Balun for Bowtie antenna

hi all
i have designed a bow tie antenna for GPR in CST that will work between 500mhz-2ghz but the reference impedance is 250 ohms and it will be having a coaxial feeding .so i need to design a BALUN that will convert 250 ohm to 50 ohms (coaxial cable).
can anyone help .

Thanks in advance

Vaibhav Thakur