Design Of Miniaturized Directional Coupler For High Power Applications

In microwave system, directional coupler is used widely in microwave components, which can extract power from the primary waveguide and output power directionally from auxiliary waveguides according to certain proportion. And it can realize the power regulation, sampling, monitoring and so on. With the continuous development of radar system, electronic reconnaissance and countermeasure, application of industrial microwave energy and other technologies, the requirements of structure, size and technical index of high power directional coupler become more and more highly. The research of new directional coupler has become one of the key directions of microwave technology. In this paper, the innovative design and simulation optimization of waveguide to microstrip directional coupler are carried out. The primary line uses the national standard BJ58 waveguide, which makes directional coupler with the characteristics of universal, high power, high mechanical strength. The auxiliary line uses a compact microstrip line structure, which reduces the section area of original waveguide type directional coupler to 50% and realizes the miniaturization design.Firstly, the processes of coupling through rectangular apertures and circular slots of two different transmission line, waveguide and microstrip, are analyzed in detail. And the constraint relations of the rectangular apertures and circular slots to the curve of coupling degree are discussed. The formula of distance of two slots is derived, which two different transmission line, waveguide and microstrip, can achieve directional coupling. Its effectiveness is verified by the simulation when rectangular apertures tend to two line sources and circular slots tend to two point sources. Two kinds of waveguide to microstrip directional couplers, waveguide to microstrip directional coupler with rectangular apertures as fishbone shape and waveguide to microstrip directional coupler with circular slots in two lines, are designed on this basis.Secondly, the high dielectric constant substrate is adopted to weaken the energy of high frequency part in the coupling curve, thus making the coupling degree realize the flatness on the design of waveguide to microstrip directional coupler with rectangular apertures as fishbone shape. The long side of the rectangular aperture is designed in arithmetic sequence arrangement, thus forming the structure of fishbone and broadening the frequency band of high directivity. In addition, the coupling port is optimized by rounding right angle of the microstrip line and the standing wave of the corner is suppressed by chamfer. The power capacity is analyzed based on this structure. The waveguide to microstrip line directional coupler with fishbone shaped apertures is finally designed, thus resulting in high power capacity, miniaturized size, high accuracy, and so on.Finally, the easily fabricated circular slots are used to replace the rectangular apertures. The coupling performance is further improved by adjusting the relative dielectric constant on the design of waveguide to microstrip directional coupler with circular slots in two lines. The structure of double-row circular slots arrangement is proposed. In this structure, the slots move vertically to the wave propagation direction from the center of microstrip line that overcomes the problems caused by the field coupling disorders at the edge of round slots. And high directivity is achieved through magnetic coupling. Meanwhile, through the method of reducing the edge of slot, the isolation energy is reduced and a high directivity of the porous coupler is achieved. The coupling port is optimized by rounding the right angle and the power capacity is analyzed. Waveguide to microstrip directional couplers with six circular slots in two lines and eight circular slots in two lines are finally designed, thus resulting in high power capacity, miniaturized size, wide band, and so on.