18 AWG *solid core* RG-6 cable involves unacceptable skin effect for line-level applications, while 18 AWG stranded conductors are not plagued with notable skin effect. Although it's somewhat unusual for cable designs to involve such a large gauge conductor for line-level applications, some Mogami, Oyaide, and other cable maker products feature 18 AWG stranded conductors that are suitable for the purpose.
Cheers, Duster
