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

"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.

On Sunday, September 1, 2013 12:40:51 PM UTC-5, Wayne wrote:
... the system will have additional loss of less than 3 dB.
And it works fine.

So the question is: Is any part of the reflected power in the
mismatch loss calculation included in that 3 dB of additional
loss? The answer is 'yes' and whether it works fine or not is
irrelevant to the argument.
--
73, Cecil, w5dxp.com

"W5DXP" wrote in message
...

On Sunday, September 1, 2013 12:40:51 PM UTC-5, Wayne wrote:
... the system will have additional loss of less than 3 dB.
And it works fine.

So the question is: Is any part of the reflected power in the
mismatch loss calculation included in that 3 dB of additional
loss? The answer is 'yes' and whether it works fine or not is
irrelevant to the argument.
--
73, Cecil, w5dxp.com

Indeed. If there is loss going one direction on a line, reflected power
going the other way suffers loss also.

The part about "works fine" was thrown in, not for you, but for those who
might be horrified by a high vswr.

On Sunday, September 1, 2013 6:09:09 PM UTC-5, Wayne wrote:
The part about "works fine" was thrown in, not for you, but for those who
might be horrified by a high vswr.

Sorry, I misunderstood what you were trying to say.
--
73, Cecil, w5dxp.com

"W5DXP" wrote in message
...

On Sunday, September 1, 2013 6:09:09 PM UTC-5, Wayne wrote:
The part about "works fine" was thrown in, not for you, but for those who
might be horrified by a high vswr.

Sorry, I misunderstood what you were trying to say.

My fault for delaying my response. I should have looked back for the
particular "read" post, instead of replying to the one "unread" at the
moment.

I was trying to add into the thread the figure 1 referenced. I look at it
as showing that at high VSWRs, reflected power takes more trips back and
forth through the line loss until the reflection becomes insignificant.