Jim,
I'm comparing like for like: TLD's loss predictions for the line with a
50 Ohm load, vs my measured results with a 50 Ohm load.
I'm pretty sure my line is equivalent to Wireman 551, and therefore
would expect Zplots should deliver similar K0,K1 and K2 coefficients to
those Dan has for 551.
I have a couple of hours free this morning so will re-run the open and
short circuit tests.
73,
Steve G3TXQ
On 10/08/2013 00:55, Jim Brown wrote:
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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