A strip line broadside hybrid coupler which is tolerant to manufacturing errors in a multi-layered LTCC substrate has been developed. The tolerance to a displacement error and a thickness variation in the multi-layered LTCC substrate can be achieved by using the tandem arrangement of diagonally shifted coupled lines with adjacent ground walls. It has been demonstrated that the coupling deviation from designed characteristics in our proposed hybrid coupler is very small.

This paper presents a printed circuit board (PCB) integrated multi-layered strip line tandem coupler, which used simple compensating ground through-hole (GTH) elements. The GTH elements on one end of the coupled line can generate additional capacitance between the signal line and the ground, which effectively compensates for the parasitic capacitance around the crossed signal lines on the opposite end of the coupled line. It has been experimentally demonstrated that the proposed coupler fabricated for the X-band is effective to improve both the reflection and the isolation characteristics.

In a multi-layered RF circuit, it is important to avoid unexpected coupling caused by a parallel plate mode excited between different ground layers. Ground via-holes that short-circuit different ground layers are used for suppressing this mode. Quantitative evaluation of relations between suppression effect and ground via-hole disposition is required for optimal design. In this paper, a simple design formula that describes the suppression ratio is derived by mode-matching technique. The results of comparison with an FEM simulation validate our proposed formula. It is shown that the technique is indispensable for designing optimal disposition of via-holes to minimize the area of the ground via-holes for desired performance.

A millimeter-wave termination which is tolerant to the resistance error of the embedded resistive film in a multi-layered LTCC substrate has been developed. The tolerance to the resistance error can be accomplished using two bifurcated strip lines overlapping with the resistive film, whose lengths are different form each other. It has been experimentally demonstrated that the proposed termination configuration is effective to enhance the tolerance to resistance error of the embedded resistive film in the LTCC substrate.

A hexagonal waveguide branch line coupler suitable for additive manufacturing is proposed in this study, and its design method is elucidated. The additive manufactured Ka-band coupler exhibits characteristics similar to those of a machined coupler, but its weight and cost are reduced by 40% and 60%, respectively. Its effectiveness is also confirmed in this study.