Helo i want to knoo if i build my own fm antena with the part 15 fm rules will i cover a good range/? and is it legal for me to conect the antena 30ft high?
You may put the antenna as high as you want.
Height gives 2 advantages. It gets the source of the signal up over obstructions that would absorb or reflect the signal, and it extends how far the signal can reach over the earth's curvature. The first is very helpful for part 15 fm. The second, not so much since the legal radiated power won't get far enough that curvature becomes a factor.
As for range, with my transmitter and antenna in an attic window, I get at most 1/8 mile, before it gets lost in the noise. That's with power turned down enough that I'm pretty sure I'm legal. I don't own a meter that can test field strength.
The problem you may have is by changing from the supplied antenna to a different antenna, the system may no longer be in compliance. You would have to verify your field strength to be sure.