Couldn't get right radiation pattern of an on-chip monopole antenan

Hello guys! I'm simulating a CPW fed on-chip monopole. But the right radiation pattern couldn?t be obtained at 79GHz even when the length of antenna is changed from 0.2mm to 0.9mm. But I could get a good radiation pattern at around 50 to 60GHz whatever the antenna?s length is.

solution freq is set at 60GHz with the monopole length 0.72mm.

I need to shift this right radiation pattern to 79GHz. So I changed the antenna length
based on the frequency proportion. But the same pattern couldn?t be obtained at 79GHz. Just got a pattern as below.

solution freq is set at 79GHz with the monopole length 0.54mm.

Can anybody tell me the reason? Thanks in advance!

Could u tell me the software used to design the antenna?

I used HFSS 15.0