The link below leads to a composite NEC plot comparing the elevation
patterns of a vertical dipole with one that is "tipped."

Sure enough, the tipped antenna has slightly more peak gain, and the
elevation null at the zenith above the vertical version is filled --
just as Art has posted.

However the reason for that is NOT that the antenna itself radiates
better when tipped. It is the result of directing more of the peak
gain of the radiation pattern launched by the antenna (which is a
constant regardless of its physical orientation) toward the earth near
the antenna. This increases the earth reflection in that direction/
sector.

Those considering only the net, far-field radiation as calculated by
NEC may think that the antenna itself radiates better when tipped.
But it is only the contribution of the earth reflection that makes it
appear so.

The radiation pattern of the antenna itself remains the same,
regardless of its physical orientation.

http://i62.photobucket.com/albums/h8...cal_Dipole.gif

RF