Improvements in or relating to electromagnetic wave lenses and mirrors

860,826. Radio refractors and reflectors. MINISTER OF AVIATION. Nov. 22, 1957 [Aug. 24, 1956], No. 25926/56. Class 40 (7). The invention relates to radio wave lenses and mirrors of a type having a large number of independent design -variables so that a wide variety of characteristics can be obtained. In the ease of lenses these comprise two assemblies of aerial elements, each assembly defining a lens surface and there being negligible free space coupling between the elements of the two assemblies. Each element of one assembly is connected by a two-conductor transmission line to a corresponding element in the other assembly. There are thus available as independent design variables the configurations of the two surfaces and the electrical lengths of the transmission line paths. Restricting consideration to a single plane this means that a typical element in one assembly positioned at (x, y) is coupled to a typical element in the other assembly at position (#, #) by a line of electrical length W. So that for a given value of y four design variables x, #, #, and W are available. As an example of the design of lenses according to the invention the Specification gives equations for the construction of lenses having four spaced focal points and proceeds from this to the case where the four focal points are made coincident. In a special lens of this type all the transmission lines are of the same length and by choosing this length to be a multiple of a half the working wavelength surface mismatches can be cancelled over the whole lens. The four independent variables can be used in other ways than for providing four focal points. The spacing between the aerial elements of the assemblies must be chosen to ensure effective continuity of the lens surface and to avoid mismatch ; it is preferably less than the working wavelength. As the bandwidth of the lens is determined only by the bandwidth that can be handled by the units employed it can be made large. The position of the focal points does not vary the frequency. If the reduction in bandwidth can be tolerated steps reducing the transmission line lengths by a whole number of working wavelengths can be introduced. It is possible to introduce amplifiers or attenuators into the transmission lines. The lenses of the invention may be constructed to provide a scan. Fig. 7 shows a simple example of a scanning lens wherein dipoles 1 backed by reflector 2 form a first and fixed aerial assembly and dipoles 3 back by reflector