Note that the input admittance or impedance defined at a gap source port is referenced to the two terminals of the voltage source connected across the gap as shown in the figure below. This is different than the input admittance or impedance that one may normally define for a microstrip port, which is referenced to the substrate's ground.
[[File:PMOM59(1).png|800px]]
Figure 1: Definition of different input impedances at the gap location.
To resolve this problem, you can place a gap source on a metal strip line by a distance of a quarter guide wavelength (λ<sub>g</sub>/4) away from its open end. Note that (λ<sub>g</sub> = 2p/Ã), where à is the propagation constant of the metallic transmission line. As show in the figure below, the impedance looking into an open quarter-wave line segment is zero, which effectively shorts the gap source to the planar structure's ground. The gap admittance or impedance in this case is identical to the input admittance or impedance of the planar structure.
[[File:PMOM60(1).png|800px]]
Figure 2: Placing a gap source a quarter guide wavelength away from the open end of a feed line to effectively short it to the ground at the gap location.
The same principle applies to the gap sources on slot traces. The figure below shows how to place two gap sources with opposite polarities a quarter guide wavelength away from their shorted ends to calculate the correct input impedance of the CPW line looking to the left of the gap sources. Note that in this case, you deal with shunt filament current sources across the two slot lines and that the slot line carry magnetic currents. The end of the slot lines look open to the magnetic currents, but in reality they short the electric field. The quarter-wave CPW line acts as an open circuit to the current sources.
[[File:PMOM61(1).png|800px]]
Figure 3: Placing two oppositely polarized gap sources a quarter guide wavelength away from the short end of a CPW line to effectively create an open circuit beyond the gap location.
The case of a probe source placed on a PEC via that is connected to a ground plane is more straightforward. In this case, the probe source's gap discontinuity is placed at the middle plane of the PEC via. If the via is short, it is meshed using a single prismatic element, which is connected to the ground from one side and to the metal strip line from the other. Therefore, the probe admittance or impedance is equal to that of the structure at a reference plane that passed through the host via.
[[File:PMOM62(2).png|800px]]
Figure 4: Input impedance of a probe source on a PEC via connected to a ground plane.
=== Exciting Multiport Structures Using Linear Superposition ===