Thus 75-ohm coax was born. This is why all those coaxes intended to carry signals, not power, not high voltage, are 75 ohms. Cables like baseband video cable, CATV/broadband cable, are all 75 ohms.

The mean between 30 and 77 ohms is 53.5 ohms. If you want to learn more about RF design, you could do worse than watch [Michael Ossmann’s] workshop (see below).