how to select the substrate

hi all,
i need a substrate that the feature of this substrate is bulky. i mean i will embeded metal post in substrate of coaxial cable. how to select this substrate, please anybody know, help me, that i can design my project a bout coaxial filter. thanks and regarded

sounds like you just need a slab of plastic.

Teflon is a good, low loss dielectric for RF/microwave.

Polyethylene is cheap:
McMaster-Carr

Emerson cumming makes a loaded polystyrene if you want to be fussy.

thanks for reply my ask. i visit the McMaster site for polyethylene, but i couldn't find relative permitivity (er=?) of material.
is there er=10.2 ?
and what is reason for select slab plastic? is there "er" for this?
i embeded bar metal (like stub short circuit) in coxial cable, perpendicular on inner conductor. whether could embeded bar in this substrate?