METALLIC DELAY LENSES 61 



space, just as parallel capacitors on a transmission line act as loading ele- 

 ments to reduce the wave velocity. Consider a charged parallel plate air 

 condenser with its electric lines of force perpendicular to the plates. Its 

 capacity can be increased either by the insertion of dielectric material or by 

 the insertion of insulated conducting objects between the plates if the ob- 

 jects have some length in the direction of the electric lines of force. This is 

 because such objects will cause a rearrangement of the lines of force (with a 

 consequent increase in their number) similar to that produced by the shift, 

 due to an applied field, of the oppositely charged particles comprising the 

 molecules of the dielectric material. The conducting elements in the lens 

 may thus be considered either as portions of individual condensers, or as 

 objects which, under the action of the applied field, act as dipoles and pro- 

 duce a dielectric polarization, similar to that formed by the rearrangement of 

 the charged particles comprising a non-polar dielectric.^ Either viewpoint 

 leads to the delay mechanism observed in the focussing action of the arti- 

 ficial dielectric lenses to be described. 



Experimental Models 



We turn now to experimental exemplifications of lenses built in accordance 

 with the principles outlined above. 

 (a) Sphere Array 



One of the simpler shapes of conducting elements to be tried was the 

 sphere. Figure 2 is a sketch of an array of conducting spheres arranged 

 approximately in the shape of a convex lens. The spheres are mounted on 

 insulated supporting rods; the microwave feed horn and receiver are shown 

 at the right. The focal length is /, the radius of the lens "aperture" is y, 

 the maximum thickness is x and not only the spacmgs 5i and ^2 but also the 

 size of the spheres are small compared to the wavelength. Rays A and B 

 are of equal electrical length because ray A is slowed down or delayed in 

 passing through the lens. Figure 3 is a photograph of the lens of Fig. 2; 

 it also portrays a similar sphere array lens made of steel ball bearings sup- 

 ported by sheets of polystyrene foam^ which have holes drilled in them to 

 accept the spheres. In both cases the balls are arranged in a symmetrical 

 lattice. It will be shown below that the polarizabiHty a of a conducting 



' Polar dielectrics have arrangements of charged particles which are electric dipoles 

 even before an external electric field is applied; the field tends to align these and the amount 

 of polarization (and hence the dielectric constant) that they exhibit depends upon tempera- 

 ture, since colhsions tend to destroy the aUgnment. Non-polar (or hetero-polar) molecules 

 have no dipole moment until an electric field is applied; the polarization of such materials 

 (and of the artificial dielectrics we are discussing) is accordingly independent of tempera- 

 ture. See, for example, Debye, "Polar Molecules", Chap. III. 



* Styrofoam (Dow) . Because of its low density (1 to 2 pounds per cu. ft.) , its contribu- 

 tion to the wave delay is neghgible (er = 1.02). 



