The mm-wave Broadwall couplers are essential components for RF systems. Couplers are normally divided into two categories based on their coupling level: the strong and weak coupling structures. Broadwall coupler falls in the category of weak coupling structures used for test and measurement purposes. Broadwall couplers can either sample in the forward or reverse direction, enabling various applications. In this project, we are interested in designing WR 28 Broadwall couplers with coaxial interfaces. There is a significant increase in the importance of such components to serve various communication systems, especially 5G/6G standards and satellite applications. This project aims to develop a wideband Broadwall coupler with a flat coupling level in the Ka-band covering from 26.5 GHz to 40 GHz. The objective unit will support a forward and reverse coupling of 30.0 dB ± 1.0 dB and directivity beyond 15 dB, which is challenging in the mm-wave band. In addition, the requested main arm return loss is below 17.7 dB, and insertion loss is above -0.6 dB over the whole operating range.
The first design challenge is the wide matching bandwidth of two-end launch coaxial interfaces at this high frequency. Tuning becomes more challenging with a two-port end launch coaxial setup. There will be a loading effect with a two-port setup, and it is difficult to determine which end is the source of matching deterioration. Moreover, the main arm return loss has a significant effect on the directivity. The second design challenge is the fabrication tolerance while making the coupling holes and wall thickness between the main and secondary arm. The minimum wall thickness must be 0.02” between the primary and secondary arm. Moreover, the minimum coupling hole diameter can be 0.15”, which limits the degrees of freedom. The final design challenge is from the mode converting section. in the end-launch coaxial interface. A pure TEM signal goes to a single ridge section with TE mode, and from a single ridge, it goes to a rectangular waveguide.