Method for reducing lowest available frequency of reverberation chamber

Abstract

The invention discloses a method for reducing the lowest available frequency of a reverberation chamber, and relates to the technical field of electromagnetic compatibility testing in a new generation of information technology, and the method comprises the following steps: S1, unit design and coding, S2, spherical surface type coding metasurface scatterer realization, S3, scatterer installation, S4, mode excitation and field uniformity improvement, and S5, LUF reduction realization. According to the invention, the small spherical surface type coding metasurface scatterer is additionally arranged in the RC, so that the effects of remarkably reducing the LUF, enhancing the uniformity of a low-frequency field, increasing the number of low-frequency modes and remarkably improving the low-frequency performance of the reverberation chamber with relatively low cost and modification are realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electromagnetic compatibility test technology in new generation information technology, in particular to the application of new artificial electromagnetic material in reverberation chamber, and more particularly to a method for reducing the lowest usable frequency of reverberation chamber. BACKGROUND

[0002] A reverberation chamber (RC) is a kind of metal shielded cavity for electromagnetic compatibility test, which can produce statistically uniform, isotropic and depolarized electromagnetic field by mode stirring or mode tuning through mechanical stirrers and antennas installed in the cavity. Generally, the reverberation chamber can only meet the requirement of field uniformity when the frequency is higher than a certain threshold, which is defined as the lowest usable frequency (LUF). The LUF can be understood as the lowest frequency required for the reverberation chamber to reach a predetermined standard of field uniformity (such as IEC 61000-4 specification). According to the standard requirements, at least 60 independent modes (satisfying Weyl formula) should be excited in the cavity at LUF, and usually LUF appears at about 3 to 6 times of the first resonant frequency of the cavity. For example, some documents require that the LUF should satisfy the condition that the total number of modes is greater than 100 and the mode density is more than 1.5 modes / MHz; in some cases, the LUF is about 5-6 times of the first resonant frequency, and the mode density should be greater than 1 mode / MHz. Due to the limitations of the size and quality factor of the reverberation chamber, the actual LUF depends on the specific cavity parameters, and often shows a higher value for small size RC.

[0003] In the under modal frequency band below LUF, the number of electromagnetic modes that can be excited in the reverberation chamber is insufficient, and the mode density is very low, resulting in a serious non-uniform field distribution and failing to meet the test requirements of statistical field uniformity. This is manifested as an increase in the standard deviation of field strength, i.e. the field uniformity becomes worse. In order to reduce the LUF of RC and increase the number of modes at low frequency band, the prior art proposes to introduce additional scatterers or change the boundary conditions in the cavity. For example, a metal spherical cap (spherical cap shape) scatterer is installed inside the RC to break the symmetry of the original field. Simulation results show that after adding the metal spherical cap, the boundary conditions of the inner wall of the RC change, the number of modes at low frequency band increases, and the field uniformity is improved to some extent. However, due to the fact that the traditional metal spherical scatterer itself only provides a single fixed phase of reflection, its frequency spectrum modulation capability is limited, and it cannot fully generate new independent modes, so it is difficult to significantly expand the low frequency working range of RC. In practice, simply relying on increasing the size of metal scatterers or stirrers to reduce LUF has limited effect, and the reverberation chamber still has the bottleneck of too high lowest usable frequency, insufficient number of modes and poor field uniformity at low frequency band, which needs to be solved urgently. SUMMARY

[0004] The main purpose of the present invention is to provide a method for reducing the lowest usable frequency of a reverberation chamber to solve the problems in the background technology.

[0005] To achieve the above objectives, the present invention is based on installing a specially designed spherical coding metasurface scatterer (Coding Spherical Cap Metasurface) inside the RC, and utilizing its multi-phase scattering characteristics to increase the number of modes in the low-frequency band and improve the field uniformity, thereby extending the lower limit of the available frequency of the reverberation chamber to low frequencies. The present invention uses digital coding metasurface technology to design a spherical cap-shaped ultra-thin scattering structure, including an array composed of several electromagnetic units. Each unit has a controllable reflection phase characteristic, and two discrete phase states are designed: 0° and 180° (π). These two types of units correspond to the digitally coded "0" and "1" binary states, respectively, and their electromagnetic responses can be precisely controlled by existing digital circuit technology. Based on this 1-bit (1-bit) coding unit, the present invention constructs three different patterns of spherical coding metasurface scatterers and installs them in the reverberation chamber. The present invention provides a method for reducing the lowest available frequency of a reverberation chamber, comprising the following steps:

[0006] S1. Unit design and encoding: Select a metasurface unit structure that meets the requirements of the predetermined frequency band. By adjusting the unit's geometric parameters or material properties, it produces two reflection phase responses with a 180° difference near the operating frequency (corresponding to the digital codes "0" and "1" respectively). Two units form a basic unit pair when the amplitude reflection coefficient is similar and the phase difference is a certain value.

[0007] S2. Implementation of Spherical Coded Metasurface Scatterers: Based on the above two units, the present invention designs three coded arrangement spherical metasurface scattering structures. First, a spherical substrate (spherical cap-shaped metal substrate) that matches the size of the RC cavity is fabricated. Multiple of the above units are arranged on the surface of the spherical substrate to form a coded metasurface scattering layer, including three typical coding arrangements:

[0008] Alternating arrangement: The two units are arranged on the surface of the spherical substrate in a periodic pattern of alternating "0 / 1", presenting a periodic coding sequence of 0101... / 0101... along two orthogonal directions. This regularly alternating coding pattern achieves a linear periodic variation of the unit phase on the spherical surface;

[0009] Checkerboard arrangement: The two units are arranged in a checkerboard pattern, with the coding sequences of adjacent rows inverted. The first row is coded 0101…, and the second row is coded 1010…, forming a two-dimensional periodic structure. This pattern alternates in two directions, forming a more complex periodic phase distribution.

[0010] Random arrangement: two kinds of units are distributed on the surface of the spherical base according to a pre-generated random sequence to form a one-bit random coding pattern without periodic rules. In this structure, the distribution of "0" and "1" units is approximately uniform but has no obvious rules, and the phase state of each unit is randomly distributed in space.

[0011] S3, scatterer installation: the above-mentioned spherical coding metasurface scatterer is fixedly installed at a proper position inside the reverberation chamber, the surface of the scatterer faces the inside of the cavity, and it is ensured that electromagnetic waves incident on the walls of the cavity and the stirrer can also be irradiated on the spherical metasurface;

[0012] S4, mode excitation and field uniformity improvement: under the normal working condition of the reverberation chamber, the introduced spherical coding metasurface scatterer produces additional multipath scattering and phase disturbance to the electromagnetic waves in the cavity. When broadband electromagnetic waves irradiate on the spherical metasurface, different reflection waves at different positions produce different phase differences due to the spatially coded phase distribution of the surface, thereby forming a more complex interference field inside the cavity. According to different coding patterns, the modulation effect of the scatterer on the electromagnetic waves is different.

[0013] S5, realization of LUF reduction: through the working mechanism in step S4, the spherical coding metasurface scatterer excites more electromagnetic modes at the low frequency band of the reverberation chamber. When the scatterer exists, the boundary conditions of the RC contain multiple phase distributions, which equivalently introduce additional degrees of freedom, so that new resonance modes appear in the originally undermoded frequency band. That is, the phase coding scatter structure not only acts as a diffuser, but also becomes a new mode source, so that the multi-mode and over-mode frequency bands of the RC are expanded to the low frequency direction. Compared with the original reverberation chamber, the introduced coding metasurface can meet the field uniformity requirements at a lower frequency.

[0014] Preferably, in step S1, the certain value of the phase difference is about π.

[0015] Preferably, in step S2, the spherical base adapted to the size of the RC cavity has a curvature radius of about 0.7 times (such as 0.7 m) the size of the cavity and a cross-sectional radius of about half (such as 0.55 m) the width of the cavity.

[0016] Preferably, in step S3, the proper position for fixedly installing the scatterer inside the reverberation chamber is a hole in the center of the top wall of the cavity for embedding installation, and the spherical cap protrudes into the inside of the cavity.

[0017] Preferably, in step S3, after the installation of the scatterer is completed, the remaining structures (such as antennas and mechanical stirrers) of the cavity remain unchanged to ensure compatibility with the existing test process.

[0018] Preferably in step S4, the different coding patterns are alternating coding, chessboard coding and random coding.

[0019] Preferably in step S4, the different modulation effects of the scatterers on electromagnetic waves are different, and the different modulation effects of the scatterers on electromagnetic waves are embodied in the following aspects: under normal incidence excitation, the alternating coding scatters the incident wave into two symmetrical main beams; the chessboard coding scatters the incident wave into four dispersed main beams; and the random coding scatters the incident energy into multiple stray beams with relatively small amplitudes.

[0020] Preferably in step S4, the different phase differences of the reflected waves in different positions are embodied in the following aspects: the unit resonant states on the spherical metasurface are periodically and repeatedly distributed in space, the phase of the scattered electric field alternately reverses between adjacent units, thereby destroying the equal-phase reflection characteristic of the planar metal surface; the two-dimensional chessboard coding introduces phase reversal periods in two orthogonal directions, so that the far-field scattered energy is distributed in more directions to form main lobes; and the random phase distribution disrupts the coherent superposition of the reflected waves, so that a scattering field that can be regarded as isotropic is generated inside the cavity.

[0021] Preferably in step S5, according to simulation and experimental results, the application of the spherical coding metasurface scatterer can reduce the LUF by 18%-22%.

[0022] Preferably in step S5, compared with the conventional case without loading or only loading metal scatterers, the standard deviation of the field strength at each measurement point significantly decreases in the low-frequency band after loading the spherical coding metasurface scatterer, and falls within the uniformity threshold specified in the IEC standard in a lower frequency range, which fully proves the effectiveness and feasibility of the spherical coding metasurface scatterer technical solution in expanding the low-frequency working range of the reverberation chamber and improving the field uniformity.

[0023] The beneficial effects of the technical solutions of the present application are: LUF is significantly reduced: by introducing a spherical coded metasurface scatterer with phase diversity in the reverberation chamber, the present application can effectively reduce the minimum usable frequency of the reverberation chamber, and simulation and experimental results show that the LUF reduction amplitude can reach about 20%; the low-frequency field uniformity is enhanced: the spherical coded metasurface scatterer can increase the randomness of the field in the low-frequency undermode range and reduce the non-uniformity of the field intensity distribution in the test space, which is manifested as a significant decrease in the field intensity standard deviation, and the reverberation chamber can meet the specification requirements of field uniformity at a lower frequency; the number of low-frequency modes is increased: unlike a single metal scatterer, the spherical coded metasurface scatterer generates multiple scattering paths through multi-phase coding, which can excite new resonance modes in the low-frequency band, so that the originally sparse mode in the low-frequency region of the reverberation chamber becomes a multi-mode state with more modes, effectively improving the mode density; the structure occupies a small space: the spherical coded metasurface scatterer of the present application is ultra-thin in thickness, and only slightly intrudes into the cavity after installation (for example, only about 5 cm deep), so it has little effect on the effective volume inside the reverberation chamber and does not significantly reduce the cavity Q value or introduce excessive loss; compatible with the original stirring mechanism: the spherical coded metasurface scatterer is fixed on the cavity wall without affecting the rotation and operation of the mechanical stirrer, and no modification is needed for the original stirring mechanism, the scatterer can work with the existing RC mechanical stirring mode to further enhance the randomness of the field while keeping the original test process unchanged. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is a specific implementation step flowchart of the present application;

[0025] Figure 2 is a schematic diagram of the mechanical stirring reverberation chamber structure of the present application with a metal spherical cap scatterer;

[0026] Figure 3 is a schematic diagram of three spherical coded metasurface scatterer structures of the present application;

[0027] Figure 4 is a simulation far-field pattern of various spherical coded metasurface scatterers of the present application;

[0028] Figure 5 is a near-field distribution comparison diagram of the electromagnetic wave modulation of the spherical coded metasurface scatterer of the present application;

[0029] Figure 6 is a comparison curve diagram of the field intensity standard deviation of the mechanical stirring reverberation chamber when loaded with and without a metal spherical cap scatterer;

[0030] Figure 7 is a field uniformity standard deviation versus frequency curve diagram of the reverberation chamber after loading the spherical coded metasurface scatterer of the present application. DETAILED DESCRIPTION

[0031] The specific embodiments of the present application are described in detail below with reference to the accompanying drawings:

[0032] In Figure 2 In the formula, L, W, H are the length, width and height of the reverberation chamber (1.2 m x 0.8 m x 1.2 m), R is the radius of the spherical cap scatterer (0.55 m), and the mechanical stirrer is composed of four metal blades with a thickness of 2 mm, each blade having a size of 0.3 m x 0.4 m, and the angle between adjacent blades being 120°; Figure 3 In the formula, (a) is an alternating coding pattern, and the coding sequence is "0101… / 0101…"; (b) is a chessboard coding pattern, and the coding sequence is "0101… / 1010…"; and (c) is a 1-bit random coding pattern. The above coding "0" and "1" correspond to two types of units with reflection phases of 0 and π, respectively; Figure 4 is the simulated far-field pattern of various spherical coding metasurfaces scatterers when the incident wave is vertically irradiated at a frequency of 700 MHz, wherein (a) is an alternating coding (#1), (b) is a chessboard coding (#2), (c) is a random coding (#3), and (d) is the far-field pattern of a traditional metal spherical cap scatterer; in Figure 5 In the formula, (a)-(d) are the electric field distributions measured in a plane perpendicular to the scatterer: (a) is an alternating coding #1; (b) is a chessboard coding #2; (c) is a random coding #3; and (d) is a metal spherical cap scatterer. (e)-(h) are the corresponding electric field distributions in another plane orthogonal to the aforementioned plane: (e) #1; (f) #2; (g) #3; and (h) metal spherical cap. The units on the surface of the scatterer exhibit different resonant states under different coding conditions, breaking the equal-phase reflection characteristic of the metal spherical scatterer, and making the scattered field more uniformly distributed in space; Figure 6 In the formula, (a) is the frequency range of 600 MHz to 1500 MHz; and (b) is an enlarged view of the frequency range of 600 MHz to 800 MHz; Figure 7 In the formula, (a)-(c) respectively give the simulation (solid line) and experimental (dashed line) curves of the standard deviation of the spatial field strength when using coding #1, #2 and #3 scatterers in the frequency band of 600 MHz-1500 MHz; and (d)-(f) are the enlarged curves corresponding to the low frequency band of 600 MHz-800 MHz. The results show that, compared with the case of not loading or only loading a metal scatterer, the standard deviation of the field strength in the reverberation chamber is significantly reduced after loading the spherical coding metasurface scatterer, especially near about 700 MHz, which is below the specification threshold, indicating that the new mode is excited in this frequency band, making the multi-mode working state of the reverberation chamber advance to a lower frequency. The trend of the experimental curve is consistent with the simulation result, verifying the effect of the spherical coding metasurface scatterer in reducing the LUF and improving the field uniformity.

[0033] Embodiment: A technique for reducing the lowest usable frequency (LUF) of a reverberation chamber by introducing a spherical cap coding metasurface scatterer

[0034] As shown in Figures 1-7 , the present application provides a technical solution for reducing the lowest usable frequency (LUF) of a reverberation chamber by introducing a spherical cap coding metasurface scatterer. This solution is based on the installation of a specially designed spherical cap coding metasurface inside the RC, which uses its multi-phase scattering characteristics to increase the number of modes in the low frequency band and improve field uniformity, thereby expanding the lower limit of the usable frequency of the reverberation chamber to low frequencies. To achieve the above purpose, the present application uses digital coding metasurface technology to design a spherical cap-shaped ultra-thin scattering structure composed of an array of several electromagnetic units. Each unit has controllable reflection phase characteristics, and two discrete phase states are designed: 0° and 180° (π). These two types of units correspond to the binary states of digital coding "0" and "1", and their electromagnetic responses can be accurately controlled through existing digital circuit technology. Based on this 1-bit coding unit, the present application constructs three different patterns of spherical cap coding metasurface scatterers and places them in the reverberation chamber, as shown in Figure 1 , and the specific technical solution steps are as follows (see Figure 2 ):

[0035] S1, unit design and coding: Select a metasurface unit structure that meets the requirements of the predetermined frequency band, and adjust the unit's geometric parameters or material properties to produce two reflection phase responses with a phase difference of 180° near the working frequency (corresponding to digital coding "0" and "1"). The design method of this 1-bit coding unit is referenced from existing digital metasurface technology literature. Two units form a basic unit pair when their amplitude reflection coefficients are similar and their phase difference is approximately π.

[0036] S2, spherical cap coding metasurface scatterer implementation: Based on the above two units, the present application designs three coding arrangements of spherical metasurface scattering structures (numbered #1, #2, #3). First, a spherical base (spherical cap-shaped metal substrate) is made to match the size of the RC cavity, with a curvature radius of about 0.7 times the cavity size (e.g. 0.7m) and a cross-sectional radius of about half the cavity width (e.g. 0.55m). Arrange multiple units on the surface of the spherical base to form a spherical cap coding metasurface scatterer. Three typical coding patterns are shown in Figure 3 :

[0037] Alternating arrangement (#1): The two units are arranged in a "0 / 1" alternating and repeating periodic pattern on the surface of the spherical base, and both show a periodic coding sequence of 0101… / 0101… in two orthogonal directions. This regular alternating coding pattern realizes a linear periodic change in the phase of the units on the spherical surface.

[0038] Checkerboard arrangement (#2): Two types of cells are arranged in a checkerboard-like pattern, with the coding sequences in adjacent rows reversed. For example, the first row is coded 0101…, and the second row is coded 1010…, forming a two-dimensional periodic structure. This pattern alternates in both directions, creating a more complex periodic phase distribution.

[0039] Random arrangement (#3): Two types of units are distributed across the surface of a spherical substrate according to a pre-generated random sequence, forming a one-bit random coding pattern with no periodicity. In this structure, the distribution of "0" and "1" units is approximately uniform but has no discernible pattern, and the phase state of each unit is randomly distributed in space.

[0040] S3. Scatter installation: The above-mentioned spherical coded metasurface scatterer is fixedly installed in an appropriate position inside the reverberation chamber, for example, it is embedded in the central hole of the top wall of the cavity so that the spherical cap protrudes a certain height from the inside of the cavity. As in the embodiment, the spherical coded metasurface scatterer extends into the cavity by about 52.62 mm in the vertical direction. This installation method ensures that the scatterer is firmly fixed and occupies only a very small internal volume, and does not significantly affect the size of the cavity itself and the original equipment layout. The surface of the scatterer faces the inside of the cavity, ensuring that the electromagnetic waves incident on the cavity wall and the stirrer can also be irradiated on the spherical metasurface. After the installation is completed, the remaining structures of the cavity (such as antennas and mechanical stirrers) remain in their original state to ensure compatibility with existing test processes.

[0041] S4. Improvement of mode excitation and field uniformity: When the reverberation chamber is operating normally (the stirrer rotates and stirs the field shape), the introduced spherical coded metasurface scatterer produces additional multipath scattering and phase disturbance effects on the electromagnetic waves in the cavity. When a broadband electromagnetic wave is irradiated on the spherical metasurface, due to the spatially coded phase distribution of the surface, the reflected waves at different positions produce different phase differences, thereby forming a more complex interference field inside the cavity. Depending on the different coding patterns, the modulation effect of the scatterer on the electromagnetic wave varies. For example, under normal incidence excitation:

[0042] Alternating encoding (#1) scatters the incident wave primarily into two symmetrical main beams. This is due to the one-dimensional periodic variation in the surface phase, which results in interference enhancement in two main directions in the far-field scattering pattern. Near-field observations show that under this encoding, the unit resonant states on the spherical metasurface are spatially distributed and periodically repeated, and the scattered electric field phase alternately reverses between adjacent units, thus destroying the isophase reflection characteristics of the planar metal surface.

[0043] The chessboard coding (#2) scatters the incident wave into four dispersed main beams. Since the two-dimensional chessboard coding introduces phase inversion period in both orthogonal directions, the far-field scattering energy is distributed in more directions to form main lobes. This structure produces more abundant scattering angles relative to #1. The near-field results show that there are various resonant modes between different units, and a more complex local field distribution is generated relative to the alternative coding.

[0044] The random coding (#3) scatters the incident energy into multiple small-amplitude stray beams. Since the surface phase distribution is random, there is no significant concentrated main lobe in the far-field pattern, but there is a certain scattering energy in each direction, forming a highly diffuse scattering field. This random phase distribution maximizes the disruption of the coherent superposition of reflected waves, resulting in a nearly isotropic scattering field inside the cavity.

[0045] The differences in the above scattering characteristics can be verified from Figure 4 and Figure 5 Among them, Figure 4 gives the far-field scattering pattern of the spherical coding metasurface scatterer under 700MHz normal incidence, compared with the scattering of the traditional metal spherical cap; Figure 4 shows the electric field intensity distribution of a certain observation plane near the surface of the spherical coding metasurface scatterer. As can be seen from Figure 5 , the spherical metasurface units under different coding conditions exhibit different resonant states, and the phase distribution of the reflected field is significantly disrupted. In comparison, the traditional metal spherical cap has a single metal phase on the surface, and the reflected wave remains essentially in phase in the local area, so the scattering direction is single. The phase diversity of the spherical coding metasurface effectively breaks this limitation, changing the statistical properties of the electromagnetic field inside the cavity - the reflected field is more randomized and uniform.

[0046] S5, realization of reducing LUF: through the working mechanism in step four, the spherical coding metasurface scatterer excites more electromagnetic modes at low frequencies in the reverberation chamber. When the scatterer exists, the boundary conditions of the RC contain multiple phase distributions, which equivalently introduce additional degrees of freedom, making the originally undermoded frequency band appear new resonant modes. That is, this phase spherical coding metasurface not only acts as a diffuser, but also becomes a new mode generation source, making the multi-mode and over-mode frequency bands of the RC expand to lower frequencies. Compared to the original reverberation chamber, the reverberation chamber can meet the field uniformity requirements at lower frequencies after introducing the spherical coding metasurface. According to the simulation and experimental results statistics, the application of the spherical coding metasurface can reduce the LUF by about 20%. Figure 6 gives the curve results of the standard deviation of the field uniformity of the reverberation chamber without loading or loading the traditional metal spherical cap scatterer with frequency, Figure 7The standard deviation of field uniformity of the reverberation chamber loaded with different coded scatterers is given, and the simulation and measurement results are compared. Figure 6 and Figure 7 It can be seen from the comparison of the curve results that, compared with the traditional case without loading or only loading metal scatterers, the standard deviation of field strength at each measurement point after loading the spherical coded metasurface scatterer decreases significantly in the low frequency band, and falls within the uniformity threshold specified by the IEC standard in a lower frequency range. This fully proves the effectiveness and feasibility of the spherical coded metasurface scatterer technical scheme in expanding the low frequency working range of the reverberation chamber and improving the field uniformity.

[0047] In view of the problem of insufficient low frequency mode of the reverberation chamber, the application introduces a spherical coded metasurface scatterer with phase diversity into the cavity. By digitally coding the reflection phase distribution of the scatterer surface, the number of effective resonance modes in the low frequency band is increased and the field uniformity is enhanced, so as to greatly reduce the minimum available frequency of the reverberation chamber; in view of the single frequency spectrum characteristic of the traditional metal spherical scattering structure, the application uses digital unit coding technology to construct an array of scattering units with multiple phase states, and forms a rich spatial phase coding pattern on the spherical coded metasurface scatterer. The structure provides multiple scattering paths for electromagnetic waves, optimizes the randomness and statistical characteristics of the field inside the reverberation chamber, and realizes the low frequency expansion effect that the traditional RC cannot achieve. By adding a small spherical coded metasurface scatterer in the RC, the application realizes the effect of significantly improving the low frequency performance of the reverberation chamber at a lower cost and with a small change, and has important engineering practical value.

Claims

1. A method for reducing the lowest available frequency of a reverberation chamber, characterized in that: The following steps are involved: S1. Unit design and encoding: Select a metasurface unit structure that meets the requirements of the predetermined frequency band. By adjusting the unit geometric parameters or material properties, it produces two reflection phase responses with a 180° difference near the operating frequency. Two units form a basic unit pair when the ratio of the amplitude reflection coefficient is between 0.9 and 1.0 and the phase difference is π; S2. Implementation of Spherical Coded Metasurface Scatterers: Based on the two units mentioned above, the present invention designs three spherical metasurface scattering structures with coded arrangements. First, a spherical substrate that matches the size of the RC cavity is fabricated, and at least two of the above units are arranged on the surface of the spherical substrate to form a spherical coded metasurface scatterer, including three typical codes: Alternating arrangement: The two units are arranged on the surface of the spherical substrate in a periodic pattern of alternating "0 / 1," presenting a periodic coding sequence of 0101... / 0101... in two orthogonal directions. This regularly alternating coding pattern achieves a linear periodic variation in the unit phase on the spherical surface. Checkerboard arrangement: The two units are arranged in a checkerboard pattern, with the coding sequences of adjacent rows inverted. The first row is coded 0101…, and the second row is coded 1010…, forming a two-dimensional periodic structure. This pattern alternates in two directions, forming a more complex periodic phase distribution. Random arrangement: The two units are distributed on the surface of the spherical substrate according to a pre-generated random sequence, forming a one-bit random coding pattern with no periodicity. In this structure, the distribution of "0" and "1" units is approximately uniform but has no obvious pattern, and the phase state of each unit is randomly distributed in space. S3. Scatterer installation: The spherical coded metasurface scatterer is fixedly installed at an appropriate position inside the reverberation chamber, with the scatterer surface facing the interior of the cavity, ensuring that electromagnetic waves incident on the cavity walls and the stirrer can also irradiate the spherical metasurface; S4. Mode excitation and field uniformity improvement: When the reverberation chamber is operating normally, the introduced spherical coded metasurface scatterer produces additional multipath scattering and phase perturbations on the electromagnetic waves in the cavity. When broadband electromagnetic waves are irradiated by the spherical metasurface, due to the spatially coded phase distribution of the surface, the reflected waves at different positions have different phase differences, thus forming a more complex interference field inside the cavity. Depending on the different coding patterns, the modulation effect of the scatterer on the electromagnetic wave varies. S5. Implementation of LUF reduction: Through the working mechanism in step S4, the spherical coded metasurface scatterer excites more electromagnetic modes in the low-frequency band of the reverberation chamber. When the scatterer exists, the boundary conditions of the RC include at least two phase distributions, which effectively introduces additional degrees of freedom, causing new resonant modes to appear in the originally under-modal frequency band. In other words, the phase-coded scattering structure not only acts as a diffuser, but also becomes a new mode generation source, expanding the multi-modal and over-modal frequency bands of the RC toward the low-frequency direction. Compared with the original reverberation chamber, after the introduction of the coded metasurface, the reverberation chamber can meet the field uniformity requirements at lower frequencies.

2. The method for reducing the lowest usable frequency of a reverberation chamber according to claim 1, characterized in that: In step S2, the spherical substrate adapted to the size of the RC cavity has a substrate curvature radius of 0.6-0.8 times the size of the cavity, and a substrate cross-sectional radius of 0.4-0.6 times the width of the cavity.

3. The method for reducing the lowest usable frequency of a reverberation chamber according to claim 1, characterized in that: In step S3, the appropriate position for fixing the device inside the reverberation chamber is to be embedded in the central opening of the top wall of the chamber and to make the spherical cap protrude from the interior of the chamber.

4. The method for reducing the lowest usable frequency of a reverberation chamber according to claim 1, characterized in that: In step S3, after the scatterer is installed, the layout of the original equipment in the cavity remains unchanged to ensure compatibility with the existing test process.

5. The method for reducing the lowest usable frequency of a reverberation chamber according to claim 1, characterized in that: In step S4, the different coding patterns are alternating coding, checkerboard coding and random coding.

6. The method for reducing the lowest usable frequency of a reverberation chamber according to claim 1, characterized in that: In step S4, the modulation effect of the scatterer on the electromagnetic wave is different. Under normal incident excitation, the alternating coding will mainly scatter the incident wave into two symmetrical main beams; the checkerboard coding will scatter the incident wave into four scattered main beams; and the random coding will scatter the incident energy into at least two stray beams with relatively small amplitudes.

7. The method for reducing the lowest usable frequency of a reverberation chamber according to claim 1, characterized in that: In step S4, the reflected waves at different positions produce different phase differences, which is reflected in the unit resonance state on the spherical metasurface being periodically repeated in space, and the scattered electric field phase is alternately reversed between adjacent units, thereby destroying the equal-phase reflection characteristics of the planar metal surface; the two-dimensional checkerboard coding introduces phase inversion periods in two orthogonal directions, so that the far-field scattering energy distribution forms a main lobe in at least two directions; the random phase distribution disrupts the coherent superposition of the reflected waves, so that a scattering field that can be regarded as isotropic is generated inside the cavity.

8. The method for reducing the lowest usable frequency of a reverberation chamber according to claim 1, characterized in that: In step S5, according to simulation and experimental results, the application of spherical coding metasurface scatterers can reduce the LUF by 18%-22%.

9. The method for reducing the lowest usable frequency of a reverberation chamber according to claim 1, characterized in that: In step S5, compared with the traditional case of no loading or only loading of metal scatterers, the standard deviation of the field intensity at each measurement point after loading the spherical coded metasurface scatterer falls within the uniformity threshold specified by the IEC standard in the frequency range below 700 MHz. This fully demonstrates the effectiveness and feasibility of the coded spherical metasurface technology solution in expanding the low-frequency operating range of the reverberation chamber and improving the field uniformity.

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