Wes Stewart wrote:
. . .
Importing their data into Dan's (AC6LA) "bestfit.xls" spreadsheet
shows very poor correlation with theoretical k1, k2 coefficients. I'm
observing something similar on some RG-142 that I am measuring with my
N2PK VNA. The '142 has a silver-coated, copper-plated steel center
conductor and I believe that the skin depth encompasses the steel at
lower frequencies.
. . .

I've definitely seen this in RG-174 and some similar diameter 75 ohm
cable, at 7 MHz. The problem with those cables is that the center
conductor is made of very fine strands of Copperweld. While the fraction
of copper relative to the wire diamter is large, the actual copper
thickness is small due to the very small diameter wire, allowing current
to penetrate into the steel at lower frequencies.

I've also seen the effect in the time domain when using RG-174 type
cable but with solid silver-plated Copperweld center conductor. The
increased loss at low frequency actually improves the step response
somewhat because of the disproportionately higher loss at lower frequency.

Roy Lewallen, W7EL