7K is the most common frequency, the.75 PIR-003 10K was used in installations experiencing frequency conflicts with neighboring containment systems. Perimeter Technologies, a leading manufacturer of electronic pet fencing, has designed a cost effective alternative to the once exclusive Invisible Fence R21 Fence dog fence collar, providing owners of the nation’s largest and best known professionally installed dog fence brand a significant savings on their replacement collar receivers.Don’t be fooled by the low aftermarket price. Perimeter Technologies is a brand you can trust with products that are loaded with exclusive features and benefits not found in the original. As a matter of fact, Perimeter Technologies' President is the same person that founded Invisible Fence brand back in 1976!.75 Compare Perimeter Technologies with your current R21Patented Comfort Contacts cushion your dog’s neck in soft rubber Patented 3-Way, Omni Directional antenna design eliminates the “holes” found in lesser pet fencing systems Waterproof and submersible collar receiver design Progressive Stimulation allows your dog to choose the fence level that is right for their personality Small and light weight collar design is a mere 1.1 oz! Long lasting replaceable lithium ion batteries Designed and Manufactured in the USA Limited Lifetime warranty 100% compatibility – guaranteed!This replacement collar receiver for 700 Series Invisible Fence systems only and will not work with 800 series or later, including new Titanium Series. The Perimeter collar is not compatible with Shields.75 Indoor Avoidance transmitters.FULLY COMPATIBLE WITH THE FOLLOWING INVISIBLE FENCE BRAND TRANSMITTERS: ICT-700, ICT-725, ICT-750, ICT-775, ICT-150, ICT-250, ICT-100, ICT-75, ICT-50, AT-950 Indoor transmitter, CT-4000.75