SOUND WAVE AND MICROWAVE SPACE PAttERNS S7l 



Structure of the lens. The phase reversal at each successive minor Igibe is 

 readily seen from the fact that the bright areas in the minor lobes line up 

 with the dark areas of the adjacent lobes. 



The addition of phase makes it possible to obtain motion pictures of 

 progressive wave motion. By taking successive movie ''stills", in which the 

 phase front pattern is advanced one-eighth of a wavelength, a complete 

 cycle is obtained. This series of eight pictures is then repeated until a rea- 

 sonable length or loop of film is obtained. 



An example of the retarding effect caused by a delay lens is shown in 

 Fig. 8. This wave pattern is a composite (two exposure) picture and was 

 obtained in the following way: the left side of the photo was scanned with 

 only the feed horn active (lens removed). The lens was then put in place and 

 the probe continued to scan the area to the right. At the top of the photo, 

 the circular wave fronts are seen to be continuous from the horn out, but 

 in the shadow region behind the lens the wave fronts are retarded. One 

 sees that the insertion of the lens has caused a delay equal to about one 

 wavelength along the axis. This results in a curved wave front emerging 

 from the lens and a consequent focusing action. 



An acoustic (or microwave) lens in which the delay elements are slanted 

 guides similar in construction to a Venetian blind^ • "^ is shown in Fig. 9. The 

 feed horn is the same as that in Fig. 8 and is set at the focus of this lens 

 so as to produce fiat rather than converging phase fronts in the emerging 

 wave. As the directional pattern of the feed horn would indicate (Fig. 8), 

 the center portion of this lens is rather strongly "illuminated," but the result- 

 ing energy concentration, in passing through the lens, is shifted upwards 

 by the guiding action of the slanted plates. The resulting dissymmetry of 

 the vertical amplitude distribution at the aperture of this lens (indicated by 

 the increased thickness of the phase lines in the upper section of the photo- 

 graph) is in a large part responsible for the unsymmetrical minor lobe struc- 

 ture as reported. "^ The straightness of the phase lines, however, indicates 

 that the lens has converted the circular wave fronts from the horn into the 

 desired plane wave fronts. 



The undesirable concentration of energy at the center portion of the lens 

 can be materially reduced by substituting, for the small feed horn of Fig. 8, 

 a full conical horn shield having its throat at the focal point of the lens. 

 The energy distribution entering the lens is then fairly uniform and the 

 slant plates cause only a slight dissymmetry in the amplitude distribution 

 of the emerging wave as shown in Fig. 10. The distribution of such 

 a "shielded" lens in the horizontal plane, however, is even more uniform 

 since it is not skewed by the slant plates in this plane (Fig. 11). 

 *W. E. Kock, "Path Length Microwave Lenses," Proc. I.R.E., 37, 852 (1949). 



