Practical_Antenna_Handbook_0071639586

472 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
Solution
P
P
=
4πr
d 2
= (1000W)
4π (1000m)
= 7.95×
10 W / m
2
–5 2
The rest of this chapter expresses antenna gains and directivities relative to isotropic
radiators.
Near Field and Far Field
Antennas are defined in terms of gain and directivity, both of which are measured by
examining the radiated field of the antenna. Published antenna patterns usually report
only far-field performance. In free space, the far field for most antennas falls off according
to the inverse square law. That is, the intensity falls off according to the square of the
distance (1/r 2 ), as in Eq. (20.19).
The near field of the antenna contains more energy than the far field because of its
proximity to the antenna radiator element, but it diminishes very rapidly with increasing
distance according to a 1/r 4 function. The minimum distance to the edge of the near field
is a function of both the wavelength of the radiated signals and the antenna dimensions:
r
min
= 2d
l
where r min = near-field distance
d = largest antenna dimension
l = wavelength of radiated signal (all factors in same units)
2
(20.20)
Example 20.6 An antenna with a length of 6 cm radiates a 12-cm wavelength signal.
Calculate the near-field distance.
Solution
r
min
= 2d
l
=
2
= 72
12
= 6 cm
(2) (6 cm)
12 cm
2