microstrip bow tie antenna

Hi Friends, please help me.
I have designed a bow-tie antenna as with dimensions given in pic also. Dims are given as below;
bow tie one arm: arm length=4.1cm, outer width=2cm, inner width=0.25cm
Gap width=0.25 cm
Sub= Width=10 cm, Length= 15 cm, height=62mil, permittivity=2.2
But I am getting maximum total gain (dB) over the frequency range is 2. How can I increase it to 4-5 dB.

Like any dipole you cannot expect much gain of it.
If you need a more directional antenna, you can add a reflector to it (a corner-reflector is even better), or build a Yagi with it. Putting it in focus of a parabolic dish is even better.
Or, make an array of more such dipoles.