The effect of air bridge height on the propagation characteristics of microstrip

The air bridge is often used for lowering the effective capacitance per unit length and fabricating crossovers in monolithic microwave integrated circuits (MMICs). The static and dynamic propagation characteristics of this type of transmission line are computed by utilizing the spectral domain technique. This analysis is used to demonstrate the significant changes in the propagation characteristic as a function of the height of the air bridge. The cases of air bridged lines on 100 mu m GaAs substrate and spiral inductors are examined. It is shown that the reduction in the capacitance of the air bridge lines is quite significant, leading to a considerable improvement in the self-resonant frequency of a spiral inductor used in a MMIC design.<>