From The RadioReference Wiki
Have ever wondered what RG means when referring to coaxial cable?
RG is an old Mil-Spec cable identification which means Radio Guide. These ratings are only used as reference today since RG is no longer used as a military standards codification. Example RG-8A/U is now codified in the Armed forces as M17/163-00001.
Line Loss Calculators
The calculator above can be used to estimate the attenuation (dB loss), power handling capability, and transmission efficiency for some of the more popular Wireless Communications and MIL Spec cables at the frequency of interest.