First of all please note that SRTAG2K-D is Not Recommended for New Design, so I strongly suggest that you use ST25TA02K-P instead. This last is smaller in terms of package and also has a bigger tuning capacitance value (50pF instead of 25pF).
About antenna design the document explaining this item is located here:http://www.st.com/st-web-ui/static/active/en/resource/technical/document/application_note/CD00232630.pdf
In practice it is about designing an inductor which in conjunction with tuning capacitor resonates around 14MHz.
Also find here a software tool that permits a first inductance calculation with respect to your design: http://www.st.com/web/en/catalog/tools/PF257518
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