A P-band RCS measurement method in a reverberation chamber

Abstract

The present invention relates to the field of RCS measurement, and particularly to a method for measuring P-band RCS in a reverberation chamber. The method includes: calibrating the field uniformity of the reverberation chamber and setting up a measurement environment; measuring the echo data in the reverberation chamber without a target; measuring the echo data in the reverberation chamber with a target; calibrating the echo data with a target by using the echo data without a target; performing two-dimensional imaging by using the calibrated echo data; and extracting the target RCS according to the two-dimensional image. By virtue of the field uniformity of the reverberation chamber, the present invention regards the transfer function of wave propagation in the reverberation chamber as a complex noise, and extracts the target RCS through processing such as calibration, imaging, calibration, and comparison method, thereby expanding the application field of the reverberation chamber and providing a new idea for P-band RCS measurement. The present invention is applicable to RCS measurement.

Description

Technical Field

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

[0002] Existing stealth and anti-stealth technologies usually focus on specific frequency bands. P-band radar is an important anti-stealth approach. In the P-band, the longer wavelength makes it difficult for the thickness of the absorbing material of stealth equipment to reach the engineering application level, resulting in a larger radar cross section of the target, increasing the possibility of detection for anti-stealth radar. The development of anti-stealth forces the design and research of stealth equipment to be further innovated to achieve more efficient stealth performance in the P-band. Therefore, RCS testing, as an essential link in stealth design, is particularly important.

[0003] Compared with the commonly used RCS test frequency bands, the main lobe of the P-band beam is wide and the sidelobe is high, and clutter interference leads to a decrease in test accuracy. Microwave anechoic chambers and compact ranges are the main places for RCS testing. When the test frequency is low, the microwave anechoic chamber needs to increase the test site to achieve far-field conditions, and the compact range needs to use an ultra-large-size high-precision paraboloid, and the test environment is expensive. The reverberation chamber is made of a metal chamber, with low cost and convenience, and the performance of the absorbing material does not need to be considered. In addition, to a certain extent, the reverberation chamber can simulate the complex random electromagnetic field in the actual combat environment. Therefore, it is of great significance to study how to measure P-band RCS 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 purpose. In the first aspect, the present invention provides a P-band RCS measurement method in a reverberation chamber, including:

[0006] S1. Calibrate the field uniformity of the reverberation chamber and build a 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 with a target;

[0009] S4. Calibrate the echo data with a target using the echo data without a target;

[0010] S5. Perform 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 shielding cavity. The rectangular cavity has dimensions of L×W×H and is equipped with a stirrer and a log-periodic antenna. The lowest available frequency of the reverberation chamber is less than 300 MHz, and the lowest available frequency is at least 3 to 4 times the lowest resonant frequency. The frequency of the resonant mode is:

[0013]

[0014] where l, m, n are non-negative integers, and only one of them can take the value of 0, F l,m,n is in MHz and represents the frequency of the resonant mode. L, M, and H represent the length, width, and height of the rectangular cavity dimensions respectively.

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

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

[0017] Under the condition of the rotating motion of the stirrer, select 8 vertices within the working area, test and record the field strength values at 3 azimuths of each vertex. Normalize the maximum field strength recorded at the 3 azimuths of each point with respect to the square root of the average input power, and take the standard deviation. The obtained standard deviation needs to meet the relevant requirements (from 4 dB to 3 dB linearly for the field standard deviation from 100 to 400 MHz; less than 3 dB for the field standard deviation from 400 MHz to 1 GHz), which means that the field uniformity is satisfied within the working area to be.

[0018] Furthermore, step S2 specifically includes:

[0019] The state of the stirrer and the antenna in the reverberation chamber remains unchanged. The total height of the foam column and the turntable is H0, the erection height of the horn antenna is H0, the normal direction of the antenna points to the foam column, and it is at a distance of L0 from the center of the foam column, where L0≥2D 2 / λ, where D is the target size and λ is the signal wavelength. There is no target on the foam column, and the rest of the test devices such as the vector network analyzer and the computer 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 2N + 1 sweep points. The turntable rotates at a constant speed by M×Δθ degrees with Δθ as the step. M is the number of points in the azimuth direction, and the echo data S is measured 0(2N+1)×M In this state, the scattered field comes from the antenna reflection coefficient and the backscattering of the reverberation chamber cavity. Then the echo data when the position of the stirrer 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, 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 radiation efficiency of the antenna, H(f i , α) is the transfer function of the interaction between the reverberation chamber cavity wall and the log - periodic antenna, and the real and imaginary parts of this function are random variables obeying the Gaussian distribution.

[0023] Furthermore, step S3 includes:

[0024] Keep the test environment in the reverberation chamber unchanged, place the test target on the foam column, the vector network analyzer emits a broadband signal with a center frequency f0, a step frequency Δf, and a sweep frequency point number of 2N + 1, the turntable rotates uniformly by M×Δθ degrees with Δθ as the step, and the measured echo data is S (2N+1)×M , in this state, the magnetic field environment in the reverberation chamber changes, only considering the change in the scattered field caused by the interaction between the target and the transmitting antenna, ignoring the scattering between the target reflected wave and the reverberation chamber wall, then when the position of the stirrer is α, the echo data is described by the BO approximation as:

[0025]

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

[0027]

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

[0029] Furthermore, step S4 includes:

[0030] Calculate the difference between the echo data with the target and without the target, and perform echo calibration according to the difference. Then the calibrated echo data is as follows:

[0031]

[0032] Regarding \(n(f i , \alpha, \theta)\) as a complex noise subject to Gaussian distribution, then:

[0033]

[0034] Furthermore, step S5 includes:

[0035] Window the calibrated echo data in the azimuth and range directions, perform zero-padding in the frequency domain for the range direction of each azimuth to achieve interpolation, sequentially read the range-direction echo data of each azimuth, and perform inverse Fourier transform respectively to obtain the one-dimensional range image;

[0036] Divide the grid \((x p , y q ) within the imaging area, calculate the distance \(R\) between the antenna phase center and the grid point, and calculate the corresponding echo time delay \(\tau p,q = 2R p,q / c, perform phase compensation on the pixel values of all grid points with a compensation amount of p,q , and perform coherent accumulation in the azimuth direction to obtain the two-dimensional image of the target.

[0037] Furthermore, step S6 includes:

[0038] After windowing the two-dimensional image, perform two-dimensional inverse Fourier transform, and interpolate and calculate the target scattering coefficient \(\gamma(f, \theta)\) at different frequencies \(f\) and different observation angles \(\theta\) for the transformed spectral domain data;

[0039] Select the size of the calibration sphere, calculate the scattering coefficient \(\gamma_0(f, \theta)\) of the calibration sphere, repeat steps S1 - S6 for the calibration sphere under the same test environment, and use the comparison method to obtain the target RCS measurement value \(\sigma(f, \theta)\).

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

[0041] By virtue of the field uniformity of the reverberation chamber, the present invention regards the transfer function of wave propagation in the reverberation chamber as a complex noise, and extracts the target RCS through processes such as calibration, imaging, calibration, and comparison method, improving the accuracy of RCS measurement, expanding the application field of the reverberation chamber, and providing a new idea for P-band RCS measurement. 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 is a schematic diagram of the layout of the measuring instrument and equipment provided by an embodiment of the present invention;

[0044] Figure 3 It is a schematic diagram of wave propagation in a reverberation chamber under targeted conditions provided by an embodiment of the present invention;

[0045] Figure 4 It 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 graph of the RCS result of a target under far - field conditions provided by an embodiment of the present invention. Specific embodiments

[0047] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions 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 method for measuring the P - band RCS in a reverberation chamber, as Figure 1 shown, specifically including:

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

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

[0051]

[0052] In the formula, l, m, n are non - negative integers, and only one of them can take the value of 0, F l,m,n The unit is MHz.

[0053] Adjust the arrangement of the stirrer and the antenna, change the structural design and stirring method of the stirrer, antenna design, etc. Conduct a field strength test every 100 MHz in the frequency range of 300 MHz - 1 GHz to calibrate the field uniformity in the reverberation chamber. The specific method is as follows: Under the rotational movement of the stirrer, select 8 vertices in the working area, test and record the field strength values at 3 azimuths of each vertex. Normalize the square root of the maximum field strength recorded at 3 azimuths of each point with respect to the average input power, and take the standard deviation and then dB (in accordance with the regulations in IEC61000 - 4 - 21 standard).

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

[0055] S2. Measure the echo data in the reverberation chamber without a target;

[0056] As Figure 2 shown, during the test, the states of the stirrer and the antenna in the reverberation chamber remain unchanged. The total height of the foam column and the turntable is H0, and the height of the horn antenna is also H0. The normal direction of the antenna points to the foam column, at a distance of L0 from the center of the foam column, where L0 ≥ 2D 2 / λ (meeting the far-field condition), where D is the target size and λ is the signal wavelength. There is no target on the foam column, and the rest of the test devices such as the vector network analyzer and the computer are arranged outside the reverberation chamber.

[0057] The vector network analyzer emits a broadband signal with a center frequency f0, a step frequency Δf, and 2N + 1 sweep points. The turntable rotates uniformly by M × Δθ degrees with Δθ as the step, and M is the number of points in the azimuth direction. 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 position of the stirrer 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 radiation efficiency of the antenna, and H(f i ,α) is the transfer function of the interaction between the wall of the reverberation chamber cavity and the log-periodic antenna.

[0060] S3. Measure the echo data in the reverberation chamber with a target;

[0061] As Figure 3 shown, the state of the reverberation chamber and the test environment remain unchanged. Place the test target on the foam column. The vector network analyzer emits a broadband signal with a center frequency f0, a step frequency Δf, and 2N + 1 sweep points. The turntable rotates uniformly by M × Δθ degrees with Δθ as the step, and 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 consider the change in the scattered field caused by the interaction between the target and the transmitting antenna, and ignore the scattering between the target reflected wave and the reverberation chamber wall. When the position of the stirrer is α st , 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, and σ(f i, θ T ) is the RCS of the target at a frequency of f i and an incident angle of θ.

[0064] Where:

[0065]

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

[0067] S4. Calibrate the echo data with a target using the echo data without a target;

[0068] According to the echo data measured in the above two cases of having and not having a target, calculate the difference between the two to achieve echo calibration, that is

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

[0070]

[0071] The reverberation chamber transfer function is independent of the target RCS and can be regarded as a complex noise obeying a Gaussian distribution. Therefore, the calibrated echo data can be expressed as:

[0072]

[0073] S5. Perform two-dimensional imaging using the calibrated echo data;

[0074] Window the calibrated echo data E (2N+1)×M in the azimuth and range directions, and zero-pad the range direction for each azimuth to achieve interpolation. The interpolated data is Read the range-direction echo data for each azimuth in turn, and perform inverse Fourier transform respectively to obtain the one-dimensional range image

[0075] Divide the grid (x p , y q ) in the imaging area, calculate the echo delay τ p,q corresponding to the distance R p,q between the antenna phase center and the grid point, and perform phase compensation on the pixel values of all grid points with a compensation amount, and perform coherent accumulation along the azimuth direction to obtain the two-dimensional image of the target.

[0076] S6. Extract the target RCS according to the two-dimensional image;

[0077] After windowing the two-dimensional image, perform two-dimensional inverse Fourier transform, and interpolate and calculate the spectral domain data to obtain the target scattering coefficient γ(f,θ) at different frequencies and different observation angles.

[0078] Under the same test environment, repeat steps S1 - 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 will be further described below in combination with simulation examples.

[0080] The simulation parameters are set as follows: Based on the actual reverberation chamber, use FEKO to establish a 1:1 reverberation chamber simulation model. Model the shielding cavity, stirrer, and periodic log-periodic antenna respectively, and calibrate the field uniformity. The size of the reverberation chamber cavity is 5.3 m × 2.9 m × 3 m (length × width × height). The transmitting antenna uses a log-dipole array, and the position of the transmitting antenna is aligned with the corner. The input power of the antenna is 100 W. The total height of the foam column and the turntable is 1.5 m, and the height of the horn antenna is also 1.5 m. The normal direction of the antenna points to the foam column, 3 m away from the center of the foam column. The frequency range of the signal transmitted by the vector network analyzer is 300 MHz to 1 GHz. As Figure 4 shown is the reverberation chamber simulation model established using FEKO. As Figure 5 shown is the RCS result of the target under far-field conditions. The abscissa represents the frequency, and the ordinate represents the RCS value.

[0081] The above are only the preferred embodiments of the present invention. It should be understood that the present invention is not limited to the form disclosed herein, should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications, and environments, and can be changed within the scope of the concept described herein through the above teachings or the technology or knowledge in related fields. And any changes and modifications made by those skilled in the art without departing from the spirit and scope of the present invention should fall within the protection scope of the appended claims of the present invention.

Claims

1. A method for measuring P-band RCS in a reverberation chamber, characterized in that, Including: S1. Calibrate the field uniformity of the reverberation chamber and set up the measurement environment; S2. Measure the echo data in the reverberation chamber without a target; The stirrer and antenna in the reverberation chamber remain unchanged, and the total height of the foam column and turntable is , the installation height of the horn antenna is , the antenna is normally pointed at the foam column, and is away from the center of the foam column. , where is the target size, is the signal wavelength. There is no target on the foam column. The vector network analyzer, computer and other test devices are set outside the reverberation chamber; The transmitting center frequency of the vector network analyzer , the step frequency , the number of sweep points of the broadband signal. The turntable rotates uniformly in steps of by degrees. That is the number of points in the azimuth direction. Measure the echo data . In this state, the scattered field comes from the antenna reflection coefficient and the backscattering of the reverberation chamber cavity. Then the echo data when the stirrer position is is as follows: ； where represents the echo data when the agitator position is , , is the frequency, is the frequency point number, is the reflection coefficient of the antenna in free space, is the antenna radiation efficiency, is the transfer function of the interaction between the reverberation chamber cavity wall and the log-periodic antenna, and the real and imaginary parts of this function are random variables obeying Gaussian distribution; S3. Measure the echo data in the reverberation chamber with a target; Place the test target on the foam column, and the vector network analyzer emits a broadband signal with a center frequency , a stepped frequency , and the number of sweep points . The turntable rotates uniformly in steps of degrees, and the measured echo data is . In this state, the magnetic field environment in the reverberation chamber changes. Only considering the change in the scattered field caused by the interaction between the target and the transmitting antenna, and ignoring the scattering between the target reflected wave and the reverberation chamber wall, when the position of the stirrer is , the BO approximation is used to describe the echo data as follows: ​ ； In the formula, represents using the BO approximation to describe the echo data, is the echo between the target and the antenna, is the RCS of the target at the frequency of and the incident angle of , where: ； Wherein, is the electromagnetic wave propagation speed, is the distance between the antenna and the target, is the initial phase of signals with different transmission frequencies; S4. Calibrate the echo data with a target using the echo data without a target; S5. Perform 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 shielding cavity. The rectangular cavity has dimensions of , and is equipped with a stirrer and a log-periodic antenna. The lowest usable frequency of the reverberation chamber is less than 300 MHz, and the lowest usable frequency is at least 3 to 4 times the lowest resonance frequency. The frequencies of the resonance modes are: ； In the formula, represents a calculation parameter, which is a non - negative integer, and only one of them can take the value of 0, with the unit of MHz, representing the frequency of the resonant mode, L, W, H respectively represent the length, width and height of the rectangular cavity size.

3. The P-band RCS measurement method in a reverberation chamber according to claim 2, wherein The calibration of the field uniformity of the reverberation chamber specifically includes: Adjust the arrangement of the stirrer and the antenna, change the structural design of the stirrer, the stirring method and the antenna design, and perform a field strength test every 100 MHz within the set frequency range to calibrate the field uniformity of the reverberation chamber. The specific method is as follows: Under the condition of the rotation of the stirrer, select 8 vertices in the working area, test and record the field strength values at 3 azimuths of each vertex, normalize the square root of the maximum field strength recorded at 3 azimuths of each point with respect to the average input power, and take the standard deviation.

4. The P-band RCS measurement method in a reverberation chamber according to claim 1, wherein Step S4 includes: Calculate the difference between the echo data with a target and the echo data without a target, and perform echo calibration according to the difference. Then the calibrated echo data is as follows: ； Let be a complex noise subject to a Gaussian distribution, then: 。 5. The P-band RCS measurement method in a reverberation chamber according to claim 1, characterized in that Step S5 includes: Apply windows in the azimuth and range directions to the calibrated echo data, and perform zero-padding in the frequency domain for the range direction of each azimuth to achieve interpolation. Read the range-direction echo data of each azimuth in turn and perform inverse Fourier transform respectively to obtain the one-dimensional range image; Divide a grid within the imaging area , calculate the distance between the antenna phase center and the grid points corresponding echo time delay , perform phase compensation on the pixel values of all grid points with the compensation amount, and perform coherent accumulation along the azimuth direction to obtain the two-dimensional image of the target.

6. 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, perform two-dimensional inverse Fourier transform, and interpolate and calculate the spectral domain data after the transform to obtain the target scattering coefficients at different frequencies , different observation angles ; ; Select the size of the calibration sphere and calculate the scattering coefficient of the calibration sphere , and obtain the measured target RCS value using the comparison method .

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