A low-profile equiangular spiral antenna backed by a cavity with an absorbing strip

An equiangular spiral antenna consisting of equiangular spiral slots cut in a finite-sized round conducting sheet (PEC sheet) located in free space is analyzed. The analysis reveals that the input impedance is not sensitive to the diameter of the conducting sheet, and remains constant at approximately 60π ohms. Frequencies from 1 GHz to 20 GHz are used for the analysis and it is found that the spiral radiates a bi-directional circularly polarized beam above 2.8 GHz. In addition, a method for transforming the bi-directional beam into a unidirectional beam is investigated, where the spiral is backed by a cavity whose depth is chosen to be small (0.07 wavelength at 3 GHz). The analysis reveals that wideband characteristics for the input impedance and axial ratio are obtained by attaching an absorbing strip to the wall of the cavity. The radiation patterns before and after the application of the absorbing strip are presented.