What's the relation between monopole length and FR4 thickness?

Hi to all!
I'm trying to simulate a printed monopole on a FR4 substrate of 1.6 mm thinkness.
Does anyone know the relation between monopole length and fr4 thikness?
Also know if dimensioning a monpole for 900Mhz can it resonate ad 1800Mhz?

thanks

Francesco

Hi on1!!
what's the structure did you used?