Efficient High Power Microwave radiating systems are required to extract high power microwaves from HPM devices. Efficiency of an HPM antenna can be improved by varying its physical shape and dielectric used in it. The most critical location in an HPM antenna is its interface from where microwave leaves and enters into the air. In this paper, the performance of an HPM antenna has been investigated as a function of its profile. It has been demonstrated that by adopting Gaussian beam distribution at the aperture of an horn antenna with an appropriate beam waist, the E-field intensity at the output window can significantly be reduced resulting into 29 % improvement in its gain. Thus, in general, improved characteristics of an HPM antenna can be achieved by adopting an appropriate horn profile, beam waist and dielectric.