In my Belkin Tunecast 3 FM transmitter disassembly article I noted that "The radio signal is attenuated before being passed to the antenna". Attenuator may be seen on the back of the circuit board between the FM transmitter IC and the antenna. Here is a closeup:
The pin at the top left corner of the IC in the bottom right is the RF output and the wire connection near the L5 marking near the top left is the antenna. Here is a schematic:
It would be simple to remove C38 and connect the internal antenna directly to pin 12. This would result in a more powerful signal. One could also connect an external antenna to pin 12. It might be better to put a capacitor between pin 12 and the external antenna; one circuit diagram uses 10pF. Note that some modifications may increase the signal beyond permitted limits.