The Nature of the Leakage from Higher Modes on Microstrip Line

Some confusion in the literature is clarified regarding the properties of microstrip line higher modes in the neighborhood of cutoff. It is shown that those modes become leaky in that range, and that the leakage occurs in two forms, a surface wave and a space wave. Numerical values obtained from an accurate analysis are presented that illustrate the nature of the leakage for microstrip lines with either open or covered tops.