coax waveport size and airbox size for microstrip patch antenna in HFSS

I am simulating a patch antenna in HFSS with a probe feed. The probe is excited by waveport on a coax similar to the HFSS example. My coax height is just 2mm and radius is 1.6mm. Coax pin is 1mm radius. My bandwidth frequencies are 2-2.8 GHz. I am using the Driven Terminal solution. I am not sure if the size of the waveport radius of 1.6mm is correct. Is their a relationship between it and the impedance of the port? Also, I am renormalizing the port to 50 ohm impedance. What should the coax and waveport size be?

Also, the airbox size should be λ/4 away. Should it be λ/4 away from bottom or top of substrate? The HFSS example is from bottom, however the substrate thickness there is only 3mm. My substrate thickness is 15mm.

Also, I tried using a λ/2 away airbox and the S-parameter results are different. I even went to 1λ to see what results I will get, and they, too, where different.

Your help is sincerely appreciated.

If you have found the answer, kindly share the same.

Hi imkochar,

A driven modal solution setup should be used, the coax dimensions can be optimized by observing the return loss. The port should be a lumped port with a value of 50 Ohms, for an appropriate setup.

The air box (radiation boundary) needs to be lambda/4 away from the nearest radiating surface in order to work properly. Most likely in kae_jolie's case, the simulation was not adequately converged.

The coax dimension is required to deliver a characteristic impedance of 50 ohm. Refer to the characteristic-impedance formula of the coax.