Do I need to use an external antenna to get the highest bandwidth?

Yes, to get the maximum bandwidth (uplink/downlink), as detailed in the table below, the external antenna has to be used.

  Thuraya IP Data Capabilities  Transmit  Receive
  Standard IP   Without External Antenna   202 kbps   Up to 444 kbps
  With External Antenna   Up to 404 kbps   Up to 444 kbps
  Streaming IP   Without External Antenna   16, 32, 64, 128, 256   kbps   16, 32, 64, 128, 256   kbps
  With External Antenna   16, 32, 64, 128, 256, 384   kbps   16, 32, 64, 128, 256, 384   kbps