Private email.
Quoting from your webpage, "I tried various methods to measure changes
in loss, including driving the line with a known power and measuring
changes in the current delivered to a load resistor. However, I found
that the most repeatable method was to measure the line loss directly as
an S21 measurement in a 50Ω system using a VNA2180 2-port Vector Network
Analyser; then, having measured the input impedance of the line, to
correct the S21 figure for the source mismatch loss."
It would appear from this that you did not correct for the excess
attenuation due to the mismatch of the load to the line. Or did you make
that correction and not notate it? That would certainly account for the
difference. Remember that Dan's parameters are for a matched line, and
are the same that Times uses to describe their cables.
Several years ago, I made point-by-point measurements of more than 1,000
ft of some very good RG8 that is rather similar to LMR400, except that
it has a solid copper center and a plenum-rated dielectric and outer
jacket. That long length is gone, but I just made up a couple of 135 ft
lengths for Field Day. I should measure those with my VNWA (I have that
German one, very nice) and feed the data to Zplots and see how it
correlates.
73, Jim K9YC
On 8/9/2013 1:56 PM, Steve Hunt wrote:
Jim,
Thanks for pointing me in the direction of Zplots - I'd not
appreciated what it can do, although I'm a big fan of AC6LA's TLD
software.
I just downloaded Zplots and ran open and short S11 tests on the line.
I plugged the values Zplots gave me for Zo, Vf, K0, K1 and K2 into
TLD, however it then predicts losses very different from my measured
losses. But, interestingly, if I use TLD's default parameters for
Wireman 551 it predicts losses very close to what I measure!
It's not clear to me yet why Zplots would give me line parameters so
different from TLD's parameters for Wireman 551.
I need to explore this some more because if Zplots can produce
believable results it would save me a lot of number crunching :)
Steve G3TXQ
On 09/08/2013 16:55, Jim Brown wrote:
Another way to get transmission line parameters from VNA or VIA
measurements is to measure S11 with the far end open and shorted, or
to measure S21 and export the data to AC6LA's Zplots spreadsheet.
I've used this with good results for 50 and 75 ohm coax, and for
closely spaced parallel wire line in the range of 90 ohms.
Although S21 data and S11 data should produce the same results, Dan
told me that users of his spreadsheet reported more consistent
results when S11 data sets were used. I've tried both, and agree that
the S11 data sets are the better way.
There's a lot of useful stuff on Dan's website. http://www.ac6la.com/
73, Jim K9YC
On 8/8/2013 2:59 PM, Steve Hunt wrote:
I managed to find time today to produce a web page on my measurements.
For anyone interested:
http://www.karinya.net/g3txq/wet_ll/
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