radiation efficiency of patch antenna in HFSS v10

Hello all.

i am facing some problems in estimating radiation efficiency of a proximity coupled patch antenna. I am using a lumped port excitation.

The problem:
The radiation efficiency reduces as i increase the dimensions of my radiation box! It is around 80% when the box just fits the patch structure along x- and y-axis. And it reduces to 30% if it is expanded.

Any idea why?

Thanks!

This is probably due to inadequate mesh on the radiation boundary. As you increase the airbox, the mesh on it get more rough. You need to seed the boundary about lambda/10. Check out this tutorial for more information:

http://emtalk.com/tut_1.htm

Thanks for the reply.

Well, I am taking care of the lamda/10 mesh requirements as well as lamda/4 radiation box requirement ...

BUT !

as soon as I increase the radiation box along x- and y- axis, not only the radiation efficiency crashes, but the resonance frequency also creeps up. So now I dont know which values to trust !

I need to expand the air-box so that I can include the casing of the antenna into my model.

You are getting changes in the resonance because the wave is reflecting back towards the patch and disturbing the current path....so take rad. boundary more than lambda/4 and the best option is use PML................it gives accurate results...................