There is no way to tell without trying. The best way to try is to disconnect the Interlink from the 798 so the 798 will not be influenced by the 997 antenna, then check the satellite status with the internal antenna setting. Next hook up your spare GPS antenna to the 798, just lay it beside the unit, and tell the unit to use it.
Compare the two status results to see if there is a big or any difference.
Your problem may be how you have the Interlink connected units set to use the GPS antennas, you may have a faulty internal antenna, or you may have a unit mounting issue.