Double ground plane for 3 band microstrip monopole

Hi to all!I'm trying to simulate a 3 band microstrip monopole for GSM/GPRS/UMTS usb modem. The problem is that the monopole is connected to the pcb but there is another pcb under the first one; I've done several simulation and I see that the second PCB has a big influence on the results: do i have to consider or not? I post some photos to see the details, sorry for bad quality but I have to make with my WebCam. NOTE: the second GND_plane doesn't extend under the antenna obviously.

Thanks.

Bye

Francesco

I didn't recocnize much details from your photos, but in general, a ground plane below a monopole that has dimensions similar to the monopole can't be actually regarded as a ground plane?! It's rather some kind of dipole. So it's plausible, that the second plane has a considerable effect on antenna impedance, radiation pattern, whatever.

Thanks for answer first. Of course the effect of GND is surely important but pay attention to the fact that this second GND doesn't extend under the monopole.

thanks

Francesco

Yes, i considered this fact. I prefer to regard the monopole and the ground plane as a kind of dipole. Whatever you assume as a model for the ground plane, because of it's small dimension it will be exited and radiate somehow. So the second plane must have a coupling with the antenna.

Yes I think so, infact considering both the GND plane the plot of S11db shows that the antenna seems to resonate at 2 frequency (1.6 and 2 Ghz). Considering only one PCB you can see only a resonance with modest bandwith at 1.7 Ghz. Now I'm doing several simulation varying the length of the monopole 'cause I want to discover the efficace index of air-PVC multistrate interface.

thanks

Francesco