Methods of reducing chromatic aberration in metal-plate microwave lenses

Microwave lenses of the waveguide type are subject to a pronounced degree of chromatic aberration because of the inherent dispersion of the waveguides. However, this aberration can be substantially eliminated over an appreciable bandwidth by making the lens of slot-loaded ridged waveguides with special properties. This type of lens design makes it possible to achieve additional benefits as well, such as improved scanning capability and elimination of discontinuities in the surfaces at the junction of adjacent "zones" or "steps." The ridged waveguides are periodically loaded by means of transverse slots in the ridges. The ridges may be relatively wide and hollow, or quite thin and solid. Additional desirable properties are obtainable by placing a relatively small strip of dielectric material above the ridge. Experimental data on the propagation characteristics of several of these structures and on the effect of varying such parameters as ridge height, slot width, slot depth, slot spacing, frequency, and size of dielectric strip are given. An analysis of the phase errors which result from changes in the operating frequency shows that the aberrations may conveniently be separated into one type in which the phase front from each zone of the lens is distorted, and a second independent type in which the desired phase relationship between the zones is destroyed. Methods of eliminating either or both of these types of aberration by suitable design of the ridged waveguides are described. In addition, it is shown how the ridged waveguides can conveniently be used to achieve "zoning" or "stepping" by changing the effective index of thereby eliminating discontinuities in the lens surfaces and avoiding the undesirable diffraction which would otherwise occur at the boundaries between adjacent zones. The superiority of this type of lens structure in wide-angle scanning applications is also discussed. Numerical examples which illustrate the advantages of the loaded ridged waveguide lens are included.