IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 16, NO. 4, APRIL 2006 149 V-Band CPW 3-dB Tandem Coupler Using Air-Bridge Structure Sung-Woon Moon, Min Han, Jung-Hun Oh, Jin-Koo Rhee, Senior Member, IEEE, and Sam-Dong Kim Abstract—We present a uniplanar coplanar waveguide 3-dB tandem coupler operating at V-band frequencies. The uni- planar structure is monolithically fabricated by using two-section parallel-coupled lines and air-bridge crossovers replacing the conventional multilayer or the bonded structures. Due to an opti- mized tandem structure and nonbonded crossovers minimizing the parasitic components, a maximum coupling of 2.5 dB is measured at 62 GHz with a 2-dB bandwidth of 83%, while a high directivity factor of 33 dB is simultaneously obtained at 58–62 GHz. Over the entire design frequency range of 30–90 GHz, we achieve good phase unbalance of 90 6.0 , return loss, and isolation lower than 23 and 16 dB, respectively. Index Terms—Air-bridge, coplanar waveguide (CPW), tandem coupler, wideband. I. INTRODUCTION T HE directional couplers are fundamental and important passive components extensively used in the realization of a variety of microwave circuits, such as balanced amplifiers, balanced mixers, data modulators, and phase shifters. Various couplers have been examined by using the circuit structures based on unit coupling length of a quarter wavelengths 4 and an appropriate analysis of the even–odd impedances to achieve the required coupling, directivity, and phase difference [1]. The frequency response depends on the coupling circuit structure. For example, the edge-coupled-line couplers have a wider bandwidth than those of branch-line and ring-hybrid couplers [2]. However, the edge-coupled-line couplers of high coupling performance require a narrow gap between the lines; therefore, a critical precision is necessary for the fabrication process. The degradation of directivity is also unavoidable in this structure due to the phase velocity mismatch between even and odd modes originated from the difference in permittivity of the materials at the top and the bottom of the transmission line [3]. When we employ a structure comprising the -section parallel-coupled lines, we can simultaneously achieve a tight coupling and a high directivity even with a low coupling co- efficient of the individual coupler. Tandem couplers take this advantage and connect the loose-coupled couplers to form a tightly coupled coupler. To fabricate this type of coupler, most research efforts have been made on the configurations utilizing Manuscript received August 18, 2005; revised November 3, 2006. This work was supported by the Korea Science and Education Foundation under the ERC Program, Millimeter-Innovation Technology (MINT) Research Center, Dongguk University, Seoul, Korea. The review of this letter was arranged by Associate Editor A. Weisshaar. The authors are with the Millimeter-Wave Innovation Research Center, Dongguk University, Seoul 100-715, Korea (e-mail: samdong@ dongguk.edu). Digital Object Identifier 10.1109/LMWC.2006.872151 Fig. 1. Calculated even–odd mode effective dielectric constants versus the cou- pling coefficient of the CPW edge-coupled line at 60 GHz. the multilayer or the bonded structure because a three-dimen- sional (3-D) crossover connection is essential for the tandem structure [4], [5]. We adopted an air-bridge structure to materialize the coplanar waveguide (CPW)-based tandem coupler as a monolithically fabricated uniplanar structure. Compared to the conventional multilayer or bonded tandem structures, this structure can be made in a smaller size, and more reliable performance is ex- pected at millimeter-wave frequencies (30 300 GHz). In this letter, we report a uniplanar 3-dB tandem coupler with two CPW parallel-coupled lines of a 8-dB coupling, which can be mono- lithically integrated with the balanced amplifiers and the mixers operating at 60 GHz. II. DESIGN OF COUPLER Directivity factor is one of important characteristics in a directional coupler. It is given by (1), where and are the powers measured at an isolation port and a coupled port, respectively. The is degraded with the decrease of coupling or the increase of mismatch between the even and the odd mode phase velocities [6] (1) Fig. 1 shows the effective dielectric constants of two different modes calculated at various coupling coefficients for the CPW edge-coupled line coupler operating at 60 GHz. In this calculation, we assumed various line widths and coupling gaps formed on a 680- m-thick GaAs substrate. In tandem couplers, 1531-1309/$20.00 © 2006 IEEE