As per the shown graph of S11 parameter ,two frequencies where it is showing below 10 dB dip in the operating frequency. But if it is an Microstrip Antenna and working on CST microwave software. Then go to optimize tab and change the dimensions(Length and Width) of patch antenna. simulate on eah change of parameters, the operating frequency with a below 10 dB will be shown on a particular dimension. This way below 10dB for a particular frequency can be obtained.