Excitation of an Infinite Microstrip Line With a Vertical Coaxial Feed

An efficient method is presented for the analysis of a vertical coaxial probe excitation of an infinite microstrip line. The novel feature of the method is that it uses a semianalytical Green's function that is derived for current sources in the presence of the infinite microstrip line. Hence, in a method of moments approach, unknown currents need only be placed on the conducting probe feed and not on the infinite strip surface. The method also utilizes an attachment mode at the contact point between the probe and line so that the correct Kirchhoff condition is automatically satisfied. Once the surface current density on the probe is known, the surface current density on the strip conductor can be readily obtained using the Green's function of the background grounded substrate. The method is valid even at high frequency, where simple transmission line theory fails to account for effects such as the continuous-spectrum current that is excited on the line. After validating the method with various commercial software simulation packages, results are presented to study the fundamental behavior of the input impedance, probe current, and current launched on the microstrip line, and to examine the high-frequency behavior of these currents.