Radiation characteristics of annular-slot arrays fed by radial waveguides

A method is established which gives the field solution for an annular slot array fed by a radial waveguide. The exciting source is placed in the central area of the feeding waveguide and its field is assumed as a summation of the radial waveguide modes. The dielectric constant of the waveguide region could be lossy or lossless. To obtain a solution, the aperture electric field is expanded into a finite series of suitable basis functions with unknown coefficients, the number of which is directly related to the electrical dimensions of the slots. The complex constants are then obtained by employing the appropriate Green´s functions and an application of the boundary conditions over the aperture. It is shown that, in general, higher order modes of the radial waveguide region excited by the slot discontinuities may have a significant effect on the solution and for a precise evaluation of the field their contribution must also be included. The possible application of the method for the design of arrays with directive beams is also discussed.