It is possible to add an external antenna connector to the modem. You may use a standard cellphone to external FME-antenna adapter. Cut off the (proprietary) cellphone connector. Carefully strip the solder resist (varnish) from the groundplane on the PCB of the adapter, where you intend to solder the coax shield. Strip the coax, fan the shield braid and solder it to the ground plane. The center connector solderes easily in between the capacitor and inductor (see picture), leading to the center connector of the above mentioned internal connector. Be careful not to short the capacitor.
Tests were performed with a small yagi antenna. Signal strenght increases significantly from "poor" to "full" signal. However, the internal antenna does not seem to perform as well as before the modification. Hence, one should consider always using some sort of external antenna when the modification is applied.
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