Thank-you for your interest in Tallysman’s antennas. The ARP (Antenna Reference Point) for the TW4721 is the bottom of the antenna. This antenna has not been calibrated by NGS nor by IGS, therefore there are no .atx files available. The data available on Tallysman’s website shows the PCV measured by an independent lab using a Satimo chamber. Unfortunately, one cannot rely upon the PCO since the antenna wasn’t accurately centred in the chamber.