I want to model an electrically small top-loaded monopole antenna being used as a receiver for distant sources, with the output being the voltage seen between antenna and ground. 

As a start I have constructed the model in HFSS just using a box with the bottom surface as a ground plane. The antenna is inside the box, just a conducting vertical cylinder with a horizontal disc on top. There is a small gap between the bottom of the cylinder and the ground plane. I am having trouble finding details online of where to go from there. 

As a first attempt (I don't know if it's correct) I have set up an incident propagating plane wave coming from one of the side planes of the box. I have used a driven terminal solution type. I have used the suggested formula from the antenna toolkit docs to get a voltage from a line drawn between the bottom of the cylinder and the ground plane. This did not work as I just get the same voltage regardless of the antenna dimensions.

Any suggestions as to what I'm doing wrong, or where to find a detailed description of how to set up this problem in ANSYS?

Cheers.