Usually the current antennas used on your current USB modem are extremely small and provide low signal so adding one of these with a 3G antenna will boost your signal greatly.
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SMA to Induction 3G Plug  | 510mm | AR3330 |
SMA Adaptor To HUAWEI E160/618 Plug | 500mm | AR3332 |
SMA Adaptor To SIERRA TS9 Plug    | 500mm | AR3334 |
SMA Adaptor To TELSTRA 4G USB Modem Lead   | 490mm | AR3336 |
Low Loss SMA Extension Lead    | 5m | WC7824 |