Also the thickness of the conductor determines the bandwidth. (thicker conductor = wider bandwidth)
for me here in the US an antenna that is 4.78 ft is good. (I used the rough center frequency of 98MHz for the FM dial. other countries find the center freq. of your fm dial and enter that in the calc )
you can cut the length of the antenna in half, but an 1/8th wave wants a ground available to function well.