I. Introduction
With the advent of wireless communication technologies, compact transceivers operating at multiple separated frequency bands are essential. For example, high-speed wireless local area networks (LANs), offering up to theoretically 600-Mbit/s wireless access service, require transceivers operating at both 2.4- and 5-GHz bands. To accommodate such multiband signal reception and transmission, conventional passive components, but operating simultaneously at multiple, especially two [1]–[5], frequency bands are attracting wide attention recently. Among various passive components, the 90° directional coupler is one of the most fundamental components in microwave circuits and is used extensively in a variety of applications including mixers, multipliers, power amplifiers, and antenna feed networks [6]. There are two major forms that the coupler can take, namely, a branch-line configuration and a coupled-line configuration. Both of them have their own advantages and disadvantages. The former has a simple uni-planar realization, but relatively large in size, whereas the later requires a multilayer realization, but compact in size.