Annular slot direction-finding antenna

The radiation characteristics of a flush-mounted, cardioid-pattern, direction-finding antenna are discussed. By use of the general equations for the far field of an annular slot in an infinite and perfectly conducting ground plane, expressions are derived for the radiation field of an antenna consisting of a single annular slot fed with a cylindrical waveguide excited with two orthogonal TE11modes and the TEM mode. The structure of the antenna and the method of exciting the desired modes are described. The effect of the cross-polarization component on the three-dimensional pattern is considered, and theoretical and experimental patterns are included for comparison.