Dominant mode power leakage from printed‐circuit waveguides

Abstract

This study is concerned with a new type of dominant mode power leakage from uniform lengths of printed‐circuit waveguides, such as microstrip line, slot line, and coplanar waveguide. These leakage effects, which are presently not recognized and can therefore be unexpected as well as undesired, occur at higher frequencies, and are therefore important for millimeter‐wave integrated circuits and high‐speed circuits. The cross talk and coupling that result can ruin the performance of a complex, high‐density circuit unless the leakage effects are understood and controlled. The leakage effects reported here occur on uniform lengths of printed‐circuit waveguide on which the dominant mode is purely bound at ordinary microwave frequencies but becomes leaky at higher frequencies; they appear in addition to the leakage produced by conversion into surface waves at various discontinuities. The nature of the leakage is described, and numerical results are presented for several structures as illustrations of the variety of ways this type of leakage can arise.

Even and Odd Modes on a Conductor-Backed Slotline

“…Their great drawback is a leakage of power in the substrate resulting in undesired coupling to the neighbouring circuit sections. That is why the CBSL has stood somewhat at the margin of the interest of designers and only a few publications [1][2][3] have worked on analyzing it.…”

The Residual-Wave Current Excited on a Printed-Circuit Line: A Source of Spurious Effects

“…As explained im [2] for the stripline problem, branch points appear in the k plane due to the multivalued nature of the current transform function in (1). The branch points are located at the propagation wavenumbers of the background modes of the structure (parallel-plate modes for the stripline structure, or surface-wave modes for the microstrip structure).…”

“…The propagation and excitation of leaky modes on printed-circuit lines has been a subject of considerable attention in recent years, for example [1]. Leaky modes are usually regarded as undesirable since they result in losses due to radiation, crosstalk phenomena, and other spurious effects such as nterference with bound modes that are also excited from the source.…”

Structural danger of printed-circuit transmission lines when they are used for millimeter-wave devices

“…We now know that this assumption is incorrect. It was found [3] that, when the strips widths of conductor-backed coplanar strips are increased, the critical frequencies f c1 and f c2 move upward and downward, respectively. As a result, there can exist a frequency range within which the bound and leaky modes can propagate simultaneously, and that the spectral gap then disappears as shown in Fig.3, where the strip width is changed from w/h = 0.375 of Fig.…”

“…From (1), we must have the relation b < k s for real-power leakage at an angle q, or, dividing by k 0 , b/k s < k s /k 0 (2) This relation (2) is the condition that must be approximately satisfied for leakage to occur [3]. For printed-circuit transmission lines, we know by experience that the plot of b/k 0 is relatively flat, with a small increase as the frequency f increases, but that the plot for k s /k 0 exhibits strongly dispersive behavior.…”