I'm designing a sleeve dipole right now. I'm just using a tube and coax cable to design this antenna (quater-wave length). Any suggestions or comment to increase the gain to 3 dBi (currently I did is 1.9 dBi)?
The best you will get out of a half wave dipole is 2.1dBi.
You could try increasing the length of the 'radiating element'. If I remember correctly A 5/8 wave ground plane antenna gives around 3dBi or so. You will have to match it though, a series inductor should do the trick.
Failing that some sort of colinear array will get you more gain.
Peter