Coaxial cables are categorized by Radio Government (RG) ratings, each adapted…

Concept — Coaxial cable is standardised by a Radio Government (RG) number. The RG number fixes the physical build of the cable: the gauge of the inner conductor, the thickness of the dielectric and the type of shielding. That build in turn fixes the cable's characteristic impedance, the ratio of voltage to current that a travelling signal sees on the line. A transmission line delivers its full power only when the source, the cable and the load are impedance-matched, so each application family standardises on a single impedance value: radio-frequency video distribution is engineered at 75 Ω, while baseband Ethernet coaxial cabling was engineered at 50 Ω.

Application — The stem fixes two properties at once: the rating RG-59 and the characteristic impedance 75 Ω. Mapping the standard catalogue of coaxial ratings onto the service each one was built for gives:

RG-59 is the 75 Ω video-grade entry of that catalogue, so the specialised function it is adapted for is cable television distribution.

Cross-check — reading the mapping the other way round confirms it is one-to-one:

• Ethernet is the name of the entire IEEE 802.3 family rather than a cable category; its variants are defined over coaxial, twisted-pair and optical-fibre media, so no single RG number belongs to it.

• Thin Ethernet (10BASE2) is built on RG-58, the flexible 50 Ω cable terminated with BNC T-connectors, with segments of up to 185 m.

• Thick Ethernet (10BASE5) runs on 50 Ω thick coaxial cable of the RG-8 family, tapped by external transceivers, with segments of up to 500 m. Some textbooks tabulate this row as RG-11; in its ordinary form RG-11 is a 75 Ω video-distribution cable, so it is the impedance rather than the RG number alone that separates the two service families.

• Only one entry in the catalogue pairs the rating RG-59 with an impedance of 75 Ω, so the mapping is unambiguous.