reg material loss and bandwidth--need urgent answers

dear all.

when i simulate the cpw antenn with fr4 substrate the gain is not uniform over the band of frequencies.
what might be the reason? how to correct it?

also i need what is gain bandwidth? impedance bandwidth? pattern bandwidth?

also what is the permittivity of fr4 with loss tangent value?


for what applications we need circular polarization?

what are the applications in 6 to 7 GHz frequencies?


with thanks in advance,

KBMani.