RE: Futaba 2.4 and Carbon Fiber models
since futaba used an industry standard coax connector for the antenna coax ends, one could buy coax extensions of the right length, i have seen em up to 10 ft, cut them in 2, strip the 21-22 mm of coax insulation off the cut ends and have a pair of much longer antennas to mount. maybe one out the top of the rudder/fin and one out a wingtip.
i would not just use the original antenna attached to the extension by connector, but rather replace the whole antenna with a longer one.