My 3G wireless modem has difficulty connecting to the Optus network for the Virgin Mobile Broadband service from my desk (seems to work okay most other places in Canberra and Sydney). So I purchased a Panorama Data Card Antenna from Allphones for $AU29.95. This works very well. The hardest part was having the sales staff check this was the right antenna.
The antenna is about 120mm high and 10 mm in diameter. It comes with a 500mm cable and a plug for the HUAWEI E169 wireless USB modem. You flip up a small circular cover on the side of the modem and plug the antenna in. There is a small clip to attach the antenna to the screen of a laptop and an alternative desk stand is included. One clever touch is that the desk stand has a tab underneath to hold the laptop clip, so it doesn't get lost. The desk stand is a bit too lightweight and the antenna tends to tip over easily. But the is the only problem I have found.
This is more antenna than I need, and it is a shame they do not make a simpler unit which is just a whip with the plug on the bottom. This would work well when the modem is in the matching Huawei D100 3G Router. Alternatively there may be a way to modify the clip or stand so the antenna could be mounted neatly on top of the router, hiding the cable and plug.