US Patent 7142167 Wireless boundary proximity determining and animal containment

A proximity detection system for determining when a single channel receiver becomes proximate to any point on a wireless closed boundary continuously generated by a transmitter. The transmitter includes a magnetic field generator broadcasting a composite, modulated, time-varying magnetic field signal of a particular carrier frequency. The receiver forms a measure of the broadcast magnetic field intensity incident to the location of the receiver or a measure of the power or energy of incident field.