75 ohm impedance

Cable tv components are designed to work well with the 75 ohm coaxial cable used for wiring houses and streets.

If you take a component designed for use n a 75 ohm system and instead use it in a 50 ohm system, it will not work as well.

SO, if you are trying to measure cable tv equipement, you get more accurate measurements with the properly designed measurement equipment--75 ohm.

75 ohm impedance is mainly used in consumer electronics. It is used for TV, CATV/MATV, video output from VCD/DVD etc. Even for FM radio.

In the coaxial cable, you need as less Cu losses as you can get. This is achieved by a 75 Ohm impedance. Of course, this value does not give maximum transmitted power, but what really matters, in TV cables, is matching....(zero reflections, not maximum transmitted power). Generally, when you need both less Cu losses and maximum transmitted power, this is achieved by a 50 Ohm impedance.

75 Ohm characteristic impedance gives lowest loss in coaxial cables, that's why 75 Ohm is used.

77 Ohm is the minimum attenuation
30 Ohm is maximum power handling

50 is a compromise between 77 and 30
75 is just an approx. for 77 (just round figure) as u dont need good power handling in TVs as much as u need low attenuation

reference the RF IC book writen by Tomas LEE

the 75 ohm impedence means that it has a good rejection of airborne noise as the impedance is low.