Transmitting one watt at 900 MHz, the modems pack quite a punch, but the stock rubber-duck antennae suck. What's worse, the internal antenna connector is nonstandard, and we've been as yet unsuccessful at identifying it. Adding better antennae makes it easy to surpass the maybe-a-mile distances achievable on the stock omnis, but first, we need a way to connect those antennae.
(Note: This Instructable originally identified the stock connector as a Hirose H.FL, but we've now confirmed that this is not the case. It's very similar, but not quite identical! If better information comes down about the connector itself, it will be posted here.)
Fortunately, it's relatively easy to replace the H.FL with the much more common (and much smaller) U.FL connector, which is found on MiniPCI wireless cards, among other places. Thus modified, the modem is easy to equip with the new antenna of your choice. The operation is straightforward, and this instructable is as much about good soldering technique as it is about the specific task at hand.
Because the radio modem was certified to meet FCC Part 15 requirements with its built-in antenna, changing that antenna breaks the certification, just like changing the antenna on an 802.11 device. This may fall under part 15.23 but I'm not a lawyer and can't say for certain whether a user-modified device counts as a home-built device. Refrain from pissing people off, and the feds should have no reason to bother you.
Or, if you're an amateur radio operator, you can operate under Part 97. Either way, if half-mile links were typical on the rubber ducks, just imagine what you'll achieve with some decently directional antennae!
This is my first instructable. Please be unsparing with your constructive criticism.