Input Impedance Matching

I designed a meander antenna in ADS layout and wish to improve the S11 by matching the input impedance at the port (50 ohm) thats Single mode transmission line excited. Any suggestions?

You might consider using stepped impedance microstrip lines for this purpose. Narrow lines provide the inductance and the wide sections provide the capacitance. A ladder LC will help improve your S11.

Note that if the S11 is poor then the imag(Zin) of the antenna is non zero. This means that the antenna is not resonant at the frequency you want it to operate. You might want to make imag(Zin)=0 before you go about including the matching network.