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?
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73, Cecil, w5dxp.com