Practical_Antenna_Handbook_0071639586

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 23
Incident wave
Less dense
More dense
(Note: a i a r )
a i
Line perpendicular
to surface
a r
Refracted wave
Reflected wave
Original path
Zone
A
Zone
B
Figure 2.8A Reflection and refraction
phenomena.
sible for both reflection and refraction
to occur in the same system. Indeed,
more than one form of refraction might
be present. More on these topics later.
Diffraction is shown in Fig. 2.8B. In
this case, an advancing wavefront encounters
an opaque object (e.g., a steel
building). The shadow zone behind the
building is not simply perpendicular
to the wave but takes on a cone shape
as waves bend around the object. The
“umbra region” (or diffraction zone)
between the shadow zone (“cone of silence”)
and the direct propagation zone
is a region of weak (but not zero) signal
strength. In practical situations, signal
strength in the cone of silence rarely
reaches zero. Also, it is not uncommon
for a certain amount of reflected signal
scattered from other surfaces to fill in
the shadow a little bit. The degree of
diffraction effect seen in any given case
is a function of the wavelength of the signal relative to the dimensions of the object and
the object’s electromagnetic Âproperties.
Dispersion can be seen in the spreading of a beam of white light into its separate
colors, such as happens with a prism. It is a direct result of the angle of refraction for a
Figure 2.8B Diffraction phenomena.