pml hfss
Catalyst
I want to stress that HFSS stands for High Frequency structure simulator. Therefore 13 MHz would not seem like a high frequency. And yes, a quarter-wavelength distance from the nearest radiating edge is required for the radiation boundary. PML would reduce your simulation time as the distance can reduce down to lamda/6 to lamda/10. In regards to setting up the PML, this is a regular query in this forum and so check out previous posts. Version 9 should be straight forward.
Alternatively, there are other simulation packages which employ the near-field boundary and the far-field is generated using Near-field to Far-field Transformation. This sort of boundary is very close to the radiating object and therefore very small total mesh in the overall simulation domain.
Sassyboy