Calculation method and system for effective shielding frequency band of trepanned shielding cavity

Abstract

The invention provides a calculation method for an effective shielding section of a perforated shielding cavity, and relates to the field of shielding of perforated shielding cavities. The method comprises the following steps: S1, establishing a space rectangular coordinate system by taking a lower left corner of a perforated surface of a perforated shielding cavity as an origin of coordinates; s2, constructing an equivalent circuit and a signal flow diagram of the trepanned shielding cavity, and obtaining a BLT calculation model of the trepanned shielding cavity; s3, according to the BLT calculation model, obtaining the TE mode voltage of the observation point in the cavity, and calculating the shielding effectiveness of the point; s4, equidistant valuing is performed on the observation frequency bands, the shielding effectiveness of the observation points in the cavity under each frequency is calculated, one frequency is selected, and the new shielding effectiveness of the frequency is calculated by adopting a symmetric window moving average method; and S5, comparing the new shielding effectiveness with the expected shielding effectiveness to obtain the effective shielding frequency band of the trepanned shielding cavity. According to the method, the symmetric window moving average method is utilized, so that the problem that the calculation of the effective shielding frequency band is influenced due to the fact that a shielding effectiveness curve obtained in the prior art often has a plurality of abnormal fluctuations can be effectively solved.

Description

Technical Field

[0001] The present invention relates to the technical field of shielding of an open-hole shielding cavity, and in particular to a method and system for calculating an effective shielding frequency band of an open-hole shielding cavity. Background Art

[0002] Electromagnetic shielding is a technology that blocks the propagation of electromagnetic fields by placing conductive materials around electronic devices. Its purpose is to prevent internal and external devices from being interfered with by electromagnetic waves. In practical applications, due to heat dissipation, wiring or other functional requirements, the shielding cavity usually needs to have holes or vents, and these holes will become channels for electromagnetic wave leakage, significantly reducing the shielding effectiveness. Determining the effective shielding frequency band refers to finding the frequency range within a certain frequency range where the shielding cavity can maintain a sufficiently high shielding effectiveness. When designing an open cavity, it is usually hoped that the shielding effectiveness will remain above a certain threshold within the critical frequency range of the device.

[0003] At present, the research on the shielding technology of the perforated shielding cavity is focused on its shielding effectiveness. There are two methods for calculating the shielding effectiveness: numerical method and analytical method. Numerical methods such as the finite difference time domain method, transmission line matrix model and moment method have the advantage of high precision, but the calculation will occupy a lot of computing resources and take a lot of time; analytical methods such as the equivalent circuit method, small hole coupling theory and multi-mode intermediate level equivalent circuit model have the advantages of simple modeling and high calculation efficiency. The shielding problem of the perforated shielding cavity can be quickly analyzed within a certain accuracy range. However, when the analytical method is used to solve the shielding effectiveness curve, due to external interference sources, that is, electromagnetic waves entering the cavity through the opening coupling will produce high-order modes and the resonance effect of electromagnetic waves in the cavity, the shielding effectiveness curve will have sudden fluctuations. These fluctuations will cause the effective shielding frequency band to be discontinuously distributed, and the shielding effectiveness will be abnormal only at the fluctuation point, while the shielding effectiveness in the adjacent frequency band meets the requirements. This situation will affect the calculation of the effective shielding frequency band, thereby limiting the scope of use of the shielding cavity.

[0004] At present, the research on the shielding technology of perforated shielding cavities focuses on its shielding effectiveness. For example, the application number is 202410164951.6, which discloses a shielding effectiveness calculation method for a multi-layer cavity with holes based on the BLT equation. The shielding effectiveness calculation method for a multi-layer cavity with holes based on the BLT equation is used to calculate the shielding effectiveness of a multi-layer shielding cavity, which can meet the shielding effectiveness calculation requirements of a multi-layer shielding scheme for a radio telescope. However, it still does not solve the problem that when using analytical methods to solve the shielding effectiveness curve, several abnormal fluctuations will occur, affecting the calculation of the effective shielding frequency band. Summary of the invention

[0005] The technical problem to be solved by the present invention is how to solve the problem that when using analytical methods to calculate shielding effectiveness, the shielding effectiveness curve obtained will have several abnormal fluctuations, thereby affecting the calculation of the effective shielding frequency band.

[0006] The present invention solves the above-mentioned problem by the following technical means: a method for calculating the effective shielding frequency band of an open-hole shielding cavity, comprising the following steps:

[0007] S1: Establish a spatial rectangular coordinate system with the lower left corner of the opening surface of the opening shielding cavity as the coordinate origin;

[0008] S2: Construct the equivalent circuit of the perforated shielding cavity and its signal flow graph, and obtain the BLT calculation model of the perforated shielding cavity;

[0009] S3: Calculate the TE mode voltage of the observation point in the cavity according to the BLT calculation model and calculate the shielding effectiveness of the point;

[0010] S4: Take values ​​of the observation frequency band at equal intervals, calculate the shielding effectiveness of the observation point in the cavity at each frequency, select one of the frequencies, and use the moving average method of the symmetrical window to calculate the new shielding effectiveness of the point;

[0011] S5: Compare the new shielding effectiveness of the observation point in the cavity with the lower limit of the expected shielding effectiveness to obtain the effective shielding frequency band of the open-hole shielding cavity.

[0012] Preferably, in step S1, the dimensions of the perforated shielding cavity are: length a, width d, height b; there is an opening on the perforated surface, and the dimensions of the opening are: length l, width w.

[0013] Preferably, in step S2, the equivalent circuit includes an equivalent opening impedance Z ap , Where a is the length of the perforated shielding cavity, b is the height of the perforated shielding cavity, l is the length of the opening size, k0 is the propagation constant of the free space outside the cavity, j is the imaginary unit, Where t is the thickness of the aperture shielding cavity material, and w is the width of the aperture size.

[0014] Preferably, in step S2, the equivalent circuit also includes the characteristic impedance Z of the cavity interior space g , Where z0 is the characteristic impedance of the free space outside the cavity, λ is the wavelength of the plane wave, and m and n are the rectangular waveguide mode indices.

[0015] The equivalent circuit and signal flow diagram constructed by the present invention can modify relevant parameters according to different opening shielding cavity sizes and different opening shapes and opening positions, and has high universality and is fast.

[0016] Preferably, in step S2, the BLT calculation model of the open hole shielding cavity is: V r,s =(S+I)(PS) -1 U, where V r,s is the node J on the pipeline r in the signal flow graph s The full voltage of node J s The scattering matrix of the pipeline r is U = [V s 0 0 0 0 0] T , Vs is the plane wave equivalent voltage source.

[0017] Preferably, in step S3, the formula for calculating shielding effectiveness is: Where SE(f) represents the frequency efficiency corresponding to the observation point Q in the cavity when the frequency is f, V Q is the TE mode voltage at the observation point Q in the cavity.

[0018] Preferably, the specific process of step S4 is:

[0019] S401: Take equidistant values ​​of the observed frequency band to obtain {f1,f2,...,f n}, calculate the shielding effectiveness {SE(f1), SE(f2),..., SE(f n )};

[0020] S402: Find the shielding effectiveness SE(fi) corresponding to the frequency fi, and then find the shielding effectiveness SE(fi) corresponding to the frequency {f i-k ,...f i-1 ,f i+1 ,...f i+k}corresponding frequency efficiency {SE(f i-k ),...SE(f i-1 ),SE(f i+1 ),...SE(f i+k )};

[0021] S403: Calculate frequency efficiency {SE(f i-k ),...SE(f i-1 ),SE(f i ),SE(f i+1 ),...SE(f i+k )}, as the average value of the frequency f i Up-to-date shielding effectiveness.

[0022] Preferably, in step S403, the frequency efficiency {SE(f i-k ),...SE(f i-1 ),SE(f i ),SE(fi+1 ),...SE(f i+k The formula for the average value of )} is:

[0023] The present invention uses a symmetrical window to reduce the lag effect caused by the moving average method, and can effectively solve the problem of abnormal fluctuations in shielding effectiveness in the analytical method while retaining the overall trend of the data, thereby affecting the calculation of the effective shielding frequency band.

[0024] Preferably, the specific process of step S5 is: if the frequency is f i The new shielding effectiveness SE MA (f i ) is greater than the expected shielding effectiveness lower limit SE min , that is, if SE MA (f i )>SE min , then the frequency f is retained i , and the frequency {f i-k ,...f i-1 ,f i ,f i+1 ,...f i+k}As the effective shielding frequency band of the open-hole shielding cavity within the observation frequency band.

[0025] Corresponding to the above method, the present invention also provides a calculation system for the effective shielding section of an open shielding cavity, comprising the following modules:

[0026] System building module: used to establish a spatial rectangular coordinate system with the lower left corner of the opening surface of the opening shielding cavity as the coordinate origin;

[0027] Obtain BLT calculation model module: used to construct the equivalent circuit of the perforated shielding cavity and its signal flow graph, and obtain the BLT calculation model of the perforated shielding cavity;

[0028] Shielding effectiveness calculation module: used to calculate the TE mode voltage of the observation point in the cavity according to the BLT calculation model and calculate the shielding effectiveness of the point;

[0029] Obtaining new shielding effectiveness module: used to take values ​​of the observation frequency band at equal intervals, calculate the shielding effectiveness of the observation point in the cavity at each frequency, select one of the frequencies, and use the moving average method of the symmetrical window to calculate the new shielding effectiveness of the point;

[0030] Filtering module: used to compare the new shielding effectiveness of the observation point in the cavity with the expected lower limit of shielding effectiveness, and obtain the effective shielding frequency band of the open shielding cavity.

[0031] The advantages of the present invention are:

[0032] (1) The present invention uses a symmetrical window to reduce the hysteresis effect caused by the moving average method, and can effectively solve the problem of abnormal fluctuations in solving the shielding effectiveness curve in the analytical method while retaining the overall trend of the data, thereby affecting the calculation of the effective shielding frequency band. That is, the shielding effectiveness curve obtained by the present invention is relatively smooth, and there is no abnormal shielding effectiveness at the fluctuation point, while the shielding effectiveness in the adjacent frequency band meets the shielding requirements, which is more conducive to obtaining an effective shielding frequency band. After obtaining the effective shielding frequency band, the effective shielding frequency band obtained can be used to assist the design of the cavity structure in the early stage of the design of the open-hole shielding cavity structure;

[0033] (2) The equivalent aperture impedance is obtained according to the aperture size and aperture position of the aperture shielding cavity, and the equivalent circuit and signal flow graph of the aperture shielding cavity are constructed by combining the cavity size, plane wave excitation source, and the characteristic impedance and propagation constant of the space inside and outside the cavity. The constructed equivalent circuit and signal flow graph can modify the relevant parameters according to different aperture shielding cavity sizes and different aperture shapes and aperture positions, and has the advantages of universality and rapidity;

[0034] (3) The present invention also compares the moving average result of the symmetric window with the expected shielding effectiveness lower limit. When the moving average result of the symmetric window is SE MA (f i )>SE min When the corresponding frequency f is retained i , taking the continuous frequencies as a combination, it is the effective shielding frequency band of the open-hole shielding cavity within the combined frequency band. If there are multiple combinations, it means that the open-hole shielding cavity has multiple effective shielding frequency bands, which improves the calculation speed of the effective shielding frequency band of the open-hole shielding cavity. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a flow chart of Embodiment 1 of the present invention;

[0036] Figure 2 This is a schematic diagram of the structure of the plane wave irradiation aperture shielding cavity in Example 1 of the present invention;

[0037] Figure 3 is an equivalent circuit of the plane wave irradiated aperture shielding cavity in Example 1 of the present invention;

[0038] Figure 4 This is a signal flow diagram of a plane wave irradiating an aperture shielding cavity in Example 1 of the present invention;

[0039] Figure 5 This is a comparison diagram of shielding effectiveness curves before and after using the symmetric window moving average method in Example 1 of the present invention;

[0040] Figure 6 This is a calculation flow chart of the effective shielding frequency band in Example 1 of the present invention. DETAILED DESCRIPTION

[0041] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] Example 1

[0043] like Figure 1 FIG. 1 is a flow chart of an embodiment of the present invention. The present invention provides a method for calculating an effective shielding section of an open shielding cavity, comprising the following steps:

[0044] S1: Figure 2 As shown, the lower left corner of the opening surface of the opening shielding cavity is used as the coordinate origin, the length of the cavity is the x-axis, and the height is the y-axis to establish a spatial rectangular coordinate system; the length of the opening is l, the width is w, and the coordinates of the center point of the opening are (x a ,y a , 0); the length of the cavity is a, the width is d, the height is b, and the thickness of the cavity material is t;

[0045] The incident plane wave propagates along the -z axis, and its electric field is parallel to the y axis; the characteristic impedance of the free space outside the cavity is Z0≈377Ω, and the propagation constant λ is the wavelength of the plane wave;

[0046] The internal space of the cavity is regarded as a rectangular waveguide with a short-circuited terminal, and its characteristic impedance is Z gmn , the propagation constant is k gmn , the expression is:

[0047]

[0048] Where m and n are the rectangular waveguide mode indices.

[0049] S2: Figure 3 As shown in the figure, the equivalent circuit of the perforated shielded cavity is constructed. The equivalent circuit consists of a plane wave equivalent voltage source V s , equivalent opening impedance Z ap , the distance d1 from the monitoring point outside the cavity to the opening, the distance d2 from the observation point Q inside the cavity to the opening, the distance d3 from the observation point Q inside the cavity to the end of the cavity, the characteristic impedance Z0 of the free space outside the cavity and its propagation constant k0, the characteristic impedance Z of the space inside the cavity g and its propagation constant k g and the voltage response V at the observation point Q inside the cavity QThe opening is equivalent to a coplanar strip transmission line with short circuits at both ends;

[0050] Equivalent aperture impedance Z ap The expression is:

[0051]

[0052] Where j is the imaginary unit,

[0053] like Figure 4 As shown, the signal flow graph of the equivalent circuit is constructed. The signal flow graph is based on the incident voltage of the node Js on the node, the pipeline and the pipeline r. and reflected voltage The structure is to reflect the voltage and current response of the impedance end, where the selection of the node depends on the position where the voltage needs to be observed in the equivalent circuit, and the pipeline is the channel for signal transmission between nodes;

[0054] tube1 represents the free space transmission channel outside the cavity, tube2 and 3 represent the rectangular waveguide transmission channels, node J1 represents the observation point outside the cavity, V s The node represents the plane wave equivalent voltage source, the J2 node represents the cavity opening, the J3 node represents the observation point in the cavity, and the J4 node represents the end of the cavity;

[0055] The transmission equation, scattering equation and full voltage equation are obtained from the equivalent circuit and signal flow graph, which are:

[0056]

[0057]

[0058] Combining formulas (5) to (7), the BLT calculation model of the open-hole shielding cavity is obtained as follows:

[0059] V r,s =(S+I)(PS) -1 U (8)

[0060] Where V r,s is the node J on the pipeline r in the signal flow graph s The total voltage of the node J is s is the scattering matrix of , and U is the node voltage source vector.

[0061] S3: According to the BLT calculation model, the TE mode voltage of the observation point Q in the cavity is obtained, and the shielding effectiveness of this point is calculated. In this embodiment, P = diag[T1 T2 T3], S = diag[ρ1 ρ2 ρ3 ρ4], U = [U0 0 0 0 0 0] T , where U0 = Vs ,

[0062]

[0063] Where, Y0, Y g , Y ap is Z0, Z gmn , Z ap The reciprocal of . Known frequency Where c is the speed of light and λ is the wavelength of the plane wave. Substituting the above expression into equation (8), we can obtain the TE mode voltage V at the observation point Q in the cavity: Q , V Q Substitute into the shielding effectiveness calculation formula: Obtain the shielding effectiveness of the observation point Q in the cavity at the corresponding frequency f.

[0064] S4: The moving average method of the symmetrical window is used to solve the new shielding effectiveness of the observation point in the cavity at a certain frequency. The specific process is:

[0065] S401: Take equidistant values ​​of the observed frequency band to obtain {f1,f2,...,f n}, calculate the shielding effectiveness {SE(f1), SE(f2),..., SE(f n )};

[0066] S402: Find the shielding effectiveness SE(fi) corresponding to the frequency fi, and then find the shielding effectiveness SE(fi) corresponding to the frequency {f i-k ,...f i-1 ,f i+1 ,...f i+k}The corresponding shielding effectiveness {SE(f i-k ),...SE(f i-1 ),SE(f i+1 ),...SE(f i+k )};

[0067] S403: Use formula Calculate the shielding effectiveness {SE(f i-k ),...SE(f i-1 ),SE(f i ),SE(f i+1 ),...SE(f i+k )} is taken as the new shielding effectiveness at frequency fi.

[0068] like Figure 5 The figure shows the comparison of shielding effectiveness curves before and after using the symmetric window moving average method.

[0069] S5: Figure 6The figure shows the calculation flow chart of the effective frequency band of the perforated shielding cavity. The specific process is as follows: Set the desired shielding effectiveness lower limit SE min , if the frequency obtained in step S4 is f i When the new shielding effectiveness is greater than the shielding effectiveness lower limit, that is, if SE MA (f i )>SE min , then the frequency f is retained i , and the frequency {f i-k ,...f i-1 ,f i ,f i+1 ,...f i+k}The effective shielding frequency band within the observation frequency band as an open-hole shielding cavity;

[0070] The obtained effective shielding frequency band is Figure 5 By comparison, it can be verified that a method for calculating the effective shielding frequency band of an open-hole shielding cavity can calculate the effective shielding frequency band of the open-hole shielding cavity and meet the set SE min Require.

[0071] This implementation takes advantage of the moving average method of the symmetric window to make the shielding effectiveness curve obtained smoother, eliminate the abnormal fluctuations in the shielding effectiveness curve, make the effective shielding frequency band distribution more continuous, and is more conducive to obtaining the effective shielding frequency band; after obtaining the effective shielding frequency band, it is possible to assist the design of the cavity structure in the early stage of the design of the open-hole shielding cavity structure by obtaining the effective shielding frequency band.

[0072] Example 2

[0073] Corresponding to the above-mentioned embodiment 1, this embodiment 2 further provides an effective shielding frequency band calculation system for an open-hole shielding cavity, comprising the following modules:

[0074] System building module: used to establish a spatial rectangular coordinate system with the lower left corner of the opening surface of the perforated shielding cavity as the coordinate origin, the length of the cavity as the x-axis, and the height as the y-axis, and obtain the characteristic impedance Z of the internal space of the cavity gmn and the propagation constant k gmn ;

[0075] Obtain BLT calculation model module: used to construct the equivalent circuit and signal flow graph of the perforated shielding cavity and obtain the BLT calculation model of the perforated shielding cavity, which specifically includes the following units:

[0076] Constructing equivalent circuit unit: used to construct an equivalent circuit, the equivalent circuit consists of a plane wave equivalent voltage source V s , equivalent opening impedance Z ap, the distance d1 from the monitoring point outside the cavity to the opening, the distance d2 from the observation point Q inside the cavity to the opening, the distance d3 from the observation point Q inside the cavity to the end of the cavity, the characteristic impedance Z0 of the free space outside the cavity and its propagation constant k0, the characteristic impedance Z of the space inside the cavity g and its propagation constant k g and the voltage response V at the observation point Q inside the cavity Q constitute;

[0077] Construct signal flow graph unit: used to construct signal flow graph, signal flow graph is composed of nodes, pipelines and nodes J on pipeline r s The incident voltage and reflected voltage constitute;

[0078] BLT calculation model unit: used to obtain the BLT calculation model V of the open shielding cavity based on the equivalent circuit and signal flow graph r,s =(S+I)(PS) -1 U, where V r,s is the node J on the pipeline r in the signal flow graph s The total voltage of the node J is s is the scattering matrix of , and U is the node voltage source vector.

[0079] Shielding effectiveness calculation module: It is used to calculate the TE mode voltage of the observation point in the cavity according to the BLT calculation model and calculate the shielding effectiveness of the point. The calculation formula is: Obtain the shielding effectiveness of the observation point Q in the cavity at the corresponding frequency f.

[0080] Obtaining new shielding effectiveness module: It is used to take values ​​of the observed frequency band at equal intervals, calculate the shielding effectiveness at each frequency, select one of the frequencies, and calculate its new shielding effectiveness using the moving average method of the symmetric window. It specifically includes the following units:

[0081] Value unit: used to obtain the value of {f1,f2,...,f n}, and calculate the shielding effectiveness {SE(f1), SE(f2), ..., SE(f n )};

[0082] Finding the symmetric window shielding effectiveness unit: used to find the frequency f i The corresponding shielding effectiveness SE(f i ), then find the frequency {f i-k ,...f i-1 ,f i+1 ,...f i+k}The corresponding shielding effectiveness {SE(f i-k ),...SE(fi-1 ),SE(f i+1 ),...SE(f i+k )};

[0083] Calculate average unit: Used to calculate average value using formula Calculate the frequency efficiency {SE(f i-k ),...SE(f i-1 ),SE(f i ),SE(f i+1 ),...SE(f i+k )} is taken as the new shielding effectiveness at frequency fi.

[0084] Filter module: used to obtain the effective shielding frequency band of the open-hole shielding cavity according to the comparison result of the new shielding effectiveness in the new shielding effectiveness module and the expected shielding effectiveness lower limit, specifically: set the expected shielding effectiveness lower limit SE min , if the frequency obtained in the new shielding effectiveness unit is f i When the new shielding effectiveness is greater than the shielding effectiveness lower limit, that is, if SE MA (f i )>SE min , then the frequency f is retained i , and the frequency {f i-k ,...f i-1 ,f i ,f i+1 ,...f i+k}As the effective shielding frequency band of the open-hole shielding cavity within the observation frequency band.

[0085] In this embodiment, a spatial rectangular coordinate system is first established through a system building module, and the characteristic impedance Z of the internal space of the cavity is obtained. gmn and the propagation constant k gmn , so that the BLT calculation model acquisition module can smoothly construct the equivalent circuit and signal flow graph and obtain the BLT calculation model of the open-hole shielding cavity, and then the TE mode voltage of the observation point in the cavity is calculated through the shielding effectiveness calculation module, and then the shielding effectiveness of the observation point Q in the cavity under the corresponding frequency f is solved, and then the new shielding effectiveness module is obtained, and the moving average method of the symmetrical window is used to calculate the corresponding new shielding effectiveness at a certain frequency, and finally the effective shielding frequency band of the open-hole shielding cavity is obtained through the filtering module.

[0086] The module for obtaining new shielding effectiveness in this embodiment, through the operation and mutual cooperation of the value taking unit, the unit for finding the symmetric window shielding effectiveness and the unit for calculating the average value, can make the algorithm faster and the shielding effectiveness curve obtained at each frequency smoother, while retaining the overall trend of the data and eliminating the abnormal fluctuation on the shielding effectiveness curve, which is more conducive to solving the effective shielding frequency band. After the effective shielding frequency band is obtained, it can also assist in the design of the open-hole shielding cavity structure.

[0087] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for calculating the effective shielding frequency band of an open-hole shielding cavity, characterized in that: The following steps are involved: S1: Establish a spatial rectangular coordinate system with the lower left corner of the opening surface of the opening shielding cavity as the coordinate origin; S2: Construct the equivalent circuit of the perforated shielding cavity and its signal flow graph, and obtain the BLT calculation model of the perforated shielding cavity; S3: Calculate the TE mode voltage of the observation point in the cavity according to the BLT calculation model and calculate the shielding effectiveness of the point; S4: Take values ​​of the observation frequency band at equal intervals, calculate the shielding effectiveness of the observation point in the cavity at each frequency, select one of the frequencies, and use the moving average method of the symmetrical window to calculate the new shielding effectiveness of the point; S5: Compare the new shielding effectiveness of the observation point in the cavity with the lower limit of the expected shielding effectiveness to obtain the effective shielding frequency band of the open-hole shielding cavity.

2. The method for calculating the effective shielding frequency band of an open-hole shielding cavity according to claim 1, characterized in that: In step S1, the dimensions of the perforated shielding cavity are: length a, width d, height b; there is an opening on the perforated surface, and the dimensions of the opening are: length l, width w.

3. The method for calculating the effective shielding frequency band of an open-hole shielding cavity according to claim 2, characterized in that: In step S2, the equivalent circuit includes an equivalent opening impedance Z ap , Where a is the length of the perforated shielding cavity, b is the height of the perforated shielding cavity, l is the length of the opening size, k0 is the propagation constant of the free space outside the cavity, j is the imaginary unit, Where t is the thickness of the aperture shielding cavity material, and w is the width of the aperture size.

4. The method for calculating the effective shielding frequency band of an open-hole shielding cavity according to claim 3, characterized in that: In step S2, the equivalent circuit also includes the characteristic impedance Z of the cavity interior space gmn , Where z0 is the characteristic impedance of the free space outside the cavity, λ is the wavelength of the plane wave, and m and n are the rectangular waveguide mode indices.

5. The method for calculating the effective shielding frequency band of an open-hole shielding cavity according to claim 1, characterized in that: In step S2, the BLT calculation model of the aperture shielding cavity is: V r,s =(S+I)(PS) -1 U, where V r,s is the node J on the pipeline r in the signal flow graph s The full voltage of node J s The scattering matrix of the pipeline r is U = [V s 0 0 0 0 0] T , Vs is the plane wave equivalent voltage source.

6. The method for calculating the effective shielding frequency band of an open-hole shielding cavity according to claim 5, characterized in that: In step S3, the formula for calculating shielding effectiveness is: Where SE(f) represents the frequency efficiency corresponding to the observation point Q in the cavity when the frequency is f, V Q is the TE mode voltage at the observation point Q in the cavity.

7. The method for calculating the effective shielding frequency band of an open-hole shielding cavity according to claim 6, characterized in that: The specific process of step S4 is: S401: Take equidistant values ​​of the observed frequency band to obtain {f1,f2,...,f n }, calculate the shielding effectiveness {SE(f1), SE(f2),..., SE(f n )}; S402: Find the shielding effectiveness SE(fi) corresponding to the frequency fi, and then find the shielding effectiveness SE(fi) corresponding to the frequency {f i-k ,...f i-1 ,f i+1 ,...f i+k }corresponding frequency efficiency {SE(f i-k ),...SE(f i-1 ),SE(f i+1 ),...SE(f i+k )}; S403: Calculate frequency efficiency {SE(f i-k ),...SE(f i-1 ),SE(f i ),SE(f i+1 ),...SE(f i+k )}, as the average value of the frequency f i Up-to-date shielding effectiveness.

8. The method for calculating the effective shielding frequency band of an open-hole shielding cavity according to claim 7, characterized in that: In step S403, the frequency efficiency {SE(f i-k ),...SE(f i-1 ),SE(f i ),SE(f i+1 ),...SE(f i+k )} is calculated as:

9. The method for calculating the effective shielding frequency band of an open-hole shielding cavity according to claim 8, characterized in that: The specific process of step S5 is: if the frequency is f i The new shielding effectiveness SE MA (f i ) is greater than the expected shielding effectiveness lower limit SE min , that is, if SE MA (f i )>SE min , then the frequency f is retained i , and the frequency {f i-k ,...f i-1 ,f i ,f i+1 ,...f i+k }As the effective shielding frequency band of the open-hole shielding cavity within the observation frequency band.

10. A system for calculating the effective shielding section of an open shielding cavity, characterized in that: Includes the following modules: System building module: used to establish a spatial rectangular coordinate system with the lower left corner of the opening surface of the opening shielding cavity as the coordinate origin; Obtain BLT calculation model module: used to construct the equivalent circuit of the perforated shielding cavity and its signal flow graph, and obtain the BLT calculation model of the perforated shielding cavity; Shielding effectiveness calculation module: used to calculate the TE mode voltage of the observation point in the cavity according to the BLT calculation model and calculate the shielding effectiveness of the point; Obtaining new shielding effectiveness module: used to take values ​​of the observation frequency band at equal intervals, calculate the shielding effectiveness of the observation point in the cavity at each frequency, select one of the frequencies, and use the moving average method of the symmetrical window to calculate the new shielding effectiveness of the point; Filtering module: used to compare the new shielding effectiveness of the observation point in the cavity with the expected lower limit of shielding effectiveness, and obtain the effective shielding frequency band of the open shielding cavity.

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