[Steve King]

"Scott Dorsey" wrote in message
...
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."

The telcos may have stopped installing open-wire transmission lines in the
20s, but the existing ones remained in use well into the 50s. Some here may
recall that that was when the deluge of green glass telco insulators in
their myriad shapes and sizes hit the collectible markets. As the wires
came off the poles to be replaced by multi-twisted pair cables the
insulators were often left in place. They were then scavanged by
enterprising people for their own collections or for re-sale. This
continued through the 60s in various parts of the country.

Steve King