antenna matching , s11 always must be 50 ohm?

s11 in designing microstrip antenna always should be 50 ohm? i design an antenna which its s11 is 70 , i change dimansions to have s11=50 but i think its performance and resonance depth was much better when s11 was 70 . we can not design antenna with s11=70?

The 50 Ohm value is a standard for tranmission lines. Most sources are 50 Ohms and fo the best results (matching), also antennas should have close to 50 Ohms.
Most antennas are mismatched a bit, so transformers and matching structures (tuners) are used to optimize matching. Read some basic antenna textbook, please.

As stated Above, If ur antenna is of 70 ohm impedance, the u need a matching section for match the standard port 50 ohm to ur antenna 70 ohm, for Good design..!!