Definition of antenna gain in HFSS & 60 GHz antenna measurement.

Hi Guys,

I've been doing 60 GHz antenna measurement these days. Things turned our weired:

The HFSS simulation showed that the 'peak realized gain' of the antenna is 10 dBi at 60 GHz. The measured maximum gain is 10.4 dBi, seemingly agree well. But the measured co-pol gain at this point is around 8 dBi. Moreover, the normalized patterns agreed quite well in the 57 GHz - 66 GHz band. The |S11| is not measured yet, which I am really worried about.

So can anyone kindly tell me that:
(1) How does HFSS define the 'peak realized gain', 'realized gain' and 'gain'.
(2) If the gain is 'realized', what is the factor according to which it is realized.
(3) Is it possible that the measured 'co-pol' gain 3 dB lower than the measured 'peak realized gain'?

(1)(2)
HFSS follows IEEE definition of gain, which does not include losses due to mismatch. Therefore:
'realized gain'(dB) = 'gain'(dB) + 10*log10(1-|S11|^2).
The work 'peak realized gain' symply means 'maximum realized gain over all directions'

(3)
How are you computing 'co-pol' gain? Remember that it is power density for certain direction and for copolar polarization, divided by the average power density for all directions and all polarizations.
If this is the case, maybe the antenna is not producing the right polarization.