SOUND WAVE AND MICROWAVE SPACE PATTERNS 



581 



next pair of pictures. In Fig. 24 the disks are arranged in an open construc- 

 tion so that sound waves as well as microwaves will pass through. An acoustic 

 amplitude pattern is shown taken at a frequency of 12 KC (X = 1.13"). 

 In Fig. 25 the disks are copper foil and are supported on polystyrene foam 

 which is transparent for radio waves but opaque for sound waves. The pat- 

 tern now shows the intensity distribution of the microwave field being 

 focused by the lens. The frequency was 9100 megacycles (X = 3.3 cm. or 

 1.3"). 



Fig. 24 — A sound field pattern of a disk array lens originally designed for 3 cm. radio 

 waves./ = 12 KC. (X = 1.13"). 



To obtain this microwave picture, the loud speaker was replaced by a 

 microwave radiator and the pickup microphone replaced by a tiny dipole 

 and crystal detector. The microwaves are modulated with 120^^ pulses so 

 that the same audio frequency amplifiers are used as before with sound 

 waves. However, with this low frequency supplied to it, the neon lamp trace 

 appears as a series of dots. 



In the next pair of pictures is shown a phase advance lens which likewise 

 is effective only for microwaves. It uses parallel conducting plates in a wave- 

 guide construction.^ Because the phase velocity is increased in passing 

 through this medium, a concave lens is required for focusing (see Fig. 26). 

 When the waveguide source (off to the left of Fig. 26) is brought in nearer 

 so as to be at the focal point of the lens, the wavefronts straighten out and 



