DIFFRACTION OK KAIJIO VVAVKS 



121 



15.4.: 



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, is the source 



. o 



X 



B 



Figure 8. Radiation wavefronts. A. Spherical wave- 

 front. B. Plane wavefront. 



of radiation, and AB a portion of the spherical 

 wavefront. From centers a, b, 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 8B 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 

 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 6) . The 

 field strength drops from a value 2 in the forward 

 direction to 1 along the line xy and to zero in the 

 backward direction (6 = 180°). While in Figure 8A 

 an envelope of secondary wavelets can also be drawn 



Figure 9. Reflection of waves from a plane surface. 

 Huyghens' construction. 



b 



SOURCE 



Figure 10. The secondary wavelet. 



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. 



13 43 Fresnel Zones 



In Figure 11, BC denotes a plane wavefront 

 moving from a distant source on the right toward 



