Richard Clark wrote in
:

....
This topic is covered at:
http://www.w2du.com/r2ch15.pdf
under the section at:
Sec 15.3 Antenna Impedances From Measured Line-Input Impedances

In S15.3, Walt lays out a method of characterising a length of line used
for measurement, and then using the characterisation, to refer
measurments of a load at the line input to the load end of the line.

TLLC at http://www.vk1od.net/calc/tl/tllc.php will perform the second
part of that using published line characteristics.

A quick check shows that TLLC produces very similar results when the VSWR
is low, and less so at the end of the data range where the VSWR is 10.
Reasons for that include that Walt used a characterisation of the cable
used for measurement, and those characteristics are not exactly the same
as published spec, and Walt's calculation engine makes some
approximations that are not made in TLLC.

The discussion highlights that the method is only as good as knowledge of
the coax characteristics, and the accuracy of the measurement instrument.
Errors will be worst when the line operates at high VSWR.

I would argue that if you use an MFJ259B, for measured values that are
not near the limits of the instrument, and |arg(Z)|70°, you will get
relatively good results from TLLC. Validating the length and loss of the
measurement coax to ensure it complies with spec is a precursor to using
TLLC with best accuracy.

Owen