dual band dipole antenna (13.5 MHz and 866 MHz) in CST

hello everyone
quite new in this field so please have some patience with me
i have to design a dual band dipole antenna resonating at 13.5 MHz and 866 MHz with port impedance 13-13.5 (real) and 120-130(imaginary) parts with dimensions not greater than 40 by 40 mm2
i have designed some antennas in CST so familiar with the software.My big dilemma is how do i start this?Am i just choosing a random shape and work on it?or there is other way of planning it before implementing in CST?
your help is much appreciated

As for the 13.5 MHz antenna, it's simply impossible. A antenna with the given size, either loop or dipole, will have a radiation resistance in the low milliohm range.
Are you actually designing a 13.5 MHz antenna? Or an inductive near-field coupler?

it is a RFID tag

For 13.56 MHz, an inductive coupler (e.g. a printed coil with 5 or 10 windings) is the only reasonable way to implement the tag "antenna". For 886 MHz, a shortened dipole is feasible.