microstrip patch antenna simulation problem

i am making square shape patch antenna at resonant frequency 1.5ghz .. but the zero iteration return loss curve is not going below 3.5 db ...... the dimensions with i took are 48 by 48 and feed is of 24 by 2 ... i have tried changing the feed length but nthin is helping much .. i dont know where the problem is .... please provide some suggestions . i am using ie3d for the simulation .

There are two ways to match the patch to 50 ohm. The first you chosse is to make a lambda/4 transformation line to the patch, the input impedance is then Zw^2/Za. Zw is the line impedance (from 24 and 2 in your example). And Za are the impedance of the patch itself.

from the length of 48 on 1.5 GHz i think you use FR4 as substrate (er=4.3). so you can calculate the length of the line (microstrip-line) for lambda/4. With the height of 1.5 mm i think the length should be 28. Then you have to modify the width of the line to change Zw. You can see if the lenght is right, when the impedance in the simulation is real (x+j0 ohm). And with the x and Zw you can calculate the impedance of your patch and then calculate the Zw for 50 ohm.

Hello,
This problem is only due to impedance matching. Please calculate the impedance correctly for your design. You didn't mention which simulation software you are using . You can see the operating frequency of the antenna only if the S11 curve is only below 3dB.