Waveguide Dual Coupler Production Technology Detailed
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>> 5.2 Positioning of matching block
>> 5.3 Dimension chain process optimization design
In accordance with the design requirements for the X-band waveguide dual directional coupler and considering the manufacturing characteristics, we have developed a logical parts splitting method. This approach ensures both the shape and positioning accuracy of the coupling hole during the processing of the waveguide cavity, while also allowing for the proper distribution of the closed ring’s size among the component rings.
The design includes a positioning step where the matching block is first attached using electron beam welding. Following this, the cover plate is vacuum brazed to create the coupler’s closed cavity. The proposed process route is both reasonable and feasible, meeting the necessary design and environmental adaptability requirements. This methodology serves as a valuable reference for the design and production of high-frequency waveguide dual directional couplers.
01. Preface
A directional coupler is an element used for power distribution that allows for monitoring or adjusting power levels through the coupled power. It consists of four ports and is made up of two transmission lines: the main transmission line (main line) and the auxiliary transmission line (auxiliary line). Directional couplers come in various structural forms, including waveguides and microstrips. The waveguide structure is capable of handling large power levels and is commonly utilized in high-power microwave transmission. The main challenge in creating a waveguide structure lies in ensuring the accuracy of the coupling slit or hole, as well as achieving a reliable connection.
As illustrated in Figure 1, a specific X-band waveguide dual directional coupler is designed to couple through a hole on the wide side of the waveguide. It is capable of transmitting high microwave power and must meet high integration and reliability requirements, which can be difficult to achieve during the manufacturing process. This document analyzes the structure and manufacturing challenges of the waveguide dual directional coupler. The final assembly of the coupler was accomplished through a design that incorporated parts splitting, positioning steps, optimization of dimensional processes, electron beam welding, and a secondary vacuum brazing process.
Inspection results indicate that the sample meets the specified dimensions in the drawing, the electrical performance fulfills functional requirements, and the overall manufacturing process is both reasonable and feasible. This work serves as a reference for the production of similar products.
02. Structural composition
The waveguide dual directional coupler consists of several CNC machining components: a mainline waveguide, a secondary line waveguide, a cover plate, a matching block, and a socket. The shape and size of the coupling holes are illustrated in Figure 2, while the matching block is depicted in Figure 3. The waveguide aperture for the X-band measures 22.86 mm by 10.16 mm, with an operating frequency range of 9 to 11 GHz. The thickness of the metal wall separating the coupling waveguides is 1 mm. A total of four coupling holes are designed in the metal wall: two holes measuring 0.52 mm by 8 mm with a radius of 0.26 mm have an inclination angle of 27.8° ± 5′, and two holes measuring 0.52 mm by 6.66 mm with a radius of 0.26 mm have an inclination angle of 28.3° ± 5′.
03. Process Difficulties
The manufacturing, storage, transportation, and installation of the dual directional coupler occur in a land environment, while its use involves coastal high mountain and island reef settings. As a result, the coupler must meet outdoor operation requirements with an ambient temperature range of -40 to +50°C. Additionally, it must withstand vibration, impact, mildew, and salt spray in accordance with the relevant provisions outlined in GJB 74A-1998. To ensure durability, the cover plate and cavity must be securely connected, with a preference for vacuum brazing.
Telecom specifications dictate that the distance between the matching block and the inner cavity should be (1.1 ± 0.03) mm. This distance forms a closed ring after assembly. Since the matching block is located within a closed cavity and cannot be mechanically processed, it can only be attached through gluing or welding. Given the environmental adaptability requirements, welding is the preferred method.
Vacuum aluminum brazing is not suitable for microwave device manufacturing in this context, primarily because the surfaces of the matching block connection and the cover plate brazing are perpendicular to each other. When using step vacuum brazing, gravity can adversely affect the quality of the brazing process under vacuum conditions. The tolerance for the closed ring is ±0.03 mm, meaning the total tolerance for the constituent rings must not exceed 0.06 mm. Given the high working frequency band and the small size of the components, precision is essential.
04. Process plan
According to the structural characteristics of the dual directional coupler, the parts are designed and divided into waveguide, cover plate, matching block, and positioning pin, as shown in Figure 1.
Plan 1: The main line waveguide and the secondary line waveguide of the directional coupler are initially formed as separate components and then vacuum-brazed together to create a complete unit. It is essential to weld the main line waveguide, the secondary line waveguide, the matching block, and the cover plate together.
However, there is an issue with the welding surfaces of the cover plate and the waveguide, as they are not perpendicular to each other. Furthermore, while the processing accuracy of the waveguide coupling hole can be maintained when it is a single part, welding the components into a complete assembly can result in a loss of angular accuracy as well as relative position accuracy.
Plan 2: The overall processing plan for the coupling body aims to be as integrated as possible, particularly with regard to the coupling hole. While the waveguide cavity is accessible for processing, the matching block is not. Therefore, the assembly is divided into three parts: the waveguide cavity, the cover plate, and the matching block. A welding connection is employed to ensure environmental adaptability and reliability.
To maintain positional accuracy, the cover plate is aligned using pins, and the matching block is connected to the waveguide cavity through a specially designed positioning step. Since the connection surfaces of the matching block and the cover plate are perpendicular to each other, vacuum aluminum brazing cannot be applied to both connections simultaneously. Instead, the matching block is welded first, followed by the cover plate.
The CNC machining block is attached using fusion welding, while the cover plate is brazed, ensuring they do not interfere with each other. Precise positioning is crucial for the fusion welding of the matching block to meet dimensional accuracy requirements. As a result, the connection surface of the matching block is extended from the part and welded using electron beam welding. This method enables the weld point to withstand the maximum temperature of subsequent vacuum brazing of the cover plate, which is 610°C.
In accordance with the process requirements for vacuum brazing, 3A21 aluminum alloy is selected as the material. The design of the waveguide cavity facilitates precise control over the position, size, and thickness of the coupling hole. After careful analysis, the overall processing plan for the coupling body has been adopted.
05. Process realization
5.1 Processing and forming
The waveguide cavity, cover plate, and matching block are produced through conventional machine tool processing methods.
5.2 Positioning of matching block
The matching block and waveguide cavity are precisely positioned using specific positioning steps. The structure of the matching block is illustrated in Figure 3, while Figure 4 shows the structure of the waveguide cavity. An enlarged view of the positioning step is provided in section I of Figure 4.
The dimensions of the matching block are 12.98 mm x 3.48 mm, and it is aligned and secured with pins simultaneously with the waveguide cavity. Additionally, a step measuring 14.98 mm x 5.48 mm is welded in place to facilitate welding accessibility. The welding and positioning processes are staggered to minimize the impact of welding heat on positioning accuracy.
5.3 Dimension chain process optimization design
In order to ensure the assembly size (1.1±0.03) mm (see Figure 4), the size of the component ring is analyzed.
Based on the dimension tolerances of 84.86 mm, (69.68 ± 0.03) mm, and 12.98 mm, as indicated in Figures 3 and 4, the calculations for the closed ring are as follows:
1) Basic size of the closed ring:
[(4.86 - 69.68 - 12.98) div 2 = 1.1 text{ mm}]
2) Tolerance zone of the closed ring:
[0.1 + 0.06 + 0.02 = 0.18 text{ mm}]
3) Upper deviation of the closed ring:
[(0 + 0.03 + 0) div 2 = 0.015 text{ mm}]
4) Lower deviation of the closed ring:
[(-0.1 - 0.03 - 0.02) div 2 = -0.075 text{ mm}]
As a result, the specified assembly size of (1.1 ± 0.03) mm cannot be guaranteed.
To ensure the size tolerance of the closed ring, the tolerances for each component ring have been adjusted. The tolerance is now distributed symmetrically in both directions, setting the tolerances to (84.86 ± 0.01) mm, (69.68 ± 0.01) mm, and (12.98 ± 0.01) mm. These processing standards can be met with the current machine tools. Therefore, the size accuracy of the closed ring can be guaranteed when the components fulfill these processing requirements.
5.4 Welding Process Design
Electron beam welding is a type of fusion welding known for its high-temperature tolerance and is often performed before the brazing process. The design specifications for the weld depth are set at 1.5 mm, with an average single weld length of approximately 41 mm. The welding current ranges from 7 to 12 mA, while the focusing current is between 500 and 530 mA. The welding speed falls within the range of 13 to 18 mm/s.
For the electron beam welding matching block, calculations indicate that the closest distance between the weld and the flange is 1.5 mm, ensuring that the electron beam will not be obstructed. Following this, the cover plate is formed through vacuum brazing. During this CNC machining process, the vacuum welding surface is maintained in a horizontal position, aided by tooling that presses and secures the components, which helps achieve an optimal welding surface.
06. Conclusion
This paper uses a directional coupler as a case study to perform a structural analysis and develop a design and assembly welding process plan. The plan involves a limited number of split components, and the positioning relationships among the parts are well-defined. High precision is maintained during the machining of the waveguide cavity. The dimensions of the closed ring are appropriately allocated to the component rings. For the two mutually perpendicular welding surfaces, positioning steps are designed to ensure that the matching block is first electron beam welded, followed by vacuum brazing of the cover plate to create a sealed coupler cavity. This process meets the standards for telecommunications and environmental adaptability of products, while also providing a reference for the manufacturing processes of similar directional couplers.
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