The answer to this really depends on what you call high frequency (900 MHz or 30 GHz). Above 900 MHz one really needs to consider laminates designed for this purpose (low loss, controlled dielectric constant). Suppliers of these materials are Rogers, Taconics and Arlon. Speaking on Rogers (fax 602 961-4533 USA) behalf, the use of a material called RO4003 (woven glass saturated with hydrocarbon plastic/ceramic loaded) is recommended for commercial applications below 15 GHz. It has electrical properties for RF/microwave applications as well as being a material that can be fabricated like standard epoxy/glass material. If you want to discuss this more please let me know ([log in to unmask]). Regards Art Aguayo Technical Services Engineer Rogers Corp.