Problems with CPW feed when simulating a loop antenna

Hi to all!
I'm trying to simulate a loop antenna for my project but I have many problem with the CPW feed. The CPW length has too big influences on the resonance frequency of my antenna, I think it could be a BALUN problem: but loop antenna should not be a balanced structure?
Please I hope that some one can help me, i need it for my project!

thanks

Francesco

There are different problems exist simultaneously.

1. Loop antenna is balanced structure and if good performance is expected you need balun.

2. CPW as any transmission line will multiply the reactance connected to one end and at the opposite end you may have a very reactive impedance. If your application is narrow band you may tune out impedance and get good results. For broadband keep line as short as it possible.

3. CPW itself is a metal structure and this structure can affect the antenna.

Consider carefully these factors and you should get the best possible performance. One more point: if you use HFSS it may give wrong results for transmission lines.