A well ranging apparatus, systems, and methods which operate to detect and determine a relative distance and/or azimuthal direction of nearby target well conductors such as pipes, well casing, etc., from within a borehole of a drilling well. An electromagnetic signal receiver is rotated in a first borehole. During the rotating, an electromagnetic ranging signal is received at the electromagnetic signal receiver. The electromagnetic ranging signal originates from an electromagnetic source in the first borehole or a second borehole. A null in an Earth electromagnetic signal spectrum associated with a signal induced by the rotating within a magnetic field of the Earth is determined. The electromagnetic source to is set to operate at an excitation frequency that is selected based, at least in part, on the determined null.