We claim the ornamental design for a RFID tag antenna, as shown and described.
The sole FIGURE is a plan view of a RFID tag antenna.
The solid black lines and solid black areas constitute the entirety of the claimed design. Broken lines depicting a RFID chip in the center of the FIGURE and broken lines on the periphery of the FIGURE illustrate environment only and form no part of the claimed design. All elements of the claimed design are flat and coplanar.