gain of an Inverted F antenna IFA

Increasing Er and decreasing the hight, both leads in increasing the gain due to higher Q.
But both situations will decrease the bandwidth.

Thanks for your answer

In my case I can't modify the substrate, I can only modify the form of the antenna, I wanna make a small antenna with meander but that affects the gain
whith my actual antenna I have a gain=-7,8 dbi with ads , is that acceptable?

-7dBi gain is not the best gain for a printed meander antenna. This antenna type can get better gain.

I think this is due to the substrate that I use, it has only a thickness of 120 um (FR4), I think I can't have a better gain with a meander antenna