In this letter, we present an improved method for synthesizing smooth-profiled horns. A direct connection between the mode compositions and far-field parameters was established by utilizing a parameterized aperture field distribution and the field decomposition method. It speeds up the optimization process by a factor of 5. The requirements for the aperture field to maintain a constant directivity were studied based on the method. A feedhorn was optimized for a W-band gyro-TWA with a 0.7 dB variation of the directivity within a bandwidth of 10%
