Crosstalk and low frequency radiation in a coupled microstrip line with a top cover

In this work we use a full-wave method to analyze the propagation characteristics of a coupled microstrip transmission line with a metallic top cover. This metallic top cover may be present in a microstrip structure due to a circuit package. We show that an even leaky mode and an improper odd mode can be excited in this structure at low frequencies. As a consequence, effects as radiation, power loss, and interference, which are usually found in microstrip transmission lines at high frequencies, appear instead in this structure at low frequencies, thus compromising the signal integrity on the line. We provide numerical results to demonstrate that signal propagation and crosstalk for this line cannot be accurately predicted by a conventional analysis based upon a quasi-TEM approximation and transmission line theory even at frequencies such that the cross section of the line is much smaller than the wavelength.