Spurious transmission effects due to the excitation of the bound mode and the continuous spectrum on stripline with an air gap

The nature of the strip current excited by a practical delta-gap source on a stripline structure with an air gap above the strip conductor is studied. This structure supports the existence of a dominant leaky mode (LM) in addition to a bound mode of propagation. It is shown that the total current on the strip excited by the source can be resolved into the sum of a bound-mode current and a continuous-spectrum current, with the latter current further represented as the sum of all physical LM currents and a "residual-wave" current. The roles of the various current components are investigated numerically as the frequency and air-gap thickness change. The appearance of a spurious sharp dip in the transmission response is demonstrated numerically and explained theoretically, and numerical results are compared with measurements. Since such a transmission dip can always occur for an appropriate set of parameter values, it is important to be able to understand and predict this effect.