How to improve S21 antenna?

Could someone kindly tell me the techniques to improve S21?
My antenna uses 2 ports: port1 uses CPW on the top of substrate, port 2 use microstrip line on the bottom of substrate
S21 is <-8 dB now, i hope it will <-15 dB and keep the same antenna's operation.
Thanks!

can u put up an image of ur model here ?

Here is the picture of 2 ports. One is on the top and one is on the bottom of substrate.
Could anyone help!

Basically an antenna is a single port device. I don't understand what exactly u r trying to do with two port ? are they both used to feed the antenna if so ? your s21 improvement, are u looking for a decrease in value of increase ? I'L take u want to make the s21 more negative? does ur antenna have a ground plane, as u mentioned microstrip on the bottom of substrate..?