The radiation boundary is implemented to abosorb any incident radiations and is best suited for normal incidence. This will only be effective if you ensure sufficient distance between the radiating element and the boundary. It is suggested to have atleast quarterwavelength distance between the two. I am wondering why you were using this boundary in your model.
Please let us know what kind of analysis you were trying to make:
- Is your structure working like a radiation source upon excitation?
- what type of excitation you are using?
- do you want to vary the distance from the radiation boundary as part of your analysis?