The study of the radiation field at any distance of a dual-shaped reflector antenna is presented

and as an example

near and far field of a dual-shaped Cassegrain antenna is calculated. The computational results show good agreement with the measured data. Therefore

a new conclusion about dual reflector antenna is obtained. Besides

a new method to calculate accurately disastrous field value round an antenna is given. It is an effective technique for solving electromagnetic compatibility （EMC） problem related to an antenna.