Unplug the power supply for the invisible fencing. Locate the terminals to which the invisible fence ground wires are connected with screws.
Loosen the contact screws with a blade screwdriver and remove the cables from the terminals.
Hold an RF choke in the round middle section so that a wire extends out to both sides. Connect a wire to each terminal of the power supply.
Attach a wire from the underground fencing to each wire of the RF choke that is connected to the terminal. Wrap the invisible fence ground wires around the RF choke wires to hold them in place.
Plug in the power supply and turn up the signal strength. Tune an AM radio to 600 mhz and begin walking the fence line following the low tone coming from the radio. Follow the line until the tone stops, indicating a break.