Gerald,
AC6LA's ZPlots implementation is an automated version of what Steve is
suggesting. You measure S11 twice for some known length of the coax you
want to test, first with the far end open and then with the far end
shorted. This produces nulls, the depth of which depend on the loss in
the line at the null frequency. You save each of these sweeps to a data
file, then export them to ZPlots, which is an Excel spreadsheet that
does the math.
So as I understand AC6LA's ZPlots, he computes the line loss from the
nulls within the frequency range of that measurement for that length of
line. These loss values are then matched to the computed loss from the
equation for loss in a line to determine the constants for that equation
needed to fit the values at those nulls. The fundamental equation is
then plotted vs frequency. The equation for loss is from the trans lines
equations, taking into account resistance of the conductors at the
frequency of interest (including skin effect). An easy place to see
that loss equation is on a data sheet for Times coax. The numeric values
in that equation are for that particular coax; what ZPlots does is
empirically determine those values for the coax we've measured.
i'm sure that Dan will correct any errors in my educated guess. :)
This is pretty easy to do if you have any decent vector impedance
analyzer that can export data and Excel running on a computer. It works
with any of the AIM units, with my VNWA, and with other comparable
analyzers. Download ZPlots, which is an Excel spreadsheet. Note that
ZPlots does NOT work with Open Office and Libre Office spreadsheets,
probably because it depends on Excel's macro language. ZPlots imports in
several text data formats, including Touchstone .s1p. I don't know about
other analyzers.
73, Jim K9YC
On Fri,8/12/2016 11:53 AM, TexasRF--- via TowerTalk wrote:
And the results are intuitively obvious for even the most casual observer,
right?
The pdf mentions 1/4 and 1/8 wavelength multiples. Can we take it that
random lengths do not work for the concept?
73,
Gerald K5GW
In a message dated 8/12/2016 1:41:50 P.M. Central Daylight Time,
steve@karinya.net writes:
http://www.karinya.net/g3txq/wet_ll/tl_formulas.pdf
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