Problem with getting S11 in HFSS of 900MHz patch antenna

Hi!
I designed a patch antenna with a center freq of 900MHz.
The calculated dimension are:
ε = 4.4 (fr4)
substrate high = 1.6 mm
w = 101.4 mm
L = 79.3 mm

and (2nd antenna)

W == L = 79.6 mm

But using hfss I can't get a decent S11 on 900MHz. I tried different patch dimensions, I changed L and W but it seem that at 900MHz Zin of the antenna is always less than 50ohm.
What am I doing wrong? What are the exact dimensions of a 900 Mhz patch antenna?
Any thoughts would be highly appreciated.

i think the patch size has little on antenna input impedance,for the port position and the port define has more effect on it.
maybe you should examine the antenna port more carefully.

I'm using waveport. Dim 10*w x 10*h (w and h of the microstrip). Microstrip is designed for 50ohm impedance (3.1 mm). I think port is correctly defined.
Deembeded Zin of the antenna is lower than 50ohm!
I understand that Zin depends mainly on L.

you are using microstrip line to feed the antenna. so you can try to reduce the width of antenna for getting biger input impedance.

hay,
is the Zin<50 or >50?
if it's >50 than you should move the port more to the center of the patch, if it's <50
then move it a little to the edge.
btw, FR4 is not a good substrate for antennas, it has a high tan(del) that means it has high loss so your antenna efficiancy will be small. you should use air, or other dialectric substrate (like RO5880, RO4003).

Thanks to everybody. I found what I was doing wrong! I'm using hfss and I made the air box too small. Airbox should be at least lambda/4 in ALL directions. I have made it lambda/4 only above the patch and not in x -y also!
Btw. I'm using RO4350 now!

Hi vfurlan

look at https://www.edaboard.com/ftopic209865.html good expl. for you.

pl