Add an additional half acre (circular) to your dog's outdoor playpen with this extra transmitter for your PetSafe Wireless Fence. For large or uniquely shaped yards, this extra unit can expand your yard coverage. Large, overlapping circular fields create even more pet paradise and give your pet more room to exercise, play, and stay safe. Completely wireless and portable, it's great for vacations or other locations. Requires the use of a PIF-300-21 Wireless Fence Receiver Collar (sold separately).