Re: 50 ohm
dont discount the natural characteristic impedances of antennas and transmission lines . for example, twin-lead is typically 300 ohms, which scales nicely to 75 ohms after a 4:1 impedance transformation. also, a half wave dipole antenna has an impedance with real part equal to 73 ohms. granted, TV and radio sets are sufficiently broadband that the antenna is not strictly half-wavelength. however, twin lead is a lot cheaper to produce than coaxial cable - an important advantage in the early days of broadcast RF electronics. these factors, in addition to the lower attenuation coefficient (which scales approximately as the ratio of impedances alpha1/alpha2 ~= Zo2 / Zo1) are the reason for the dominance of 75 ohm impedances in commercial RF electronics.