gain dB = gain dBi For plotting the gain over frequency you need to make a frequency sweep and activate the "save field" option. Then you select under plot Far Fields Reports -> Rectangular plot. Now you can plot the gain

You must have defined at least one radiation or PML boundary in the design for HFSS to compute antenna.

1. Right-click the Infinite Sphere icon in the project tree, and then click Compute Antenna Parameters on the shortcut menu.

The Antenna Parameters dialog box appears.

2. Under the Solutions tab, select the solution for which you want HFSS to compute antenna parameters.

3. Under the Intrinsic Variables tab, select the solved frequency point at which you want HFSS to compute gain.