[TowerTalk] Open wire measurments

[Jim Brown]

Some analyzers are better than others to produce this data. OZ7OU has 
produced some excellent applications notes on the topic of VIA and VNA 
calibration for S11 measurements, and has developed an excellent test 
once the instrument is calibrated. This one is specifically written for 
the DG8SAQ VNWA, but the concept applies universally.

http://www.hamcom.dk/VNWA/How%20to%20check%20the%20calibration%20for%20perfection%20of%20the%20VNWA%20or%20any%20other%20VNA.pdf

After calibration, a short length (1-2m) of coax is measured from very 
low frequency to 100 MHz, and S11 in dB and the Smith chart are 
examined. If calibration is good, S11 should always be negative and fall 
smoothly with increasing frequency; the Smith trace should start at the 
right edge of the outer limit of the chart,and smoothly circle around 
the chart with a gradually falling radius due to increasing loss at high 
frequencies.

And I completely agree with both Wes's and Paul's comments.

There is also the need to recalibrate to whatever fixture is used to 
connect the line. I use a BNC female to dual male banana (Pomona) 
adapter with alligator clips pushed onto the bananas, and calibrate to 
the tips of the clips.

73, Jim K9YC

On 9/3/2019 1:30 PM, Paul Christensen wrote:
> Wes,
> 
> "As has been suggested elsewhere in this thread, you might be tempted to try connecting the balanced line to the unbalanced test port of the analyzer, in the belief that since it is a "floating" device it's also "balanced".  This has not worked for me."... "So the line radiates; it's part transmission line and part antenna."
> 
> No argument there, but that doesn’t mean the CIA cannot report back a valid impedance measurement.  I have yet to find an instance where it hasn’t worked with a complex Z on a balanced line.   I routinely reverse the balanced termination on the unbalanced reflection port and achieve the same result.  Moreover, I obtain the same (or nearly the same) result when using a 1:1 Mini-Circuits RF transformer, then OSL calibrate after the transformer.  By contrast, I often cannot obtain identical results when changing line polarity when: (1) using a "bench" type VNA device where powering comes from electrical mains; and (2) a transformer isn’t used.
> 
> Paul, W9AC
> 
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