For bandwidth from 2-16 GHz, FR-4 layers with relative dielectric constant of 4.4, and a loss tangent of 0.025 are utilized as substrate in the following research with simulated and experimental results. I think this research may help you.
Article Enhanced asymmetric transmission due to Fabry-Perot-like cavity
Dear Meenakshi, for flexible designs, flexible polyethylene terephtalate (PET) film can be used as substrate in GHz, more information can be found in the proposed article.
Article Transparent and Flexible Polarization-Independent Microwave ...