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A straight forward way, depending on which version of HFSS, is to define a radiation boundary by clicking HFSS => Model => Create Open Region and then selecting Radiation. This will then create a radiation box with dimensions corresponding to the operating frequency you choose for it in that window.
It will also create three far field setups called "3D", "Elevation" and "Azimuth". You can select these in the far field reports for the plots you want. Also, if you want to plot radiation patterns at multiple frequencies, use a disrete frequency sweep with your solution setup. You may then select "Sweep" in the report windown - It is on the left hand side of the window.