Ground waves travel horizontally away from the antenna just above the ground, therefore the goal of most mast designs is to radiate a maximum amount of power in horizontal directions. An ideal monopole antenna radiates maximum power in horizontal directions at a height of 225 electrical degrees, about 5/8 or 0.625 of a wavelength (this is an approximation valid for a typical finite thickness mast; for an infinitely thin mast the maximum occurs at formula_14 = .637formula_15) As shown in the diagram, at heights below a half wavelength (180 electrical degrees) the radiation pattern of the antenna has a single lobe with a maximum in horizontal directions. At heights above a half wavelength the pattern splits and has a second lobe directed into the sky at an angle of about 60°. The reason horizontal radiation is maximum at 0.625formula_15 is that at slightly above a half wavelength, the opposite phase radiation from the two lobes interferes destructively and cancels at high elevation angles, causing most of the power to be emitted in horizontal directions. Heights above 0.625formula_15 are not generally used because above this the power radiated in horizontal directions decreases rapidly due to increasing power wasted into the sky in the second lobe.