Hi,

Can anyone suggest the optimum size of the radiation boundary box for an antenna in HFSS? If I use the default radiation box, HFSS places the box at the edge of the board. In one of the manuals for an earlier version of HFSS, they have used a box that is quarter-wavelength bigger than the board on 3 sides for a patch antenna, while in the manual for version 9, the box is a lot closer to the patch.

As expected, my results vary depending on the size of the radiation boundary box. Can anyone help me on this?

Karthik.

I have a feeling that a quarter-wave is pretty close... for microstrip circuits I try and keep the airbox 10x the substrate thickness - i.e. 2.5 mm for a 10 mil alumina substrate... so while I'm not sure where the optimum placement is, you could also try and keep moving the box further and further away from your antenna until the results don't change very much.

I have a dipole in front of a ground plane. If I take the 'substrate thickness' to be the total depth of the arrangement - i.e, pcb thickness plus the distance between the dipole and the ground plane (this total thickness works out to roughly 20mm), my air box is still 8x this size. However, my results still vary. The gain varies by about half a dB, and as long as the box is symmetrically big on all sides, the pattern doesnt change. The input impedance varies quite a bit.

Another question was regarding interpreting the results in HFSS. When you calculate the far-field characterisitics, HFSS gives results in terms of Directivity, Gain and 'Realized Gain'. What does 'realized gain' mean? I was confused, since, if you look at the efficiency factor, it appears this is calculated using Directivity and Gain and not realized gain.

Sorry, I don't know about the realized gain...

Def. Realized Gain:
When mismatch loss occurs, as it usually
does, this loss must be subtracted from the
power gain of the antenna to yield realized
gain. Realized gain is important to the systems
engineer, for it reveals how much signal
will be available at the input to the receiver
for a given field strength.

See: http://www.wj.com/pdf/technotes/Direct_antennas.pdf

Thanks!!