[aprssig] Why is coax 50 ohm ?

[Wyatt Foard]

Andrew Rich wrote:
> Is there a reason why coax is 50 ohm ?
>  
> And 75 ohm ?
>  
> And 92 ohms ?
>  
> Is it historical or is there a scienific reason why ?
Both 50 ohms and 75 ohms have been selected for specific reasons. Going back to Maxwell's equations for coax as a transmission line, maximum power transfer is at Z=30ohms and mimimum loss in the transmission line is at Z=77ohms. 50ohm coax is a good trade off for general purpose applications, as well as a nice round number. Video signals are not high power and so use 75ohms to minimize loss, again picking a round number.

I have no idea about 92 ohms, but would suspect there is another spec that was minimized or maximized by its selection.

Thanks,
Wyatt Foard - KR5WF