Why Do TV Cables Use 75 Ohm Impedance?

[freydawg56]

we learned it could be a trade off between loss and flexibility. Because if it was around 50 ohms or so it would be too thick of a wire.

and the we learned the minimum loss is around about 77 ohms.

What do you guys think.

 

[Bob S]

Many years ago I recall calculating the optimum impedance and found ~77 ohms to be lowest attenuation for a fixed coax OD and dielectric type (solid, foam, etc.). The minimum attenuation impedance may depend on the propagation velocity (i.e., dielectric type). As I recall, at other impedances the skin effect losses on the center conductor were higher. Foam polyethylene dielectric is better than solid. See

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=14693

Bob S

 

[Antiphon]

The real reason is that 75 ohms is closer to the real part of the impedance of a dipole at around 73 ohms.