Abstract: The design of radial line helical rectangular sub-array is the basis of the investigation of high-power high gain radial line helical array antenna.An X-band 16-element double-layer radial line helical square sub-array is proposed and designed.Its fundamental conception of operation is presented,the radiation elements,coupling probes and feeding system are designed,and the sub-array model is simulated.In the range of 8.6~10.1 GHz,this sub-array shows a reflection coefficient of below 0.1.In the range of 9.0...