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). Features Include:Cover an adjustable circular area up to 1/2 acre (180 feet in diameter)completely wireless/portable locate indoors 2-4 feet above ground level Includes AC power adapter. The PetSafe if-100 Can can be expanded up to 5 working units overlapping property boundaries.