Reference to Nearest Negative Signal
When you setup excitations using the Edge Port Definition window, select to select the Reference option and specify the nearest negative signal. This option explicitly forces the reference of a port to be located on the nearest negative signal plane, directly below (or above) the port. A vertical trace connecting the port to its reference is added to the structure.
The Reference to a nearest negative signal option should be used when the port is over one-tenth of a wavelength on the negative signal plane. If the port is closer than one-tenth of a wavelength, referencing to the nearest negative signal is inappropriate.
In a stripline environment, the additional vertical current that results from using the Reference option to the nearest negative signal may excite the parallel plate mode — which could create inaccurate results.