[Don Pearce[_3_]]

On Mon, 25 May 2009 11:40:48 -0500, "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 impedance of free space is 377 ohms (120 pi)

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.

Minimum loss (at which copper loss and dielectric loss cross) comes at
about 67 ohms. There are cables at that impedance, but I've never seen
one.

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.


Very true.

d