Antenna directivity problem in HFSS simulation of a circular patch loaded antenna

Hi Friends,
Recently I use hfss to simulate a circular patch loaded with annualr ring antenna. S parameter is ok, but the directivity is below than 1. I seed the air with lambda/10, and the boundary is lambda/4 away from the radiation element, but the result is same. Anyone can give some advice? Thanks a lot!

The parameter from hfss:

antenna directivity

hi pudding,

Would you mind attaching your model? I want to have a look and take care of it.

Re: antenna directivity

HI nhomuathu_hanoi,
Thank you. I have uploaded the hfss design. And I found that in this case the slot on the ground also radiate, so that means I put the boundary too close to the radiation element( I didn't realise that before, so sorry). After I increase the air box below the ground, the problem solved. But would like to talk with me about this structure? I alsl uploaded the paper. I don't know what element radiate. I think the slot radiate, because when I change the slot length , the resonant frequency move accordingly. But when I change the radius of the ring, or the circular patch, the resonant frequency change to oppesite direction. That means, I increase the radius, but the resonant also increase. It is really wierd. I need your help. Thanks!
pudding

antenna directivity

hi Pudding,

Sorry for my late reply. From your attachment, I thinh you're trying to design either a circular polarization dual or broad band antenna. I agree with you that two slots at ground plane can radiate, in general case any discontinuation can radiate as well. Futhermore, I think, they're major factors that determine your antenna polarization characteristic and resonated frequency. In this stuation, maybe, a couple phenomenon is created between circular patch and two slots, and we can assume the ring as a parasitic element. For this reason, the rule of each element can be explained as the affection that you met.
Another one, I wonder why did you make the air box size so wide (width and length). I think equals to substrate's size is Ok.

Does anyone has some opinion?

Re: antenna directivity

HI nhomuathu_hanoi,
This is a single band cp antenna according the paper. I made the airbox lambda over 4 apart from the radiation element. So it looks a little bigger. Now I can get accurate resonant frequency, but the S11 is about -7db, larger than the paper said. Do you have any opinion? I think the match is not good, but all the parameters are from that paper, except the 50ohm coaxial line. I still don't know where the problems is. I think HFSS should accurate enough to reproduce the paper's result. Thanks
pudding