Contents I. Introduction
Since several microwave and millimeter-wave devices and circuits make use of rectangular waveguide bends, these components have been very widely investigated in the past. All the proposed configurations aim at compact size, low-cost manufacturing, and high return loss over a broad frequency band. In particular, quite common waveguide bends are the mitered ones. These components are very compact, but the return loss could be poor for broadband applications [1], [2]. Another bend type is the constant-curvature (uniform) one. These bends are low-cost devices, but are not the optimum solution in terms of both compact size and high return loss. In order to achieve better performances, two solutions were proposed in the literature. In [3], matching elements such as steps, stubs or septa were introduced in order to match the uniform bend. In this way, a high return loss can be obtained but the presence of matching elements leads to a more critical manufacturing at millimeter-waves. Variable-curvature (nonuniform) bends were proposed in [4], [5]. In these bends, the curvature of the waveguide is increased gradually from zero (straight waveguide) to a maximum (at the center of the bend). Thanks to the gradual variation of the bend radius, a higher return loss over a very large frequency band can be achieved, possibly with a slight increase of the bend size. Due to its features, this geometry seems to be very appealing for millimeter-wave applications. The only drawback, with respect to a uniform bend, is the increased complexity of the manufacturing process, related to the numerical definition of the curvature profile.
