An animal containment system is described that comprises a transmitter unit and a receiver, the transmitter unit comprising a transmitter coil and signal generator. The signal generator transmits a modulated activation signal through the coil and generates a uniform magnetic field within a region around the coil. A receiver travels through a portion of the uniform magnetic field and uses information of periodically emphasized positively directed portions of the signal to detect a first polarity of the magnetic field in a first detection area and a second reversed polarity of the magnetic field in the second detection area. A detected reversal of polarity indicates a boundary crossing event including the receiver transitioning across the boundary.
