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/ß), where ß 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]]