Amplifier design technique has made it possible to provide a usable band of about 18 MHz while still
keeping the repeater spacing of about 4.5 km as defined in Recommendation G.332; the CCITT has therefore defined an 18 MHz system which offers a transmit
ITU-T G.334-1989 history
1989ITU-T G.334-1989 18 MHz Systems on Standardized 2.6/9.5 mm Coaxial Cable Pairs - International Analogue Carrier Systems (Study Group XV) 9 pp