SOUND WAVE AND MICROWAVE SPACE PATTERNS 573 



the slant plate lens ensures that plane wave fronts will be produced from 

 14 KC down to the lowest frequency contemplated for use, and almost any 

 size can be employed since microwave lenses of 10 and 20 foot apertures^ 

 have been built and found quite satisfactory. 



In the three preceding photographs, variations in the intensity of the 

 emerging waves (variations in the thickness of the phase lines) can be ob- 

 served. To show this effect more clearly and to indicate the symmetry of 

 these amplitude variations, Fig. 11 was retaken with the phasing signal 



Fig. 12 — Removal of the phase signal in Fig. 11 shows more clearly the amplitude variations 

 present in this horn-lens pattern./ = 9 KC. 



removed so as to obtain a pure amplitude pattern (Fig. 12). As mentioned 

 in connection with Fig. 4, these patterns simply show the intensity varia- 

 tions which are always present in the close-in or Fresnel field of any plane 

 wave source having a finite cross-sectional extent. For a given wavelength 

 and cross-sectional dimension, these variations can be calculated from 

 diffraction theory and evaluated with the aid of Cornu's spiral. It is interest- 

 ing to compare Fig. 12 with Fig. 13 which is a Schlieren photo of the sound 

 field in front of an ultrasonic quartz radiator of 14 wavelengths aperture 

 dimension.^ Although the aperture dimensions in wavelengths in the two 

 cases are different and the actual wavelengths are quite different, there is a 



* W. E. Kock, "Metal Lens Antennas," Proc. I.R.E., 34, 828 (1946). 

 ® K. Osterhammel, "Optische Untersuchung des Schallfeldes Kolbenformig schwingen- 

 der Quarze," AkusHsche Zdts. 6, 82. (1941). 



