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

Abstract

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.