Wireless Fence Extra Transmitter by PetSafe

PetSafe says:

This extra wireless fence unit can be used to expand the limits of your existing Wireless Fence System. Set up overlapping circular fields to give your pet greater room to roam. Requires use of a PIF-275 Wireless Fence Receiver Collar (sold separately).

    *  Covers an adjustable circular area up 1/2 acre (180 feet in diameter)
    * Completely wireless/portable
    * Locate indoors 2 to 4 feet above ground level
    * Includes AC Power Adaptor