Help... I'ts about microstrip patch bowtie...

Hi there... I have a problem in simulating antenna microstrip patch bowtie using HFSS... I try to put the antenna step by step based on it's layer in order to understand the effect of each layer. First I simulated the antenna bowtie using PEC. Second I put substrate (using FR4) under the bowtie.. Then I put groundplane (using PEC) under the substrate. The significant changes are the different gain pattern and the impedance. The impedance is in problem. The imaginer part is a lot bigger than the real part... Any suggestion about my antenna?

My antenna is designed for ISM purpose, working at 2400-2484MHz. I use this antenna coz I've read that patch bowtie have narrow bandwidth and good for ISM purpose.

I have another question. Why if we work in high frequency, the imaginer part of impedance is no longer affect the performance of the antenna?

Thank you...