Beam width of an antenna gives an idea of the antennas focus. Beamwidth is listed in degrees for both planes in a polar point graph.
The beam width is calculated by finding the strongest point on a plot, somewhere on the outer circle.
The plot is followed in either direction until the value decreases by 3dB, the point where the signal is half of the strongest power.
A line is drawn from the centre of the plot to intersect at each of the two points, and an angle is measured.
The resulting angle shows the beam width.