Direct modal transition from space wave to surface wave leakage on microstrip lines

Abstract

In this paper it is shown for the first time that a novel transition may occur for dominant (quasi transverse electromagnetic (quasi‐TEM)) leaky modes on microstrip line, between a leaky mode that leaks into the TM0 surface wave mode and one that leaks both into the surface wave and into space. The modal evolution of leaky modes as a function of frequency is studied by means of a full‐wave spectral domain approach, and it is shown that such transitions occur on microstrip lines that have a critical strip width, which is relatively large. When the strip width is equal to the critical value, a transition frequency will exist at which the attenuation constant of the leaky mode drops exactly to zero and the leaky mode has a real propagation wave number exactly equal to the free‐space wave number. At this frequency, the leaky mode transitions from one that leaks into only the surface wave to one that also leaks into space. In a frequency neighborhood of the transition frequency, very large spurious effects may be produced because of interference between the fundamental quasi‐TEM mode and the continuous spectrum current.