I would suggest reading over pages 176 - 191 of the product manual. All of the approved antennas are listed.
See http://www.digi.com/resources/documentation/Digidocs/90001477/Default.htm#reference/r_antenna_options.htm%3FTocPath%3DCertifications|Agency%2520certifications%2520-%2520FCC%2520(United%2520States)|_____2