Practical_Antenna_Handbook_0071639586

476 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
3 dB BW
Main lobe
Sidelobes
0
Figure 20.22B Graphically presented pattern.
Directivity D is a measure of the power density at the peak of the main lobe to the
average power density through the entire spherical surface surrounding the antenna:
or, referred to isotropic,
0111057 FIG 18-23B
D =
P PMAX
AVG
(20.24)
4 π
D = (20.25)
Φ
where D = directivity
P MAX = maximum power
P AVG = average power
F = solid angle subtended by main lobe
The term F is a solid angle, which emphasizes the fact that antenna patterns must be
examined in at least two extents: horizontal and vertical.
A common method for specifying antenna directivity is beamwidth (BW). The definition
of BW is the angular displacement between points on the main lobe (see Figs.
20.22A and 20.22B) where the power density drops to one-half (–3 dB) of its maximum
at the peak of the lobe. The angle between the –3 dB points is a, as shown in Fig. 20.22A.