I don't think it has. Bandwidth depends on the diameter to
length ratio of the antenna. The greater the diameter of the
elements the wider the bandwidth. The inductance goes down and the
capacitance goes up, giving the antenna a lower Q. the folded
dipole has a greater effective diameter (at least double for the
same materials). You can increase a normal dipole's bandwidth by
increasing the diameter, hence the old time birdcage aerials.