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Range on the PN532 is a few inches, almost touching basically. It's range is already maximized. Modifying the antenna changes the inductance of it. Even if you wanted to supply more power to it, the board doesn't support it. 13.56 MHz is meant for close range. I would probably stick to using a EM4100I guess you could also use a active tag(powered) but I don't think the cat would appreciate a project enclosure on it's collar..