An excitation theory for bound modes, leaky modes, and residual-wave currents on stripline structures

The nature of the current on a general multilayered printed-circuit stripline structure excited by a delta-gap source is investigated. The current is obtained through the construction of a semianalytical three-dimensional (3-D) Green's function, which accounts for the presence of the infinite conducting strip and the layered background structure. The 3-D Green's function is obtained by Fourier transforming the delta-gap source in the longitudinal (z) direction, which effectively resolves the 3-D problem of a delta-gap source into a superposition of 2-D problems, each of which is infinite in the z direction. The analysis allows for a convenient decomposition of the strip current into a sum of constituent parts. In particular, the strip current is first resolved into a set of bound-mode current waves and a continuous-spectrum current. The continuous-spectrum current is then represented as a set of physical leaky-mode currents in addition to a set of "residual-wave" currents, which arise from the steepest-descent integration paths. An asymptotic analysis reveals that the residual-wave currents decay algebraically as z(-3/2). Far away from the source, the residual-wave currents dominate the continuous-spectrum strip current. Results are shown for a specific type of stripline structure, but the analysis and conclusions are valid for arbitrary multilayer stripline structures.