Practical_Antenna_Handbook_0071639586

R N-1
C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 25
Isophase
spherical
wave
surface
B 1
D 2
r b
r b
r b
r a
r a
r a
A
S
C
r a
r a
r b
r b
B 2
D 1
D 1 A-S
D S-C
R N
S
(Rotated view)
Figure 2.9B Fresnel zone geometry.
Using ray tracing, we see rays r a incoming to plane [B l -B 2 ] and rays r b outgoing from
plane [B l -B 2 ]. The signal seen at C is the algebraic sum of all rays r b. . The signal pattern
will have the form of an optical interference pattern with wave cancellation occurring
between r b waves that are a half wavelength apart on [B l -B 2 ]. The ray impact points on
plane [B l -B 2 ] form radii R n called Fresnel zones. The lengths of the radii are a function of
the frequency and the ratio of the distances D 1 and D 2 (see Fig. 2.9B). The general expression
is