Rob, as Chris pointed out, the length of an FM antenna should be a multiple of the FM wavelength. The wavelength at the middle of the FM band is about 10 feet. For a single wire antenna to work best(but still not all that great)a half-wavelength length, i.e., about 5 feet, is usually suggested. So if you happen to have some copper wire around, you might twist a couple more feet on to the end of the present wire to see if it makes any difference.
But so much for theory. For the real world you have to study
this survey from the Berkeley boys.
