[isw]

In article ,
"mcdonaldREMOVE TO ACTUALLY REACH wrote:

Scott Dorsey wrote:


You want goofy, look up where the 50 and 75 ohm transmission line
standards came from...


That's not goofy. The impedance of free space is (about) 75 ohms, as
is, not accidentally, the impedance of a matched dipole antenna.

The minimum loss of a coaxial transmission line with air insulation
occurs at 75 ohms (for the same reason!) while the minimum
loss for a coax line with plain polyethylene insulation is at
50 ohms approximately. Foam insulation line is intermediate.

75 ohms answers the question "what impedance has the lowest attenuation
per unit length for a given outside diameter?". I believe that is true
*regardless* of the dielectric.

The fact that 75 ohms (and 300 ohms) are antenna impedances is
convenient, but not the main reason for the prevalence of 75 ohm cable
-- the preponderance of antennas are vertical quarter-wave devices, and
those run around 50 ohms.

50 ohms (sort of) answers the question "what impedance has the greatest
power handling capacity for a given outer diameter?". I believe that is
true *regardless* of the dielectric.

The precise answer is around 37 ohms, but the curve is very broad, and
50 (or 51.5 or 52) ohms is useful for (vertical) antennas, so that's the
impedance cable is built to. Incidentally, the lower DC resistance of 50
ohm cable made it the best choice for Ethernet (over 75 ohm's lower
attenuation) because it makes collision detection work better.
Propagation delay limits the length of an Ethernet segment anyhow, and
that doesn't vary greatly with impedance.

It is a pain in the butt that TV (cable and receiving antennas) uses 75 ohm
lines while almost all other RF electronics equipment is 50 ohm.

I suspect that the length of coax in use for cable TV RF plus baseband
video far, far exceeds all other uses of any other impedance of cable,
and in those uses, low transmission loss is more important that anything
else. Plus, of course, the major antenna type used for TV is the
(folded) dipole, which, at 300 ohms, has an impedance that is
"convenient" for use with 75 ohm coax.

Isaac