The rubber ducky antenna available for sale from Parallax (#32410) has a very limited range when attached to an XBee Pro SB3 900MHz module. After a few tests, I found it very difficult to communicate further than 1/4 of mile. On the Parallax page for the module, it claims up to 9 miles with a dipole antenna and 28 miles with a high gain antenna. So I started trying to find an appropriate antenna. After a lot of research without finding a definitive answer online, I went ahead and bought a yagi antenna from digikey: OSCAR3A/X/FMEM/S/S/19-ND. Here's the datasheet for it: https://www.siretta.co.uk/download.php?id=709&cmd=view
. It sort of made me nervous because it claims a max power rating of 200W so I wasn't sure what would happen with 0.25W driving it. Also I needed a Female FME to RPSMA Male cable. The antenna is about 6 by 24 inches in size.
So my setup was 2 XBee modules with 2 antennas about 4 feet off the ground. One powered via walworth supply and the other via USB. I haven't full tested the range on this setup since I found it suitable to my application, but I found it reaching 1.5 miles without any problem and without ideal conditions. Even with the antenna facing backwards and invisible below a hill, I could communicate with it over a mile away. At 1.5 miles, with one antenna turned backwards, it still had only a 10% packet loss. I'm guessing its reasonable to expect 2 to 3 miles line-of-sight.
So I hope this post helps someone googling for hours like I was. I'll update it if I ever learn more or do more range tests. I wish Parallax would sell a directional antenna or at least give a recommendation, because your application space opens up greatly with 1+ mile unlicensed radio communication.