Probe Fed Patch Antenna : S-Parameters improvement using HFSS. Help needed.

Hello all. This is my first post in this forum. In advance , many thanks to everyone here , since I have found many posts really helpful , concerning HFSS and its issues that come across when designing. After doing some research in the forum posts , I did not find something matching my situation. So , I have the following "problem" :

I have successfully simulated a probe fed patch antenna , following the exact instructions in the HFSS manual. The S-parameters are quite good , about ~-20dB , depending on the central frequency. When the antenna is simulated "by itself" (using an Airbox) the total gain radiation pattern is as expected. However , when the antenna is used inside air (air surrounding the whole antenna , in order to simulate real conditions) , there is a significant secondary radiation field at the bottom of the patch , at the coax feed.

My question is : Are there any ways in order to further decrease the S-parameters of the patch antenna , or are there any design tips in order to reduce the secondary field? In addition , if anyone has any papers to suggest for further reading that would help to resolve this issue , I would really appreciate it.

Thanks in advance!

Hello, can you explain more about the conditions you are using to simulate the antenna? What do you mean by Airbox and real air conditions?
Thx

By airbox , I mean radiation boundary conditions. I created a box (material : vacuum) above the substrate , in order to get the radiation pattern.

When I mentionned "real air conditions" (a maybe not so correct definition) , I mean that the whole antenna was surrounded by air. (air is the material) I have run several simulations and when the antenna is surrounded by air there is a secondary radiation pattern at the bottom , which i s caused by the excitation. (waveport)

As for the other conditions , my design of the patch antenna is as mentionned in the official hfss manual. I hope I covered the information you requested. If you need any more let me know. Thanks for your reply.

So for the definition of the Airbox, try to check the permittivity that you used for the air. Then ther is another parameter which invoke your probe. Because if your probe is not well designed you will have reterned losses and the probe will radiate.
Another question: did you defined a metallic ground plan?
Can you send to me an image of your structure for 3 shots: bottom, profile (3D) and top.
Cheers

Hello, I m having problems trying to simulate a probe feed patch antenna as explained by the hfss manual. I have tried pretty much eveything as directed by the manual. Please help me out.
Regards