For Mike Rivers Scott ecc......history of 600 ohm lines
Mike Rivers wrote:
To give you the right answer would be to ask why the telephone company
decided to make this their standard, as we got a lot of our standards
and practices from telephony. I don't know why they chose 600 ohms
other than that's what was what they made at the time.
It's because an open-wire transmission line with five inches between
18 ga. conductors has a characteristic impedance of 600 ohms.
On long cross-country telephone circuits you care about characteristic
impedance because you don't want to see reflections from the termination
hundreds of miles away. Transmisson line effects DO become a big deal
when you're running a line from New York to LA.
Of course, the phone company stopped using open-wire transmission lines
by the 1920s, and went to twisted pair systems. That's why you ALSO
see the 150 ohm standard (as used at CBS radio). 150 ohms happens to
be the characteristic impedance of typical cotton-insulated 20 ga.
twisted pair.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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