Radiation Pattern Lobes

A radiation pattern lobe is a section of the radiation pattern surrounded by regions of relatively weak radiation intensity.

Major lobe / main beam
The lobe in the radiation pattern containing the direction of maximum radiation.
Side lobe
The lobe in any direction other than the intended lobe and usually represent radiation in undesired directions.
Back lobe
The lobe in the radiation pattern that is in the opposite direction of the major lobe.

Figure 1. Characteristic values of an antenna, related to the field pattern.
Characteristic values:
20 log ( b )
Side lobe attenuation
20 log ( c )
Back lobe attenuation
2 φ M
3dB angle of main lobe
2 φ S
3dB angle of side lobe