78 TECHNICAL SURVEY 
Wave Propagation 
For most purposes the antenna may be considered 
as a point source of radiation. Near the antenna 
the wavefront (the locus of points of constant transit 
time) is spherical, but at great distances it is prac- 
tically plane. According to Huyghens’ principle each 
point of a wavefront may be considered as a source 
emitting wavelets whose envelope at a given time 
is the new wavefront. In Figure 8A, O is the source 
Ficure 8. Radiation wavefronts. A. Spherical wave- 
front. B. Plane wavefront. 
of radiation, and AB a portion of the spherical 
wavefront. From centers a, 6, and c, secondary waves 
spread out as shown by the dotted lines and are 
enveloped in the new front A’B’. A similar construc- 
tion is made in Figure 28 for a plane wavefront. 
Another example showing how waves are reflected 
from a plane surface is given in Figure 9. A wavefront 
AB is descending in an oblique direction on the 
FicureE 9. Reflection of waves from a plane surface. 
Huyghens’ construction. 
reflecting surface AB’ Points ACDEB’ are struck 
successively and in turn become centers of new 
wavelets. In the time required for B to reach B’ 
the wavelet from A spreads to a radius AA’, the 
distance it would have traveled if there were no 
reflector. Other wavelets have lesser radii which, in 
spreading, form a new wavefront. This is the reflected 
wavefront, and its angle with the reflecting surface 
is the same as that of the incident wavefront. 
The secondary wavelet from a point on a spherical 
wavefront (AB in Figure 10) does not produce the 
same effect in all directions. The field strength in a 
direction ac varies in proportion to (1 + cos @). The 
field strength drops from a value 2 in the forward 
direction to 1 along the line zy and to zero in the 
backward direction (@ = 180°). While in Figure 8A 
an envelope of secondary wavelets can also be drawn 
to the right of AB so as to produce a convergent 
wave traveling back to zero, it can be shown that 
this backwave does not exist. Only waves in the 
forward direction should be considered. 
x 
B 
f) 
Pa mya Neale SOURCE 
=) a 
W2 | 
7 I A 
y, 
Cc | 
| 
y 
Figure 10. The secondary wavelet. 
Fresnel Zones 
In Figure 11, BC denotes a plane wavefront 
moving from a distant source on the right toward 
a point P to the left. It is desired to know the effect 
at P of the secondary wavelets emanating from the 
wavefront. A straight line is drawn from the distant 
source to point P cutting the wavefront at C. In the 
wavefront with C as a center are drawn circles such 
that the first is a half wavelength further from P than 
C is, the second is 2 half wavelengths, etc., so that 
the secondary disturbance from any circle will reach 
P half a wavelength ahead of those from the circle 
enclosing it. 
If PC = b, the radius 7; of the first zone may be 
obtained from 
\E 
(643) oe ne. 
