Abstract: This paper applies the principe of local epuivalence to the analysis of the wave-form mode in an infinite conical epuiangular spiral antenna, and discusses the active regi-on of the T_-1 waveform. The design concept is presented that a two-arm conical equi-angular spiral antenna with small cone angles can be applied to the GPS antenna throughproperly reduced active region of the T_-1 waveform. The engineering design method ofthe antenna has been introduced, and the performance index and application informationhas been given. It provides a kind of receiving antenna of good properties for the per-fect operation of GPS.