A lot of very good theory above none of this i doubt but from practical observation of the two most common ADF,s in use in GA the king KR87 has considerably less tendancy to "dip" than the Collins ADF650 this i find surprizing because they both use the same type of system to find the sorce of the radio signal to which the unit is tunned and both units have a good reputation with the avionics guys that i know.
The King KR87 recever has a tighter band width than the ADF650 but in theory this should not effect the dip when near the station ,i am left to speculate that it is something to do with the "damping" of the signal within the unit but this all goes far beond my electrical know how.
Comments from an avionics engineer please.