Miniaturized 3dB bridge building method and directional coupler

Abstract

The invention discloses a miniaturized 3dB bridge building method. The method comprises the steps that S1, a traditional # imgabs0 # branch line 3dB bridge model is acquired; s2, under the constraint of meeting specific impedance and electrical length, selecting a specific loading position by loading branches with different widths, so that the distribution of the characteristic impedance and the electrical length meets a relational expression; s3, fragmenting the loaded branches, distributing the fragments into a specified design space, and obtaining design parameters of the miniaturized # imgabs1 # branch line 3dB bridge model; s4, obtaining an approximation model of the miniaturized # imgabs2 # branch line 3dB bridge model and the traditional # imgabs3 # branch line 3dB bridge model; s5, obtaining a minimum # imgabs4 # branch line 3dB bridge model; and S6, carrying out rapid construction on the minimum # imgabs5 # branch line 3dB bridge model. On the premise that the 3dB bridge designed by the invention has an extremely small structural size, the scattering coefficient index is not deteriorated relative to a classical structure.

Description

Technical Field

[0001] The present invention relates to the technical field of 3dB bridges, and in particular, to a method for building a miniaturized 3dB bridge and a directional coupler. Background Art

[0002] A 3dB bridge is a passive device widely used in wireless / microwave systems, mainly for sampling and separating radio frequency signals in microwave systems. It is particularly widely used in high-end equipment manufacturing scenarios such as satellites, radars, and missile seekers that require signal splitting or high-frequency signal isolation. Traditional bridge design methods have been fully developed on various microwave transmission lines such as coaxial cables, waveguides, striplines, and microstrip lines. With the development of microwave high-density integration technology and the continuous requirements of various complex design environments (such as high-performance detection systems and deep-space vehicles), 3dB bridges designed using traditional theories are often large in size, which will affect the high-density integration of electronic systems. Therefore, it is necessary to design a miniaturized directional coupler that meets the performance index requirements using new methods and technologies.

[0003] A 3dB bridge is mainly characterized by coupling degree, isolation degree, and directivity. Traditional design methods include parallel coupling, branch coupling, aperture coupling, and matching structures. Considering engineering usability, most current structures mainly focus on the first two. How to reasonably miniaturize the 3dB bridge is the research focus at home and abroad.

[0004] Reference [1] (Tang C W, Chen M G. Synthesizing microstrip branch-line couplers with predetermined compact size and bandwidth[J]. IEEE Transactions on Microwave Theory and Techniques, 2007, 55(9):1926-1934) studied the miniaturization of 3dB directional couplers from the perspective of high-impedance microstrip line loading; Reference [2] (Liao S S, Peng J T. Compact planar microstrip branch-line couplers using the quasi-lumped elements approach with nonsymmetrical and symmetrical T-shaped structure[J]. IEEE Transactions on Microwave Theory and Techniques, 2006, 54(9):3508-3514.) proposed a method for miniaturization design using meandered branch lines; Reference [3] (Tsai K Y, Yang H S, Chen J H, et al. A miniaturized 3dB branch-line hybrid coupler with harmonics suppression[J]. IEEE Microwave and Wireless Components Letters, 2011, 21(10):537-539.) studied the 3dB bridge from the perspective of slow-wave structure using the structure of loaded interdigital transmission lines.

[0005] The above design methods can solve the problem of stub miniaturization and utilize the internal space composed of branch lines under the guidance of relevant theories. However, the existing microwave transmission line theory cannot give a more miniaturized design method for 3dB bridges through what kind of structure under the limitation of specified performance indicators. Summary of the Invention

[0006] The present invention aims to solve at least one of the technical problems in the related art to some extent. To this end, an object of the present invention is to propose a method for building a miniaturized 3dB bridge, under the premise of having an extremely small structural size, the scattering coefficient index will not deteriorate compared with the classical structure.

[0007] In a first aspect, a method for building a miniaturized 3 dB bridge proposed by the present invention is as follows:

[0008] S1: Obtain a traditional branch-line 3 dB bridge model;

[0009] S2: Define the transmission performance of the miniaturized branch-line 3 dB bridge model to be the same as that of the traditional branch-line 3 dB bridge model. Under the constraints of specific impedance and electrical length, by loading stubs of different widths and selecting specific loading positions, make the distribution of characteristic impedance and electrical length satisfy the relational expression;

[0010] S3: Fragment the stubs loaded in step S2, distribute the fragments into the specified design space, and update the design parameters of the miniaturized branch-line 3 dB bridge model when the fragments are distributed into the specified design space;

[0011] S4: Define the circuit performance of the miniaturized branch-line 3 dB bridge model updated in step S3 to be the same as that of the traditional branch-line 3 dB bridge model, and obtain an approximation model of the miniaturized branch-line 3 dB bridge model and the traditional branch-line 3 dB bridge model;

[0012] S5: According to the approximation model obtained in step S4, search for the ratio of the electrical length in the approximation model to the electrical length in the traditional branch-line 3 dB bridge model. Identify the miniaturized branch-line 3 dB bridge model corresponding to the minimum ratio as the most miniaturized branch-line 3 dB bridge model;

[0013] S6: Perform machine learning on the result obtained in step S5, and quickly construct the most miniaturized branch-line 3 dB bridge model after machine learning.

[0014] Preferably, in step S2:

[0015] The transmission matrix expression of the transmission line of the traditional branch-line 3 dB bridge model is as follows:

[0016]

[0017] Among them, A is the voltage ratio, representing the voltage transmission characteristic of the transmission matrix network; B is the transfer impedance, representing the impedance characteristic of the transmission network under short-circuit conditions; C is the transfer admittance, representing the admittance characteristic of the transmission network under open-circuit conditions; D is the current ratio, representing the current transmission characteristic of the transmission matrix network; θ Δ is the electrical length; Z Δ is the characteristic impedance of the transmission line; the square of j is equal to -1;

[0018] The transmission matrix expression of the transmission line miniaturized using a loaded stub is as follows:

[0019]

[0020] θ α and θ γ are respectively the equivalent characteristic impedances of the transmission line miniaturized using a loaded stub; Z α and Z γ are respectively the equivalent electrical lengths of the transmission line miniaturized using a loaded stub; θ β is the characteristic impedance of the loaded stub of the transmission line miniaturized using a loaded stub; Z β is the electrical length of the loaded stub of the transmission line miniaturized using a loaded stub;

[0021] Let Equation (1) and Equation (2) be equal to obtain the constraint conditions of the miniaturized transmission line regarding the characteristic impedance and electrical length (Z β , θ β ), and the expression is as follows:

[0022]

[0023] When stub loading is performed in the middle of each branch line, Z α = Z γ , θ α = θ γ . At this time, Z β , θ β are only related to Z α , Z Δ , and the expression is as follows:

[0024]

[0025] The expression of the microstrip line impedance Z is as follows:

[0026]

[0027] Among them, Z air = 376.8 (Ω), ε r$\varepsilon$ represents the dielectric constant of the microstrip line substrate material, $w$ is the width of the microstrip line, and $h$ is the thickness of the attached material. As can be seen from Equation (5), when the attached material of the bridge remains unchanged and the thickness remains unchanged, the characteristic impedance of the microstrip line is determined by its width;

[0028] That is, under the constraints of meeting specific impedance and electrical length, by loading stubs with different widths and selecting specific loading positions, the distribution of characteristic impedance and electrical length can satisfy Equation (4), and the miniaturization of the classic 3dB bridge can be achieved by loading stubs.

[0029] Preferably, for miniaturization In the branch-line 3dB bridge model, for the four transmission lines formed by the branch lines, the loaded stubs corresponding to each transmission line cannot intersect.

[0030] Preferably, in step S4:

[0031] The miniaturized The circuit performance of the branch-line 3dB bridge model is defined as the same as that of the traditional branch-line 3dB bridge model, and the expression is as follows:

[0032]

[0033] $\theta$ represents the electrical length of the equivalent transmission line in the traditional branch-line 3dB bridge model, $L$ n represents the equivalent inductance of the loaded debris, $C$ n represents the equivalent capacitance of the loaded debris, and $\omega$ represents the frequency.

[0034] Preferably, in step S5:

[0035] The expression of the impedance of the loaded stub is as follows:

[0036]

[0037] Substituting Equation (7) into Equation (6) to obtain the optimal impedance and electrical length distribution, the expression is as follows:

[0038]

[0039] Among them, $Z$ i represents the characteristic impedance of the $i$-th debris, $\theta$ i represents the electrical length of the $i$-th debris.

[0040] Preferably, in step S6:

[0041] The objective function expression of machine learning is as follows:

[0042]

[0043] Among them, f1(x) represents the first objective function, and |S 31 | dB represents the scattering coefficient from the input port to the coupled port in the transmission line model, and |S 21 | dB represents the scattering coefficient from the input port to the isolated port in the transmission line model, and f2(x) represents the second objective function, and |S 11 | dB represents the scattering coefficient from the input port to the input port in the transmission line model;

[0044] Machine learning is used to perform clustering analysis on the common structural features existing in the obtained 3dB coupler, and summarize the structural features of the results that meet the regulations, so as to realize the rapid construction of the branch-line 3dB coupler model with miniaturization.

[0045] Preferably, in the branch-line 3dB coupler model with miniaturization, there are two coupled lines.

[0046] In the second aspect, a directional coupler proposed by the present invention includes any one of the miniaturized 3dB couplers built by the above-mentioned miniaturized 3dB coupler building method.

[0047] The beneficial effects in the present invention are as follows:

[0048] (1) Based on the classical structure of the branch-line directional coupler, by adopting an irregular fragmented filling structure in the area surrounded by the branch lines, the area composed of the branch lines is utilized to the greatest extent, and structural design is carried out, so that the electrical length of the ports of the coupler meets the design requirements given by the design method, and a miniaturized 3dB coupler is realized without reducing the coupling performance and isolation performance.

[0049] (2) By using the design method of the present invention, various miniaturized 3dB couplers with different frequency bands can be conveniently designed based on different design spaces, and their structural dimensions are generally miniaturized by more than 70% compared with the couplers designed by the classical method.

[0050] (3) The designed miniaturized 3dB coupler can be widely applied to various microwave systems, especially in high-end equipment manufacturing fields such as aviation, aerospace, and underwater where there are high requirements for device miniaturization, and has broad application prospects. Description of the Drawings

[0051] In the drawings:

[0052] Figure 1 is a comparison diagram of the traditional 3dB coupler and the miniaturized 3dB coupler proposed by the present invention;

[0053] Figure 2 Schematic diagram of the stub-loaded miniaturized 3dB bridge in step S2 proposed by the present invention;

[0054] Figure 3 Schematic diagram of the stub-fragmented miniaturized 3dB bridge in step S3 proposed by the present invention;

[0055] Figure 4 Equivalent circuit schematic diagram of stub loading and fragment loading proposed by the present invention;

[0056] Figure 5 Schematic diagram of the fragmented miniaturized 3dB bridge with unequal density loading using fragments of different sizes;

[0057] Figure 6 Example comparison diagram of the fragmentation miniaturization of the transmission line with a certain stub loading;

[0058] Figure 7 Example diagram of the fragment-loaded miniaturization in step S5 proposed by the present invention;

[0059] Figure 8 Example diagram after the pre-setting of the coupled line using machine learning in step S6 proposed by the present invention. Detailed implementation method

[0060] Referring to Figure 1 , a method for building a miniaturized 3dB bridge proposed by the present invention, the steps of the method are as follows:

[0061] S1: Obtain a traditional branch-line 3dB bridge model;

[0062] In this embodiment, referring to Figure 1 , the traditional branch-line 3dB bridge is composed of a square area surrounded by branch lines. The branch lines are in the form of classic transmission lines, and the length of the branch lines is

[0063] S2: Define the transmission performance of the miniaturized branch-line 3dB bridge model to be the same as that of the traditional branch-line 3dB bridge model. Under the constraints of specific impedance and electrical length, by loading stubs of different widths and selecting specific loading positions, make the distribution of characteristic impedance and electrical length satisfy the relational expression,;

[0064] In this embodiment, referring to Figure 2 , the traditional transmission matrix expression of the transmission line of the branch-line 3dB bridge model is as follows:

[0065]

[0066] Among them, A is the voltage ratio, representing the voltage transmission characteristic of the transmission matrix network; B is the transfer impedance, representing the impedance characteristic of the transmission network under short-circuit conditions; C is the transfer admittance, representing the admittance characteristic of the transmission network under open-circuit conditions; D is the current ratio, representing the current transmission characteristic of the transmission matrix network; θ Δ is the electrical length; Z Δ is the characteristic impedance of the transmission line; the square of j is equal to -1;

[0067] The transmission matrix expression of the transmission line miniaturized using a loaded stub is as follows:

[0068]

[0069] θ α and θ γ are respectively the equivalent characteristic impedances of the transmission line miniaturized using a loaded stub; Z α and Z γ are respectively the equivalent electrical lengths of the transmission line miniaturized using a loaded stub; θ β is the characteristic impedance of the loaded stub of the transmission line miniaturized using a loaded stub; Z β is the electrical length of the loaded stub of the transmission line miniaturized using a loaded stub;

[0070] Let Equation (1) and Equation (2) be equal to obtain the constraint conditions of the miniaturized transmission line regarding the characteristic impedance and electrical length (Z β , θ β ), and the expression is as follows:

[0071]

[0072] When loading a stub in the middle of each branch line, Z α = Z γ , θ α = θ γ At this time, Z β , θ β are only related to Z α , Z Δ , and the expression is as follows:

[0073]

[0074] For a traditional long 3dB branch-line bridge, its Z Δ is a fixed value, that is, by adjusting the characteristic impedance and electrical length of the loaded stub (Z β , θ β ) and the characteristic impedance between the original branch lines (Z α , θ α) The relationship can achieve the same transmission performance as the original transmission line.

[0075] The characteristic impedance of the transmission line can be characterized by the width of the transmission line. The impedance Z expression of the microstrip line is as follows:

[0076]

[0077] Where Z air = 376.8 (Ω), ε r represents the relative permittivity of the microstrip line substrate material, w is the width of the microstrip line, h is the thickness of the attached material. It can be seen from Equation (5) that when the attached material of the bridge remains unchanged and the thickness remains unchanged, the characteristic impedance of the microstrip line is determined by its width;

[0078] That is, under the constraints of meeting specific impedance and electrical length, by loading stubs with different widths and selecting specific loading positions to make the distribution of characteristic impedance and electrical length satisfy Equation (4), the miniaturization of the classical 3dB bridge can be achieved by loading stubs.

[0079] S3: Fragment the stubs loaded in step S2, distribute the fragments into the specified design space, and update the design parameters of the miniaturized branch-line 3dB bridge model when the fragments are distributed into the specified design space;

[0080] In this embodiment, referring to Figure 3 , fragmenting the stubs loaded in step S2 can achieve a wide range of design freedoms for characteristic impedance and electrical length, and thus obtain a 3dB bridge with a higher degree of miniaturization.

[0081] More specifically, the specific engineering implementation method of fragmentation is as follows: First, use the fragment sizes that can be achieved by the process to distribute the fragments into the specified design space; second, the fragments should be connected in a partially overlapping form; third, the fragment forms are indefinite and can exist in the form of triangles, quadrilaterals, polygons, circles, irregular shapes, and the filling density per unit area can be defined by a function; fourth, the loaded stubs of the four transmission lines formed by the branch lines cannot intersect; fifth, the loaded stubs can exhibit forms of central symmetry, axial symmetry, or asymmetry.

[0082] S4: Define the circuit performance of the miniaturized branch-line 3dB bridge model updated in step S3 to be the same as that of the traditional branch-line 3dB bridge model, and obtain an approximation model of the miniaturized branch-line 3dB bridge model and the traditional branch-line 3dB bridge model;

[0083] In this embodiment, for the transmission line implemented using the fragmented structure and the transmission line implemented using the classical stub structure, their respective circuit models are as follows Figure 4 As shown, define the circuit performance of the miniaturized branch-line 3 dB bridge model updated in step S3 to be the same as that of the traditional branch-line 3 dB bridge model. The expression is as follows:

[0084]

[0085] θ represents the electrical length of the equivalent transmission line in the traditional branch-line 3 dB bridge model, L n represents the equivalent inductance of the loaded fragment, C n represents the equivalent capacitance of the loaded fragment, and ω represents the frequency.

[0086] S5: According to the approximation model obtained in step S4, search for the ratio of the electrical length in the approximation model to the electrical length in the traditional branch-line 3 dB bridge model, and identify the miniaturized branch-line 3 dB bridge model corresponding to the minimum ratio as the most miniaturized branch-line 3 dB bridge model;

[0087] In this embodiment, the expression of the loaded stub impedance is as follows:

[0088]

[0089] Substitute formula (7) into formula (6) to obtain the optimal impedance and electrical length distribution. The expression is as follows:

[0090]

[0091] Among them, Z i represents the characteristic impedance of the i-th fragment, and θ i represents the electrical length of the i-th fragment.

[0092] S6: Perform machine learning on the result obtained in step S5, and quickly construct the most miniaturized branch-line 3 dB bridge model after machine learning.

[0093] In this embodiment, according to the multi-objective optimization theory, different objective functions have different influences on the pareto front of such open optimization problems. The objective function disclosed in this application is summarized based on a large amount of simulation data. The specific objective function expression is as follows:

[0094]

[0095] Among them, f1(x) represents the first objective function, |S 31 | dB represents the scattering coefficient from the input port to the coupled port in the transmission line model, |S 21 | dB represents the scattering coefficient from the input port to the isolated port in the transmission line model, f2(x) represents the second objective function, |S 11 | dB represents the scattering coefficient from the input port to the input port in the transmission line model, as Figure 2 shown;

[0096] Based on more than a thousand optimization results calculated according to the above formula, the present invention uses a machine learning method to perform clustering analysis on the common structural characteristics existing in the obtained 3dB bridge, and summarizes the structural characteristics of the better-performing results. The summary shows that introducing two isolation lines into the 3dB bridge is helpful for obtaining better miniaturization indexes, as Figure 8 shown.

[0097] To more clearly illustrate the solutions and effects of this embodiment, an example is given in combination with the accompanying drawings:

[0098] Figure 1 The schematic diagram of transforming the branch-line 3dB bridge into a fragmented miniaturized 3dB bridge is given. These fragmented structures generally have parasitic capacitance and inductance, so that the fragmented branch-line structure has the constraints of specified characteristic impedance and electrical length. This application also discloses that using a preset coupled line is beneficial to the realization of the miniaturized 3dB bridge. The 3dB bridge using this structure can achieve the miniaturized design of the branch line.

[0099] Among them Figure 5 is a specific implementation example of the fragmented filling structure. This structure uses black and white grids to represent the existence or non-existence of the fragmented structure. This kind of fragmentation shows the characteristics of disorder and diversity. The biggest advantage is that it can make full use of the design space to traverse the branch-line structure. Its fragmented forms include various two-dimensional forms such as polygons and circles. The sizes of the fragments in each region can be set, and the distribution probability can be specified separately.

[0100] To verify the method, Figure 6 An example of miniaturizing a branch of the 3dB bridge using a fragmented structure is given. The original traditional branch loading area was 6mm × 6mm. After improvement, the fragmented branch loading area was divided into: 5.5mm × 3.8mm. It can be seen from the figure that the designed miniaturized branch achieved a 58% size reduction compared to the original branch. Figure 7An example of miniaturizing a classic branch-line 3 dB bridge using a fragmented structure is given. The original 3 dB bridge operates at 2.8 GHz. The 3 dB bridge implemented based on the method provided in this application achieves an 80% size reduction compared to the classic 3 dB bridge. The following table shows the performance parameters of this 3 dB bridge, and the performance parameters of this bridge do not deteriorate compared to those of the classic branch-line 3 dB bridge.

[0101] Table 1 Comparison table of bridge performance parameters

[0102]

[0103] Figure 8 Based on the conclusions of machine learning, it is shown that introducing two pairs of coupled lines has a beneficial effect on obtaining a miniaturized 3 dB bridge.

[0104] This application discloses a design method for a miniaturized 3 dB bridge. Based on the classic structure of a branch-line directional coupler, by adopting an irregular fragmented filling structure in the area enclosed by the branch lines, the area formed by the branch lines is utilized to the greatest extent possible to carry out the structure design, so that the electrical length of the ports of the bridge meets the design requirements given by the design method, and a miniaturized 3 dB bridge is realized without reducing the coupling performance and isolation performance. Using the design method of the present invention, various miniaturized 3 dB bridges with different frequency bands can be conveniently designed based on different design spaces. Generally, the degree of miniaturization of their structural dimensions can reach more than 70% compared to the bridges designed by the classic method. The designed miniaturized 3 dB bridge can be widely applied to various microwave systems, especially in high-end equipment manufacturing fields such as aviation, aerospace, and underwater, which have high requirements for device miniaturization, and has broad application prospects.

Claims

1. A method for building a miniaturized 3dB bridge, characterized in that: The method steps are as follows: S1: Get Tradition Branch line 3dB bridge model; S2: Miniaturization The transmission performance of the branch line 3dB bridge model is defined as The branch line 3dB bridge model is the same. Under the constraints of satisfying specific impedance and electrical length, branches of different widths are loaded and specific loading positions are selected so that the distribution of characteristic impedance and electrical length satisfies the relationship; S3: Fragment the branches loaded in step S2, place the fragments into a specified design space, and update the fragments to miniaturize them when they are placed in the specified design space. Design parameters of branch line 3dB bridge model; S4: Miniaturize the updated data in step S3 The circuit performance of the branch line 3dB bridge model is defined as Branch line 3dB bridge model is the same, miniaturization is achieved Branch line 3dB bridge model and traditional Approximation model of branch line 3dB bridge model; S5: Based on the approximation model obtained in step S4, search for the electrical length size in the approximation model and the traditional The ratio of the electrical lengths in the branch line 3dB bridge model, and the corresponding miniaturization when the ratio is the minimum The branch line 3dB bridge model is the smallest Branch line 3dB bridge model; S6: Perform machine learning on the result obtained in step S5, and perform the miniaturization on the result obtained by machine learning. The branch line 3dB bridge model can be quickly constructed.

2. A method for building a miniaturized 3dB bridge according to claim 1, characterized in that: In step S2: Tradition The transmission matrix expression of the branch line 3dB bridge model transmission line is as follows: Among them, A is the voltage ratio, which represents the voltage transfer characteristics of the transmission matrix network; B is the transfer impedance, which represents the impedance characteristics of the transmission network under short-circuit conditions; C is the transfer admittance, which represents the admittance characteristics of the transmission network under open-circuit conditions; D is the current ratio, which represents the current transfer characteristics of the transmission matrix network; θ Δ is the electrical length; Z Δ is the characteristic impedance of the transmission line; the square of j is equal to -1; The transmission matrix expression of a transmission line miniaturized using loading branches is as follows: θ α ,θ γ are the equivalent characteristic impedances of the transmission line miniaturized by using loading branches; Z α , Z γ are the equivalent electrical lengths of the transmission lines miniaturized using loading branches; θ β The characteristic impedance of the loading branch of the transmission line miniaturized by using the loading branch; Z β The electrical length of the loading stub for the transmission line miniaturized using the loading stub; Let formula (1) and formula (2) be equal to obtain the characteristic impedance and electrical length (Z) of the miniaturized transmission line: β ,θ β ) constraint, the expression is as follows: When from each group When branch loading is performed in the middle of the branch line, Z α =Z γ ,θ α =θ γ At this time, Z β ,θ β Only with Z α ,Z Δ The expressions are as follows: The expression of microstrip line impedance Z is as follows: in, Z air =376.8(Ω),ε r represents the dielectric constant of the microstrip line substrate material, w is the width of the microstrip line, and h is the thickness of the attachment material. From formula (5), it can be seen that when the bridge attachment material and thickness remain unchanged, the characteristic impedance of the microstrip line is determined by its width; That is, under the constraints of satisfying specific impedance and electrical length, by loading branches of different widths and selecting specific loading positions so that the distribution of characteristic impedance and electrical length satisfies formula (4), the miniaturization of the classic 3dB bridge can be achieved by loading branches.

3. A method for building a miniaturized 3dB bridge according to claim 1, characterized in that: In step S3: miniaturization In the branch line 3dB bridge model, the four transmission lines formed by the branch lines cannot intersect with each other in the loading branches corresponding to each transmission line.

4. The miniaturized directional coupler according to claim 1, characterized in that: In step S4: Minimize the updated The circuit performance of the branch line 3dB bridge model is defined as The branch line 3dB bridge model is the same, and the expression is as follows: θ represents traditional The electrical length of the equivalent transmission line in the branch line 3dB bridge model, L n represents the equivalent inductance of the loaded fragment, C n represents the equivalent capacitance of the loaded fragment and ω represents the frequency.

5. A method for building a miniaturized 3dB bridge according to claim 4, characterized in that: In step S5: The expression of the loaded branch impedance is as follows: Substituting formula (7) into formula (6) yields the optimal impedance and electrical length distribution, as shown below: Among them, Z i represents the characteristic impedance of the i-th fragment, θ i represents the electrical length of the i-th fragment.

6. A method for building a miniaturized 3dB bridge according to claim 1, characterized in that: In step S6: The objective function expression of machine learning is as follows: Among them, f1(x) represents the objective function 1, |S 31 | dB represents the scattering coefficient from the input port to the coupled port in the transmission line model, |S 21 | dB represents the scattering coefficient from the input port to the isolation port in the transmission line model, f2(x) represents the objective function 2, |S 11 | dB It represents the scattering coefficient from the input port to the input port in the transmission line model; Machine learning is used to cluster the common structural features of the obtained 3dB bridges, and the structural features of the results that meet the requirements are summarized to achieve the smallest size. Rapid construction of branch line 3dB bridge model.

7. A method for building a miniaturized 3dB bridge according to claim 6, characterized in that: In the smallest There are two coupled lines in the branch-line 3dB bridge model.

8. A directional coupler, characterized in that: It comprises a miniaturized 3dB bridge constructed by the miniaturized 3dB bridge construction method as described in any one of claims 1 to 7.

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