An air-buried microstrip line (ABMSL) is monolithically fabricated on a glass substrate using a cost-effective multilayer process. The insertion loss of the line is 0.07 dB/mm and it shows high-isolation characteristics between close transmission lines. Using the ABMSL, a branch-line coupler is monolithically fabricated. Because the ABMSL structure employs air-dielectric medium and ground conductor walls surrounding a strip conductor line, it is very effective for reducing dielectric loss and parasitic coupling between close transmission lilies over a high-frequency range. The fabricated branch-line coupler shows coupling and transmission losses of 3.6 and 3.9 dB, respectively, at the center frequency of 29 GHz. In the frequency range front 26 to 32 GHz (20% bandwidth), the return loss is below than 15 dB and the isolation is better than 14 dB. (c) 2005 Wiley Periodicals, Inc.