Main lobe: Of an antenna radiation pattern, the lobe containing the maximum power (exhibiting the greatest field strength).

Note: The horizontal radiation pattern, i.e. , that which is plotted as a function of azimuth about the antenna, is usually specified. The width of the main lobe is usually specified as the angle encompassed between the points where the power has fallen 3 dB below the maximum value. The vertical radiation pattern, i.e. , that which is plotted as a function of elevation from a specified azimuth, is also of interest and may be similarly specified. Synonym main beam.

Source: from Federal Standard 1037C and from MIL-STD-188