what is S11 in patch antenna?

S11 is the input reflection factor: how much of the input signal is actually reflected back to the transmitter.

You can convert S11 to the VSWR, which is another parameter to describe antenna matching:
http://www.giangrandi.ch/electronics/anttool/swr.html

.... and about VSWR you can read here: http://www.microwaves101.com/encyclopedia/vswr.cfm or http://en.wikipedia.org/wiki/Standing_wave_ratio


it is about Return loss: http://en.wikipedia.org/wiki/Return_loss

in this case you: s11 = RL, it is the same.

If you know exactly, what is the s11, try search "Scattering Matrix" or "S-PARAMETERS", for example: http://www.odyseus.nildram.co.uk/RFM...Parameters.pdf

I hope, that I helped you