Wim mentioned the high angle lobe you get with a 5/8 wave antenna. It
produces a sky wave which can, if skip conditions are present, interfere
with the ground wave signal and cause fading. This could be a very
common and serious problem at higher frequencies where the ground wave
is weaker and the sky wave stronger. You can reduce this by keeping the
radiator height to 1/2 wavelength or less. Antennas in that height range
don't have a high angle lobe, although they do produce some radiation at
high angles. Radiators near a half wavelength in height produce less
high angle radiation than shorter antennas, and they have more
horizontal gain, so I'd think they would be best. I'm sure the AM
broadcasters have other "anti-fading" techniques -- other newsgroup
contributors would know much more about them than I do.

Roy Lewallen, W7EL