Lecture 49 : Radiation Characteristics of an Antenna
Half Power Beam Width(HPBW) of an antenna
The main beam is the angular region where primarily the radiation goes. The effective width of the antenna main beam called the HPBW is defined as the angular separation between directions where the field strength reduces to of its maximum value.
Since the power density of a wave is proportional to the square of the electric field, when the electric field reduces to of its maximum value, the power density reduces to of its maximum value. That is, the power density reduces by 3-dB. The HPBW therefore is also referred to as the 3-dB Beam width.
There two HPBWs, one for the E-plane pattern and other for the H-plane pattern.
For the Hertz dipole, the E-plane HPBW is and the H-plane HPBW is not defined since the radiation pattern is constant in the H-plane.
The HPBW is a better measure of the effective width of the main beam of the antenna compared to BWFN because there are situations when the effective width of the antenna beam changes but the BWFN remains same.
Side-Lobe Level(SLL)
The local maxima in the radiation pattern are called the side-lobes of the radiation pattern.
Since ideally the antenna should radiate along the direction of the main beam the side-lobes essentially indicate the leakage of power in undesired directions. The side-lobes in general is an undesirable feature in a radiation pattern.
The ratio of the main beam to the highest side-lobe is called the SSL of the radiation pattern. For a good communication antenna the SLL lies in the range of 30-40 dB
Since the Hertz dipole has only one maximum in the radiation pattern, there are no side-lobes for the Hertz dipoles.
of its maximum value. 
of its maximum value, the power density reduces to 
of its maximum value. That is, the power density reduces by 3-dB. The HPBW therefore is also referred to as the 3-dB Beam width. 
and the H-plane HPBW is not defined since the radiation pattern is constant in the H-plane. 
