Page 160 - Satellite Communications, Fourth Edition

140 Chapter Six

z

P

r
θ

y

φ

x
Figure 6.3 The spherical coordinate system.

which gives the distance between point P and the antenna; is the
angle measured from the x axis to the projection of r in the xy plane; and

is the angle measured from the z axis to r.
It is important to note that the x, y, and z axes form a right-hand set.
What this means is that when one looks along the positive z direction,
a clockwise rotation is required to move from the positive x axis to the
positive y axis. (This is the same as the right-hand set introduced in
Sec. 5.1) The right-hand set rotation becomes particularly significant
when the polarization of the radio waves associated with antennas is
described.

6.4 The Radiated Fields
There are three main components to the radiated electromagnetic fields
surrounding an antenna: two near-field regions and a far-field region. The
field strengths of the near-field components decrease rapidly with
increasing distance from the antenna, one component being inversely
related to distance squared, and the other to the distance cubed. At com-
paratively short distances these components are negligible compared
with the radiated component used for radio communications, the field
strength of which decreases in proportion to distance. Estimates for the
distances at which the fields are significant are shown in Fig. 6.4a.