Differential Microstrip Antenna Simulation in HFSS (Incorrect Results)

I have designed a dual layer differential microstrip antenna .This is my first HFSS simulation model and also I am very new to Antenna theory. This was designed based on a research paper . The model is running but the results aren't as they should be. (Like VSWR is highest at working frequency of 900 MHz; real and imaginary part of Z0 are coming zero for all frequencies.

Can someone kindly check this and say where is the error in this design. ? I have attached the HFSS file and the VSWR plot

Hi
You should put the Radiation Boundary at least lambda/4 away from the radiating parts of the structure. Calculate the distance and draw a bigger radiation box.

Hi rizoeda,

Thank you for your response. Yes I implemented it and also my model was incorrect and made some changes. I am getting realistic results now.
But the main problem was that I was getting linear graphs of this kind. It happened because I added only a few steps in my frequency sweep. Its solved as of now.

Best Regards