How to create a 13.56MHz PCB antenna in Eagle?
SparkFun have this RFID Evaluation Shield that works on 13.56MHz.  They provide the Eagle files which contain the antenna portion.  You could either use this for ideas or take the antenna portion from the .brd file.

Copying this sort of antenna is not a simple case of copy and paste.  If you look at the schematic you will see this section with C2-C7:



This relates to the following portion of the board:



These capacitors and two resistors (used as jumpers) tune the PCB antenna to work well on the chosen frequency.  If you choose to copy the antenna then make sure it's as close to the original as possible.  This means trace width, spacing, the feed line from your driver etc, even the PCB manufacturing process might make a difference.  If you keep it virtually identical to the original then you might get away with the capacitor values used by Sparkfun.

You might like to check their licensing model if your application is commercial.

Images in this post from Sparkfun, licensed CC BY-NC-SA 3.0.