Impedance Boundary for HFSS Antenna Array project + Scan Angle Question

Hello,

I've recently completed an HFSS tutorial demonstrating how to setup an antenna array in HFSS (Example ? Endfire Antenna Array). In the tutorial you build a unit waveguide antenna, set up the master/slave boundaries and then setup an antenna array. From there you can easily plot directivity and vary the direction of the beam.

I have a basic question - perhaps not so much about HFSS - but about the modeling and/or antenna basics.

In the tutorial we place a perfectly matched layer (PML) on top of the the air box which sits directly above the waveguide - the radiating end of the antenna. This makes sense to me since it is not practical or possible to place an infinitely large airbox at the firing end of the antenna.

But, in addition to the PML we also place an impedance boundary on top of the PML using the following expression for impedance:

377*cos(theta_scan)

Where theta_scan is used in the scan definition of the antenna array setup (HFSS-->Radiation-->Antenna Array Setup . . . --> Regular Array (tab)

So, I can see that the impedance boundary provides the impedance of free space only for theta_scan = 0 and then at the extreme, 90 degrees, that impedance goes to zero.

I don't really get why this is done. 1) I would have thought the PML would act as a terminating load snubbing any and all energy before reaching the impedance boundary. And, 2) why must the impedance also vary with angle - and not simply remain 377 ohms?

Why would this equation + impedance boundary need to be placed here?


This is more of an HFSS question:

I'm fuzzy about how HFSS uses the scan angle of an array antenna. I know that the scan angle for an antenna array is the angle made between the plane of the antenna array and the beam - hence the name scan angle - it is the angle at which the beam is scanning. Now, I know that complex weights are used for individual feeds of the antennas - this causes the beam to have a maximum gain at the scan angle - varying the angle of the complex weights will vary the scan angle of the beam.

Now, (assuming I have this right) in HFSS is seems that you sort of skip details of weighting and go straight to scan angle - entering the angle you desire and - like magic - you're done. That seems convenient. But, how would you find details of the individual weighting values (the angles/delays) needed for a given scan angle? That is, how would one know the value of each delay for for a particular scan angle?

Thanks in advance!