A cavity-backed spiral antenna with an unbalanced feed system - Analyses using FD-TD and moment methods

The FD-TD method and the moment method are used to analyze a two-wire square spiral antenna backed by a conducting square cavity (SAC). The SAC is fed from a coaxial line without a balun circuit. First, the SAC is analyzed using the FD-TD method. The SAC design is based on radiation characteristics of a two-wire square spiral antenna backed by a conducting plane reflector (SAR). It is found that the frequency bandwidth for a 3-dB axial-ratio criterion is 12% (18% for SAR), with a gain of 7 dBi (8 dBi for SAR). Second, the moment method is used to analyze a wire-grid model of the SAC. Grid sizes appropriate for the analysis are determined on the basis of the FD-TD solutions. It is revealed that the grid size for the cavity side plates is 1.3 times larger than that for the cavity bottom plate. Third, a two-SAC array is analyzed to reveal the mutual coupling. Calculation shows that the decoupling factor is larger than that for a two-SAR array by approximately 3 dB. The computation time and memory size required for the analyses are compared between the two cases for the FD-TD and the moment methods.