"W5DXP" wrote in message
...

On Saturday, August 31, 2013 11:37:20 AM UTC-5, Jeff Liebermann wrote:
Therefore, if I replace a length of 50 ohm coax, with a physically
similar length of 75 ohm coax, the I^2*R losses do not change.

# What you may be missing is that the RMS value of the current is higher
when reflections are present than when
# they are not present. Therefore, the I^2*R losses in the transmission line
are higher when reflections are
#present. Part of the reflected energy from the load (used to calculate
mismatch loss) is dissipated
# as heat in the I^2*R of the copper transmission line as illustrated by
the following example.

# Consider 200 ft. of RG-58 used on 440 MHz driving a 291.5 ohm load. The
mismatch loss at the load
# is 3dB but the loss in the coax is 29.4 dB and the impedance looking into
the coax at the
# source is 50.12-j0.19 ohms, almost a perfect match. Would you still argue
that none of the power
# involved in the mismatch loss is dissipated in the coax?
--
# 73, Cecil, w5dxp.com

This chart has been around a long time and indicates what is going on.
See figure 1 at:

http://www.arrl.org/files/file/Techn...f/q1106037.pdf

As a practical example, my elevated vertical (on a metal patio cover) is fed
with about 20 feet of RG-8. Matching is via a tuner right at the rig, and
the vertical element connects to the coax with no other matching.

RG-8 has a loss of about 0.55 db per 100 feet.

Assume that my 20 foot feedline has a full 0.55 dB of loss when matched. On
bands where the VSWR is 20:1,
according to the chart, the system will have additional loss of less than 3
dB.

And it works fine.