Increasing Plane Wave Coupling to a Microstrip on a GTEM Cell Wall in Radiated Susceptibility Measurement

Abstract

Despite many published studies on coupling of plane wave to a microstrip line, an important special case of incident angle parallel to the ground plane has received much less attention. This special case is essential in radiated immunity testing where the excitation voltage to the device under test (DUT) is delivered through a microstrip line on a printed circuit board mounted on the gigahertz/transverse electromagnetic (GTEM/TEM) cell wall. We point out that the published analytical coupling formula for this special case must be corrected by a factor of 1/2 since in the GTEM/TEM the incident electric field does not double as opposed to that assumed in the infinite ground plane for which the formula was derived. Next, we introduce a method to increase the coupling factor by approximately a factor of two or more by meandering the microstrip, which recovers the value expected from the analytical formula without the 1/2 correction. We demonstrate that the increase of coupling is due to the meander line exhibiting a finite impulse response high-pass filter with increased responses compared to the straight line. Full-wave simulations and GTEM measurements verified our analysis. The meander line permits ample voltage to induce an upset event in a radiated immunity test using a low-cost GTEM 5402-SAE at a modest RF power of 50 W.