How can waveguide couplers achieve a perfect balance between high directivity and low insertion loss in millimeter-wave communication?
Publish Time: 2025-11-14
With the rapid development of high-frequency applications such as 5G/6G communication, satellite communication, and radar systems, the millimeter-wave band is becoming a key resource for wireless communication. In this context, the performance of waveguide couplers, as one of the core passive components of microwave transmission systems, directly affects the efficiency and stability of the entire system. A waveguide coupler is a four-port device, typically including an input port, a through port, a coupling port, and an isolation port. Its core indicators include coupling strength, directivity, insertion loss, and bandwidth. High directivity ensures signal isolation capability, while low insertion loss guarantees efficient transmission of the main channel signal. However, in the millimeter-wave band, due to the extremely short wavelength, sensitivity to manufacturing tolerances, and significantly increased material losses, achieving a "perfect balance" between high directivity and low insertion loss becomes extremely challenging.1. Physical Mechanisms and Design Strategies for High DirectivityDirectivity is a key parameter measuring the ability of a waveguide coupler to distinguish between forward and reverse waves, defined as the ratio of the power at the coupling port to that at the isolation port. Ideally, the isolation port should have no output, meaning its directivity tends towards infinity. However, in practical millimeter-wave systems, even minute structural asymmetries, surface roughness, or dielectric inhomogeneities can cause a sharp decrease in directivity.To improve directivity, traditional waveguide couplers often employ structures such as multi-hole coupling, branched waveguides, or multimode interference. For example, multi-hole coupling structures use multiple spacing-based coupling holes between the main and secondary waveguides, leveraging the phase superposition principle to enhance forward coupling and cancel reverse leakage, thus significantly improving directivity. In the millimeter-wave band, this structure requires precise control of the hole size and position to avoid phase mismatch caused by manufacturing errors.Furthermore, the emerging substrate integrated waveguide (SIW) technology has provided new ideas for high directivity design. SIW simulates the electromagnetic characteristics of traditional rectangular waveguides in planar circuits, retaining the low-loss advantages of waveguides while facilitating integration with planar circuits. By optimizing the length, width, and distribution of SIW coupling slots, directivity exceeding 30 dB can be achieved in the millimeter-wave band while maintaining good broadband characteristics.2. Key Factors for Reducing Insertion LossInsertion loss refers to the energy loss of a signal from the input port to the through port, primarily originating from conductor loss, dielectric loss, and radiation loss. In the millimeter-wave band, the skin effect concentrates current on the conductor surface, leading to a significant increase in conductor loss; simultaneously, the dielectric loss tangent of the dielectric material becomes a significant factor at high frequencies.To reduce insertion loss, firstly, high-conductivity metals should be selected as waveguide wall materials, and surface roughness should be reduced through electroplating or surface polishing. Studies have shown that when the surface roughness is less than the skin depth, conductor loss can be effectively suppressed.Secondly, in dielectric selection, low-loss, low-dielectric-constant materials should be prioritized, such as polytetrafluoroethylene (PTFE), quartz, or high-frequency ceramics.Finally, the structural design should avoid unnecessary corners, abrupt changes, or discontinuities, as these can cause mode conversion and energy reflection, thereby increasing loss. Gradual transitions, rounded corners, or impedance matching sections can effectively mitigate these problems.3. The Path to Balance: Collaborative Optimization and Advanced Simulation TechnologyHigh directionality often relies on complex coupling structures, while complex structures can introduce additional losses. Therefore, achieving a balance between the two hinges on collaborative optimization design. Modern electromagnetic simulation software, combined with parameter scanning and optimization algorithms, can quickly explore the structural parameter space in the early design stages, finding the optimal solution that balances directionality and insertion loss.Furthermore, advancements in 3D printing and micromachining technologies have made the manufacture of high-performance couplers possible. Metal 3D printing can achieve complex internal cavity structures that are difficult to achieve with traditional machining, while ensuring high precision and consistency, thus supporting the realization of theoretical designs at the physical level.
Waveguide couplers are a systems engineering project involving electromagnetic theory, materials science, precision manufacturing, and intelligent optimization. By rationally selecting structural forms, optimizing material parameters, controlling machining accuracy, and supplementing with advanced simulation methods, current technology can meet stringent performance requirements in most millimeter-wave applications.
