The most common impedance in actual practice for the sort of transmission line you have in mind is 75 ohms, not 50. Most of the cable TV network, and thus cabl

The most common impedance in actual practice for the sort of transmission line you have in mind is 75 ohms, not 50. Most of the cable TV network, and thus cable Internet, runs on this. That's why RG-6 and RG-11 grows on trees. There is no big secret behind all of this. Transmission lines by definition have some impedance. Practitioners long ago adopted a handful of standard values that suit the common use cases given prevailing constraints, mostly driven by cost.