 1, characterized in that: Step S3 includes: The test target is placed on the foam column. The vector network analyzer transmits a broadband signal with a center frequency of f0, a step frequency of Δf, and a sweep frequency of 2N+1. The turntable rotates at a constant speed of M×Δθ degrees with a step of Δθ. The measured echo data is S (2N+1)×M In this state, the magnetic field environment in the reverberation chamber changes. Only the scattering field changes caused by the interaction between the target and the transmitting antenna are considered, and the scattering between the target reflected wave and the wall of the reverberation chamber is ignored. When the stirrer position is α, the echo data is described by BO approximation as follows: In the formula, S(f i , α, θ) means using BO approximation to describe the echo data, is the echo between the target and the antenna, σ(f i ,θ) is the target at frequency f i , RCS at an incident angle of θ, where: In the formula, c is the propagation speed of electromagnetic waves, R is the distance between the antenna and the target, φ i is the initial phase of signals with different transmission frequencies.

6. The P-band RCS measurement method in a reverberation chamber according to claim 1, characterized in that: Step S4 includes: Calculate the difference between the echo data with and without a target, and perform echo calibration based on the difference. The calibrated echo data is as follows: n(f i ,α,θ) is a complex noise that obeys Gaussian distribution, then:

7. The P-band RCS measurement method in a reverberation chamber according to claim 1, characterized in that: Step S5 includes: The calibrated echo data is windowed in azimuth and range, and the frequency domain is zero-filled in the range of each azimuth to achieve interpolation. The range echo data of each azimuth is read in turn, and inverse Fourier transform is performed respectively to obtain a one-dimensional range image. Divide the grid (x p ,y q ), calculate the distance R between the antenna phase center and the grid point p,q The corresponding echo delay τ p,q =2R p,q / c, the pixel values ​​of all grid points are The phase compensation is performed by the compensation amount, and coherent accumulation is performed along the azimuth direction to obtain a two-dimensional image of the target.

8. The P-band RCS measurement method in a reverberation chamber according to claim 1, characterized in that: Step S6 includes: After windowing the two-dimensional image, a two-dimensional inverse Fourier transform is performed, and the transformed spectral domain data is interpolated to obtain the target scattering coefficient γ(f,θ) at different frequencies f and different observation angles θ; Select the calibration sphere size, calculate the scattering coefficient γ0(f,θ) of the calibration sphere, and use the comparison method to obtain the target RCS measurement value σ(f,θ).

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